This is a newly redesigned Water-Research.net page
Page Archive

Water Quality Index Calculator for Surface Water Guide

General Guidance for Educational Purposes and Information on Our Version of the Water Quality Index Calculator for Surface Water

Stream Flow and Real-Time Water Quality Monitoring QC/QA Programs

Stream Assessments, Biological Monitoring, Remote Data Logging

The Know Your H20™ Water Quality Index Calculator for Surface Water

This calculator is based on the original WQI Calculator that was developed through a consensus method in the 1970s by the National Sanitation Foundation (NSF). For this process to work, we strongly suggest reviewing this information sheet regarding the data requirements and how the process assumes you are entering the data. After you enter the data, it is used to calculate a “Q” value for each parameter. The “Q value” for each parameter you enter is then multiplied by a weighting factor to attempt to account for the role or influence of the individual parameters. We have modified the equation to calculate weighted values if all the data are not entered.

We hope that you find our version of the surface water quality tool helpful.  We added a few elements to the original tool, such as:

1. Updating the equations.
2. Clarifying the input values.
3. Providing the ability to georeference a site, i.e, a mapping feature with GPS location.
4. Adding Country and Zip Code (Note: This tool is used in at least 50 countries).
5. Adding a report feature.
6. Restructuring the tool for teaching applications.

If you used this tool in the past, it is possible we have a copy of your previous data and information and if you create an account using an email address you may be surprised to find your historic data entry.

We hope to permit adding other data to the tool, such as: iron, manganese, acidity, secchi-disk depth, chloride, sulfate, and chlorophyll.  We are currently working on tools to estimate these values from the other data and a tool to make it easier to calculate percent saturation.

With time, we hope to create a specific calculator for "sea water.”

The following are notes on the specific parameters:

Percent Saturation – Dissolved Oxygen (DO)

You do not enter the measured dissolved oxygen content of the water for this parameter but rather the degree of saturation of oxygen in the water. The maximum solubility of oxygen in water is a function of barometric pressure, elevation, water salinity, degree of reaeration, and other biological/chemical processes. The minimum value is zero and a reasonable maximum value is 200%.  The Q Index value for an oxygen saturation of over 140 % is equivalent to 50.

You might wonder how it is possible to have as much as double the amount of oxygen dissolved in water as the water can hold (the saturated concentration, 100%). A simple analogous example is carbonated soda in which the concentration of dissolved CO2 in the soda is many times the saturated concentration at normal air pressure. The amount of dissolved CO2 in the soda is no longer in equilibrium once pressure is released by opening the container and, over time, the excess CO2 will leave the soda until the dissolved CO2 is reduced to the saturated concentration under the new (lower pressure) conditions (the soda goes flat).  You can have a similar situation with DO in water.  

Calculating the Degree of Saturation of Oxygen 


PS = Percent Saturation ( %) = ([DO]/[DOsat]) x 100


[DO] = observed or field measured oxygen concentration (mg/L).

‍[DOsat aka DO1] = saturated concentration of oxygen at the local temperature (and possibly other conditions such as  altitude, barometric pressure, and salinity or conductivity).‍

The brackets, [ ], mean ‘concentration of,’ so that [DO] means the concentration of (measured or actual) dissolved oxygen and [DOsat] means the concentration of dissolved oxygen under saturated conditions.

You can measure the dissolved oxygen, [DO], directly from the handheld monitoring equipment you are using (a DO meter) and calculate the saturated concentration of oxygen, [DOsat], from a standard table of water oxygen saturation levels based on the altitude, barometric pressure, temperature, and salinity of the water, from a lookup monograph, or by using tools generated and maintained by the USGS. We are in the process of developing an online calculator to take into consideration the elevation of the sampling site and the temperature.

Method 1: Spreadsheet and Calculations

Equation #1 - Calculate Atmospheric Pressure from Elevation Data assuming you are not at sea level or  at an atmospheric pressure of 1 atm.

