How Do You Calculate BOD and COD?

The measurement of BOD (Biochemical Oxygen Demand) and COD (Chemical Oxygen Demand) is crucial in determining the level of organic pollution in water bodies. Both BOD and COD tests are widely used in environmental analysis to assess water quality. In this article, we will delve into the details of how these measurements are calculated.

What is BOD?

BOD is a measure of the amount of dissolved oxygen required by aerobic microorganisms to decompose organic matter in water. It indicates the level of organic pollution present in the water. The higher the BOD value, the greater the amount of organic material present, which can deplete oxygen levels and harm aquatic life.

How is BOD Calculated?

To calculate BOD, a laboratory test called the BOD5 test is typically performed. This test measures the dissolved oxygen (DO) levels before and after a period of five days under controlled conditions.

The calculation formula for BOD is:

BOD = DOi – DOf


  • DOi represents the initial dissolved oxygen concentration (in mg/L) at the beginning of the 5-day incubation period.
  • DOf represents the final dissolved oxygen concentration (in mg/L) after 5 days.

What is COD?

COD is another important parameter used to assess water quality. It measures the amount of oxygen required to chemically oxidize both biodegradable and non-biodegradable organic compounds in water. Unlike BOD, COD provides a more comprehensive evaluation by including substances that cannot be broken down by microorganisms.

How is COD Calculated?

The calculation of COD involves a laboratory test called the COD test. In this test, a strong oxidizing agent, such as potassium dichromate (K2Cr2O7), is added to the water sample. The oxidizing agent reacts with the organic matter, producing a color change.

The formula to calculate COD is:

COD = (V1 – V0) Γ— N Γ— 8000 / V


  • V1 represents the volume (in mL) of the titrant solution used in the blank determination.
  • V0 represents the volume (in mL) of the titrant solution used in the sample determination.
  • N represents the normality of the titrant solution.
  • V represents the volume (in mL) of the water sample taken for analysis.

Difference Between BOD and COD Measurements

The main difference between BOD and COD measurements lies in their principles. BOD measures the oxygen consumed by microorganisms during biological degradation, while COD measures both biodegradable and non-biodegradable organic compounds by chemical oxidation. Consequently, BOD takes longer to determine compared to COD since it relies on microbial activity over a period of five days.

In conclusion,

BOD and COD measurements play a vital role in assessing water quality and determining the level of organic pollution. By understanding the principles and calculations behind BOD and COD, environmental analysts can effectively evaluate the impact of organic matter on water bodies.

By implementing these calculations, we can better monitor, manage, and protect our valuable water resources for a sustainable future.

Photo of author

Lindsay Collins