BOD from dissolved oxygen before and after incubation, with dilution.
Biochemical Oxygen Demand (BOD) Calculator: Free Online Tool
Biochemical oxygen demand, or BOD, shows how much oxygen micro-organisms use while breaking down organic matter in water over a set time. It is one of the main tests used to judge the strength of sewage and the quality of treated effluent or river water.
This calculator works out BOD from dissolved oxygen readings taken before and after incubation, using up to three sample dilutions in a bottle of known volume. It also applies a seed correction if you used seeded dilution water. It averages only the dilutions that give valid oxygen depletion. Environmental engineers, plant operators and students use it because choosing valid dilutions by hand is easy to get wrong. The tool works in feet and inches or in metres and millimetres, so it suits both older site practice and metric drawings, and every result can be rechecked by hand.
When to use this calculator
Use it after the incubation period is over and you have measured the final dissolved oxygen in each bottle. It suits sewage treatment plant laboratories, effluent compliance checks, river water studies and academic lab work. It is most useful when you have set up several dilutions of the same sample, because it tells you which ones are valid and gives one averaged answer. The test is usually run for 5 days at 20 degrees C. Leave a dilution blank if you did not prepare it.
How to use the Biochemical Oxygen Demand (BOD) Calculator
- Open the calculator above on your phone, tablet or computer. Nothing needs to be installed and no sign-up is required.
- Enter your values in the input boxes: BOD bottle volume (ml), Seed correction (mg/L, 0 if unseeded), Sample in bottle (ml), Initial DO D1 (mg/L), Final DO D2 (mg/L). The boxes already hold sample values, so overwrite them with your own.
- Press Calculate. The result appears straight away under the button.
- Change any value and press Calculate again to compare options. Press Reset to go back to the sample values.
- Check the result against your drawings, project specification and local code before you order material or approve work.
How the calculation works
The calculator computes BOD (mg/L) = ((D1 - D2) - seed correction) / P, where P = sample volume / bottle volume. D1 is the initial dissolved oxygen and D2 the final dissolved oxygen, both in mg/L. The defaults are a 300 ml bottle and no seed correction. For example, 10 ml of sample gives P = 0.0333, and a drop of 2.1 mg/L gives 63.0 mg/L. Only dilutions that deplete at least 2 mg/L of DO and leave at least 1 mg/L are averaged. With the defaults, dilution 3 is marked not valid and the result is 63.5 mg/L.
Worked example
When you open the page the calculator is filled with sample values. Pressing Calculate with those values gives the result below, which you can use to check that you are reading the output correctly.
| BOD of each dilution | |
|---|---|
| Dilution 1 (P = 0.0333) | 63.0 mg/L |
| Dilution 2 (P = 0.0500) | 64.0 mg/L |
| Dilution 3 (P = 0.0833) | 90.0 mg/L (not valid) |
| Result | |
| BOD (average of valid dilutions) | 63.5 mg/L |
Tips and common mistakes
- Choose dilutions so that the oxygen depletion is at least 2 mg/L and the remaining oxygen is at least 1 mg/L. Other bottles are ignored by the calculator.
- Enter the seed correction in mg/L only if you used seeded dilution water. For unseeded samples, keep it at 0.
- Use the same bottle volume for all dilutions, and measure D1 and D2 with a calibrated DO meter or the same titration method.
- Incubate the bottles for 5 days at 20 degrees C in the dark. This calculator cannot check that the incubation conditions were correct.
- Compare the BOD from different valid dilutions. If they differ widely, the sample may contain toxic matter or the dilution water may be of poor quality.
Frequently asked questions
What does P mean in the BOD formula?
P is the dilution fraction, equal to the sample volume divided by the bottle volume. For 10 ml of sample in a 300 ml bottle, P is 0.0333. The oxygen drop is divided by P to give BOD.
Why is one of my dilutions marked not valid?
The calculator accepts a dilution only if the oxygen drop is at least 2 mg/L and the final oxygen is at least 1 mg/L. Bottles outside these limits are shown but left out of the average.
When should I enter a seed correction?
Enter a seed correction when you used seeded dilution water, for example for sterilised or chlorinated effluent. It is subtracted from the oxygen drop before dividing by P. For unseeded samples, enter 0.
Can I use a bottle size other than 300 ml?
Yes. Change the bottle volume field to match your bottles. The calculator then recalculates P for each dilution using your bottle volume in ml, so use the same units for sample volume.
How many dilutions do I need?
You can use up to three. More than one is recommended, because it increases the chance that at least one gives a valid oxygen depletion. Leave unused rows blank.
Important note
This tool gives estimates for planning, study and quick checks. It does not replace design by a qualified engineer, site-specific testing, or the requirements of your project specification and local building code. Always confirm final quantities, test results and acceptance limits with the responsible engineer before using them for construction or payment.