Elevation (E) – meters

Equivalent Pressure for Elevation in Pascals (P1) - Pascals (the standard metric unit for pressure)

Equivalent Pressure for Elevation in atmospheres (P2) - Atmospheres (atm) or bars

P1 =101325* (1 - 0.0000225577*(E))^5.25588 (Pascals) - Note 1 atm = 101,325 Pascals

P2 =P1*0.000009869233 (atm) - Note: 1 Pascal = 0.000009869233 atm

Equation #2 - Calculate Saturated Oxygen Concentration, [DO1],  at 1 atm (DO1 is  in mg/L)

t = Water Temperature (Celsius, or °C) - Note that °C = (5/9)(°F – 32)

T = t + 273.15 = Water Temperature (Kelvins, or K)

P3 = Partial Pressure of Water Vapor at Water Temperature (T), in atm

ln P3 = 11.8571 – (3840.7/T) – (216961/T^2) – Note: ln means natural log of; T^2 means T squared

P3 = EXP(ln P3) – Note: EXP( ) means raise e to whatever power is in parentheses 

[DO1] = EXP(7.7117 – 1.31403*ln (t+45.93)) – Note: * means multiplied by

Equation #3 - Calculate Saturated Oxygen Concentration, [DO2] in mg/L, at Sampling Site Elevation and Pressure

DO2 = = DO1 * P1 * { (1 – (P3/P))(1 – XP1) / (1 – P3)(1 – X) }

X Value = 0.000975 - (0.00001426 t) + (0.00000006436 t^2)

For Example:

Elevation – 500 feet or 152.4 meters

P1 = 99507.539 P

P2 =0.98206 atm

t = 5

DO1 = 12.77 mg/L (at 1 atm)

DO2 = 12.54 mg/L (corrected for elevation or pressure)

Equation # 4 - Percent Saturation (%)

DO in the field = 8 mg/L at 5 °C

Percent Saturation (%) = (100 * DO) / DO2

Percent Saturation (%) = (100 * 8 mg/L) / 12.54 = 63.8 %

For a range of water temperatures at the same site:

Table 1 | Correct DO1 Maximum Oxygen Saturation Value for Elevation, i.e., Calculate DO2

Pressure 1 atm 0.98 atm (P2)
Water Temperature (t) (°C) DO1 (mg/L) DO2 (mg/L)
0 14.63 14.36
2 13.83 13.58
4 13.11 12.87
5 12.77 12.54
8 11.84 11.63
10 11.29 11.09
12 10.78 10.58
14 10.31 10.12
16 9.87 9.69
18 9.47 9.3
20 9.1 8.93

- A detailed equation for the calculation of DO2, dissolved oxygen corrected for pressure,  that can be used to develop a spreadsheet calculation is as follows:

DO2 = ((EXP (7.7117 – 1.31403 ln(A+45.93))) * B * (1 – EXP (11.8571 – (3840.7/(A+273.15)) – (216961 / ((A+273.15)^2))) / B) *(1 – (0.000975 – (0.00001426 A) + (0.00000006436 (A^2)))B)) /  (1-EXP(11.8571 – (3840.7 / (A+273.15)) – (216961 / ((A+273.15)^2)))) /  (1 – (0.000975 – (0.00001426 A) + (0.00000006436 * (A))))


A = t: Water Temperature °C (this is not the temperature difference, but the actual water temperature).

B= P2; Atmospheres (atms) as a function of elevation

Percent Saturation Calculation at Non-Standard Elevation

Percent Saturation (PS) = (100 * DO measured, in mg/L) / DO2

Method 2: Graphical Method (Salinity is not a factor - Freshwater)

Using a Monograph for Water at 1 atm (760 mm Hg) Temperature of water 0 to 30 °C and a Salinity of Zero, see (PDF – Dosatruation.pdf). When using the monograph, you know the water temperature in Celsius and the field-measured dissolved oxygen in mg/L; you draw a straight line that connects the two data points, and then you see where this drawn line intersects with the line marked Percent Saturation.

