Chemical Oxygen Demand Calculator

Calculate COD from blank and sample titration data to assess oxidizable pollution in water and wastewater.

COD titration data

Enter titrant volumes, titrant normality, and the digested sample volume using consistent milliliter units.

Blank titrant volume (mL)

Sample titrant volume (mL)

Titrant normality (N)

Sample volume (mL)

Result unit

About chemical oxygen demand

Chemical oxygen demand, abbreviated COD, estimates the amount of oxygen equivalent consumed when oxidizable material in water is chemically oxidized. It is widely used to characterize wastewater strength, monitor treatment performance, and detect changes in industrial discharges. Results are normally reported as milligrams of oxygen per liter, allowing samples and treatment stages to be compared on a common basis. A common closed-reflux method digests the sample with a known excess of strong oxidant under acidic conditions. After digestion, the remaining oxidant is measured by titration. A reagent blank undergoes the same procedure without sample. The blank normally consumes more titrant than the sample because the sample's oxidizable matter used part of the original oxidant. The difference between blank and sample titrant volumes is therefore proportional to oxygen demand. For the conventional titration calculation, COD in milligrams per liter equals the blank-minus-sample volume in milliliters, multiplied by titrant normality and 8000, then divided by sample volume in milliliters. The factor 8000 incorporates the equivalent mass of oxygen and unit conversion. This calculator also converts the result to grams per liter by dividing milligrams per liter by 1000. All entered volumes must refer to the method's actual aliquots and must use milliliters. COD is not identical to biochemical oxygen demand. COD uses a chemical oxidant and can include compounds that microorganisms do not readily degrade, while biochemical oxygen demand measures microbial oxygen consumption over a specified incubation period. COD can usually be completed much faster, making it useful for routine process control, but the relationship between the two measures depends on wastewater composition. Reliable results require representative sampling, correct preservation, careful reagent standardization, complete digestion, and a valid method blank. Chloride and other substances can interfere unless the prescribed method controls them. Very high COD samples may need dilution so the titration remains in range. This calculator handles only the arithmetic after a valid laboratory procedure; it cannot determine whether digestion, endpoint detection, interferences, or quality-control checks were acceptable. Follow the approved method used by your laboratory and retain appropriate significant figures.

COD calculation examples

Worked titration values demonstrate the blank correction and unit conversion.

Titration data	COD	Calculation
Blank 20 mL; sample 12.5 mL; 0.1 N; sample 50 mL	120 mg/L	The 7.5 mL difference is multiplied by 0.1 and 8000, then divided by 50.
Blank 25 mL; sample 5 mL; 0.25 N; sample 20 mL	2000 mg/L or 2 g/L	A larger titrant difference and normality indicate a high oxygen demand.
Blank 10 mL; sample 8 mL; 0.025 N; sample 100 mL	4 mg/L	A small blank correction produces a low COD result.

How to calculate COD

Enter the titrant volume measured for the reagent blank.
Enter the titrant volume measured for the digested sample.
Provide titrant normality and the sample aliquot volume in milliliters.
Choose milligrams or grams per liter and select Calculate COD.

Chemical oxygen demand FAQ

What does a high COD value indicate?

A high value indicates a larger quantity of chemically oxidizable material in the sample. In wastewater this often corresponds to a greater organic pollution load, though some inorganic reducers also contribute.

Why is a blank measurement required?

The blank establishes titrant consumption when no sample demand is present. Subtracting the sample titration from the blank isolates oxidant consumption associated with the sample.

Is COD the same as biochemical oxygen demand?

No. COD measures chemical oxidation under specified test conditions, while biochemical oxygen demand measures oxygen used by microorganisms during incubation.

Why might sample titrant exceed the blank?

That pattern usually signals an input reversal, endpoint problem, contamination, or procedural issue. It would imply negative COD, so this calculator asks you to check the measurements.

Can COD be converted directly to organic carbon?

There is no universal conversion because different compounds have different oxidation states and oxygen requirements. A site-specific empirical relationship may be developed from paired measurements.