BOD / COD Calculator

BOD = (DO_initial − DO_final) × dilution factor; COD via dichromate titration.

Last updated: October 2026How we build & verify

BOD

Initial DO (mg/L)

Final DO (mg/L)

Dilution Factor

COD (Dichromate)

Blank titrant vol. (mL)

Sample titrant vol. (mL)

Titrant (FAS) normality

Sample Vol (mL)

BOD

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COD

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BOD = (DO initial − DO final) × dilution factor (dilution factor = bottle volume ÷ sample volume). COD = (blank − sample titrant mL) × N × 8000 ÷ sample mL, with the titrant being ferrous ammonium sulfate (standard methods, closed/open reflux dichromate).

How it works

BOD and COD both quantify how much oxygen a water sample will consume, by two different routes. BOD tracks the dissolved oxygen that microbes actually use over an incubation, classically five days in the dark at 20 °C, so it measures the biodegradable fraction. COD oxidises the sample chemically with dichromate and captures organics the bacteria will not touch. COD returns in about three hours; BOD takes five days, which is why most plants run both.

Formula

BOD₅ (mg/L) = (DO_initial − DO_final) × Dilution factor
COD (mg/L) = (V_blank − V_sample) × N × 8000 / mL of sample

Variables

DO_initial, DO_finaldissolved oxygen in the bottle on day 0 and day 5, mg/L
Dilution factortotal bottle volume ÷ volume of sample in it (5 mL in a 300 mL bottle gives 60)
V_blank, V_samplemL of ferrous ammonium sulphate titrant used on the reagent blank and on the digested sample
Nnormality of the FAS titrant
8000milligrams of oxygen per equivalent (8 g/eq × 1000 mg/g)

Worked example

Inputs: 5 mL of effluent in a 300 mL BOD bottle, DO 8.4 mg/L on day 0 and 3.1 mg/L on day 5. COD on the same effluent: 10 mL digested, blank titre 10.0 mL, sample titre 7.0 mL, 0.25 N FAS.

Dilution factor: 300 / 5 = 60
Oxygen depletion: 8.4 − 3.1 = 5.3 mg/L
BOD₅ = 5.3 × 60 = 318 mg/L
COD = (10.0 − 7.0) × 0.25 × 8000 / 10 = 600 mg/L
Ratio: 600 / 318 = 1.89

Result: BOD₅ 318 mg/L, COD 600 mg/L, a COD to BOD ratio of 1.89

Notes

A dilution only counts if it lands inside the working window: at least 2.0 mg/L of oxygen consumed and at least 1.0 mg/L still remaining on day 5. Outside that, the bottle either ran out of oxygen or barely moved, and the number is not defensible. Labs set up three or four dilutions per sample precisely so that at least one qualifies.
COD exceeds BOD for effectively every real sample, and the ratio is the diagnostic. Raw municipal sewage typically runs 1.5 to 2.5, and 1.89 above sits in the middle of it. Ratios climbing past about 4 point to industrial input or an inhibitory load and warn you that a biological stage alone will not meet consent.
Nitrifying bacteria begin drawing oxygen in some samples before day 5, inflating the result with demand that has nothing to do with carbon. Adding a nitrification inhibitor and reporting CBOD is standard for nitrified effluent, and mixing CBOD and BOD figures across a dataset produces a trend that is not real.

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Frequently Asked Questions

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