(add image)

Other Resource: (PDF – Maximum-Dissolved-Oxygen-Satruation-Table.pdf)

Method 3: USGS Tools

Different Elevations and Salinity – Use this Resource - You can do individual calculations or produce values for a range of conditions (Weblink to https://water.usgs.gov/software/DOTABLES/)

Range of Conditions:

Water temperature: 0 - 40 °C or 32-104 °F

Barometric pressure: 380-836 mm Hg, 14.97-32.91 in Hg, 507-1114 mbar, 51-112 kPa, or 0.5-1.1 atm

Salinity or SC:  0-40 ‰ salinity, or 0-59118 µS/cm specific conductance

Fecal Coliform

The units areColonies per 100 ml, cfu/100 ml, or Most Probable Number per 100 ml;for the purpose of this analysis, you can enter Fecal Coliform or E. coli. counts in units per 100 ml. If the count is over 100,000 colonies per 100 ml, the Quality Index Value uses a default Q Index Value of 2. The fecal coliform group includes all of the rod-shaped bacteria that are non-spore forming, Gram-Negative , lactose-fermenting in 24 hours at 44.5 °C, and which can grow with or without oxygen and are typically associated with warm-blooded animals. In contrast, the total coliform parameter includes bacteria found in the environment, soil, and insects and the bacteria will ferment lactose at 37 °C. E coli. are fecal coliform bacteria which live inside the intestine of warm-blooded animals; whereas the coliform bacteria in general can be either fecal coliforms or non-fecal coliforms that live in the soil. Since we are typically evaluating the impact associated with wastewater, sewage, manure management, biological sludges, agricultural runoff, and other animal waste, we typically recommend testing for fecal coliform and E. coli. The General recommendation is that the fecal coliform count be < 200 colonies per 100 ml, but the lower the better.


The technical definition of pH is that it is a measure of the activity of the hydrogen ion (H+) and is reported as the reciprocal of the logarithm of the hydrogen ion activity, [H+]. Therefore, a water with a pH of 7 has 10⁻⁷ moles per liter of hydrogen ions; whereas, a pH of 6 is 10⁻⁶ moles per liter. The pH scale ranges from 0 to 14. In general, a water with a pH < 7 is considered acidic and with a pH > 7 is considered basic. The normal range for pH in surface water systems is 6.5 to 8.5 and for groundwater systems 6 to 8.5. The pH of pure water (H₂0) is 7 at 25 °C, but when exposed to the carbon dioxide in the atmosphere this equilibrium results in a pH of approximately 5.2. Because of the association of pH with atmospheric gasses and temperature, it is strongly recommended that the water be tested as soon as possible and within 15 minutes.

The pH of the water is not a measure of the strength of the acidic or basic solution and alone does not provide a full picture of the characteristics or limitations with the water supply. Alkalinity is a measure of the capacity of the water to resist a change in pH that would tend to make the water more acidic. The measurement of alkalinity and pH is needed to determine the corrosivity of the water. A pH < 2 or greater than 12 is given a Q Index Value of 0. For most aquatic systems a pH between 6.0 and 9.0 is optimal, but for flowing systems it may be wise to have a pH between 6.5 and 8.2. A pH at or near 5.5 may suggest a water body that has a very low total dissolved solids and the low pH is related to the dissolved carbon dioxide concentration of our atmosphere. If the total dissolved solids and sulfates, iron, and manganese are high it may be related to acid mine drainage or acidic rainfall.

Environmental Effects: A reduction in pH (more acidic) may allow the release of toxic metals into solution, metals that would otherwise be adsorbed onto sediment particles. Once mobilized, these metals are available for uptake by organisms, the amount of which is related to the rate of biological activity and level of the pollutant in the environment. Metal uptake can cause extreme physiological damage to aquatic life. Aluminum concentrations of 0.1 - 0.3 mg/l will increase mortality, retard growth, gonadal development, and egg production of fish. Even if the aluminum availability is low, recent studies have shown that acidity alone may cause mortality in developing brook trout.

Acidification of the aquatic system can shift the biological community to one that is less desirable for recreation and aesthetic uses, can reduce decomposition rates and nutrient cycling, reduce the variety and distribution of the biological organisms that create a health ecosystem, and make other compounds like ammonia and trace metals more toxic.


Biochemical oxygen demand (BOD),sometimes called CBOD, is the amount of dissolved oxygen needed by aerobic biological organisms in a body of water to break down organic material present in the sample to carbon dioxide. This process is a function of the biological population, temperature, time, and constituents in the water. BOD5 is the Oxygen Demand, excluding nitrification, over a 5-day period when the sample is incubated at 20 °C . Therefore, this relates to the readily available amount of decomposable organic material. The units of BOD are typically expressed in mg/L. The measurement of BOD over a default limit of 30 mg/L is a Q Index Value of 2. This level of BOD is equivalent to typical wastewater effluent that has undergone secondary treatment.


The Q calculation uses the change in temperature between the test site and the reference site for the project. This uses the classic idea of comparing two watersheds or systems by comparing your site to an ideal or unimpacted site. If you do not have a reference site, you can enter the Q value as zero. The datum that is entered is the difference in degrees Celsius between the site and the reference stream; the number you enter can be positive or negative. A value over 30 °C will have a Q score of 0. For most aquatic systems, when the Temperature exceeds 30 °C there is a significant stress on fish. Coldwater fisheries should be < 18.9 °C and warm water fisheries < 30 °C.

Warm Water Fish

These fish prefer water temperatures ranging between 18-29 °C (65-85 degrees °F); it includes fish such as smallmouth bass, largemouth bass, and bluegill.

Cold Water Fish

Fish such as trout and salmon prefer water temperature ranges between 7-18 °C (45-65 degrees °F). Cool water fish, such as striped bass, northern pike, and walleye, have a range between that of cold water and warm water fish.

Total Phosphate as Total Orthophosphate

The total phosphate data should be entered as phosphate in the form of total orthophosphate “PO₄⁼” and not “PO₄-P, P, or phosphate” in mg/L. The notation, mg PO4-P/L, means, or should be read to mean, mg of orthophosphate in the form of “P” per liter. For most systems, phosphate is the growth-limiting nutrient that controls the primary productivity of the system. Phosphorus is one of the key elements necessary for the growth of plants and animals and in lake ecosystems it tends to be the growth-limiting nutrient and is a backbone of the Krebs Cycle and production of DNA. Phosphates exist in three forms: orthophosphate, metaphosphate (or polyphosphate), and organically-bound phosphate; each compound contains phosphorus in a different chemical arrangement. The value of the total phosphate in lakes can be used to estimate or calculate a lake trophic status (TSI) using this equation: TSI = 14.42 * ln [TP] + 4.15, where TP is µg P/L or ppb as P, not as PO₄.

If you are using a calculator and your phosphate is in the form of “P,” you should correct your data by doing the following:

mg PO4-P/L * 3.0664 = mg PO4/L 

Conversion factor: this value,  3.0664, is the molecular weight of PO4 ( 30.97 + 63.9976) divided by the molecular weight of P (30.97)  or  (( 63.9976 + 30.97) / 30.97))

mg PO4/L * 0.3262 = mg P04-P/L

For flowing waters, it would be ideal if the orthophosphate concentration were < 0.01 mg P/L. For non-polluted waters, total phosphorus is normally < 0.1 mg P/L.

For Lakes
Oligotrophic Conditions < 0.02 mg P/L
Eutrophic Conditions > 0.03 mg P/L
Ideal Conditions < 0.01 mg P/L


The nitrate value used in the Q calculator is assuming that the nitrate is expressed as NO₃ and not as Nitrogen “N." In most cases, nitrates are not normally a growth-limiting nutrient because blue-green algae can fix or convert nitrogen gas into bioavailable nitrate, but in extreme pollution cases, nitrate can be a growth-limiting nutrient when the bioavailable phosphate is extremely high. The value of the total phosphate parameter in lakes is that it can be used to estimate or calculate a lake trophic status. If your data is in the form of “NO₃-N, nitrate as N, or N in mg/L,” you should use the following conversion factors:

mg N03-N/L * 4.43 = mg NO3/L 

Conversion factor: this value,  4.43, is the molecular weight of NO3 ( 47.99982+14.0067) divided by the molecular weight of N (14.0067)  or  (( 47.99982+14.0067) / 14.0067))

mg N0-3/L * 0.226 = mg N/L

In a healthy lake the level of nitrate is normally < 0.05 mg/L and in streams is rarely above 1 mg/L.

For the nitrogen series, the primary concern is ammonia. Ammonia (NH3) is a colorless gas with a strong pungent odor. Ammonia will react with water to form a weak base. The term ammonia refers to two chemical species which are in equilibrium in water: unionized NH₃ and the ionized ammonium cation, NH₄⁺. The relative proportion of the two forms present in water is mainly determined by pH. Un-ionized ammonia is the toxic form and predominates when pH is high.

In non-polluted streams the level of ammonia is normally < 1 mg/L. The EPA criterion for ammonia is 0.02 mg/L for freshwater, especially if the pH > 8.5. The acute lethal levels for fish range from 0.2 to 2.0 mg/L. Fish may suffer a loss of equilibrium, hyperexcitability, increased respiratory activity and oxygen uptake, and increased heart rate. At extreme ammonia levels, fish may experience convulsions, coma, and death. Experiments have shown that the lethal concentration for a variety of fish species ranges from 0.2 to 2.0 mg/L. Trout appear to be the most susceptible of these fish to ammonia and carp the least susceptible. Different species of fish can tolerate different levels of ammonia but in any event, less is better. Rainbow trout fry can tolerate up to about 0.2 mg/L while hybrid striped bass can handle 1.2 mg/L.

The value of the total nitrogen (TN), as mg N/L, in lakes can be used to estimate or calculate a lake trophic status (TSI) using the following equations:

TSI (TN) = 56 + 19.8 × ln (TN)

TSI2 (TN) = 10 × [5.96 + 2.15 × ln (TN + 0.001)]

(for systems in which nitrogen is the growth-limiting nutrient)


Turbidity is not commonly used in surface water monitoring but we find it to be a great tool. At this point, the Q calculator does not use secchi depth because this is commonly only used in lakes. For the Q calculator, we are using turbidity as measured in nephelometric turbidity units (ntus), but these are about the same as formazin turbidity units (ftus) and also known as Formazin Nephelometric Unit (fnu). Turbidity is a measure of the amount of light that is scattered by particles that are suspended in water and it is a function of the scatter angle for the detector and wavelength of the light source. The “ntu” and “ftu” detectors both use a 90° scatter angle, but the “ntu” detectors use a white light in the range of 400 to 680 nm and the “ftu” detectors use an infrared light source of 780 to 900 nm. In general: When the turbidity is over 100 ntu, it is excessive, but surface water is considered slightly turbid at 50 ntu.  Turbidity is basically a measure of the amount of light intercepted by a given volume of water due to the presence of suspended and dissolved matter and microscopic biota. Increasing the turbidity of the water decreases the amount of light that penetrates the water column, which can then cause changes in the aquatic ecosystem.

These changes could result in a reduction in photosynthetic activity of phytoplankton, algae, and macrophytes, which would reduce the primary productivity of the system and may result in causing less favorable Cyanobacteria (blue-green algae) to become established. Turbidity can also result in the reduction of dissolved oxygen, destroying the habitat of macroinvertebrates, and can cause gill damage/abrasion in fish.

Based on some available data, we believe a good relationship between Secchi Depth in meters and turbidity in ntu, may be defined by the following:

For turbidity values < 20 ntu or a Secchi Depth > 0.2 m

Calculated Secchi Depth = – 0.061 * (Turbidity) +1.524

For Turbidity values > 20 or a Secchi Depth < 0.2 m

Calculated Secchi Depth = – 0.0034 * (Turbidity) +0.3471

We found a USGS Study that had the following correlation:

Secchi Depth (feet) = 11.123 x Turbidity (ftu) ^(–0.637)


Secchi Data

We hope to add a field very soon to the WQI calculator, where you can enter secchi depth data. We believe the relationship between the Q score and secchi depth (meters) is described by this equation:

Q Value for secchi = 23.756 * (ln (Calculated Secchi Depth) + 84.663 (r2 value 0.9738)

Secchi depth data has also beem correlated to lake trophic status using the equation:

( TSI - S = 60 – 14.41 * ln [Secchi] ), where the Secchi Depth reading is in meters.

Secchi Disk (SD) - Lake Systems
Excellent > 15 feet
Poor < 2 feet
Oligotrophic > 26 feet
Mesotrophic > 13 to < 26 feet
Eutrophic > 6.5 to < 13 feet

Secchi Disk (SD)- Lake Systems

Excellent - > 15 feet

Poor < 2 feet

Oligotrophic > 26 feet

Mesotrophic > 13 to < 26 feet

Eutrophic > 6.5 to < 13 feet

Total Solids or Total Dissolved Solids (TDS)

Water is a good solvent and picks up impurities easily. Pure water -- tasteless, colorless, and odorless -- is often called the universal solvent. Dissolved solids" refer to any minerals, salts, metals, cations, or anions dissolved in water. Total dissolved solids (TDS) comprise inorganic salts (principally calcium, magnesium, potassium, sodium, bicarbonates, chlorides, and sulfates) and some small amounts of organic matter that are dissolved in water. In general, the total dissolved solids concentration is the sum of the cations (positively charged) and anions (negatively charged) ions in the water. Therefore, the total dissolved solids test provides a qualitative measure of the number of dissolved ions but does not tell us the nature of ion relationships. Typically, unpolluted water has a total dissolved solids of < 30 mg/L, but total dissolved solids does not usually pose a significant concern in freshwater until the concentration exceeds 1500 mg/L.

The total dissolved solids concentration can be related to the conductivity of the water, but the relationship is not a constant. The relationship between total dissolved solids and conductivity is a function of the type and nature of the dissolved cations and anions in the water and, possibly the nature of any suspended materials. For example, NaCl and KCl solutions with the same conductivity of 10000 umhos/cm will not have the same concentration of NaCl or KCl and they will have a different total dissolved solids concentration. Conductivity is measured through the use of a meter and is usually about 100 times the total cations or anions expressed as equivalents and the total dissolved solids (TDS) in ppm usually ranges from 0.5 to 1.0 times the electrical conductivity. We strongly recommend measuring the field conductivity of water samples in the field and in the laboratory at the time the total dissolved solids testing is being conducted. This way you can calculate a more representative or unique ratio of TDS to Conductivity for a specific site.

Environmental Effects

The introduction of excess organic matter or soluble organic materials may result in a depletion of oxygen from an aquatic system through chemical or biological oxygen consumption or demand. Exposure to low dissolved oxygen levels (< 5 to 6 mg/l ) may not directly kill an organism, but will increase its susceptibility to other environmental stresses. Exposure to < 30% saturation (<2 mg/l dissolved oxygen) for one to four days may kill most of the biota in a system. If oxygen-requiring organisms perish, the remaining organisms will be air-breathing insects and anaerobic (not requiring oxygen) bacteria.


If all oxygen is depleted, aerobic decomposition ceases and organic decomposition or processing is accomplished through anaerobic reactions. Anaerobic microbes obtain energy from oxygen bound to other molecules such as sulfate compounds and can result in the mobilization of many otherwise insoluble compounds, such as happens in Acid Mine Drainage. The breakdown of sulfate compounds will often impart a "rotten-egg" smell (from hydrogen sulfide, H₂S) to the water, affecting its aesthetic value and preventing recreational use.

No items found.

Additional Resources

Get Informed

No items found.

Get Tested

Level 1
Level 2
No items found.
Level 3
No items found.
Level 4

Get Treatment

Short Term
No items found.
No items found.
No items found.

Get Tools

Get Training

Archive Page Reference
This is a newly redesigned Water-Research.net page. To reference related archived Water-Research.net page(s) click the link(s) below:
No items found.