Chemical Oxygen Demand (COD) Calculator
Compute the Chemical Oxygen Demand of a water or wastewater sample from ferrous ammonium sulfate (FAS) back-titration data — the standard dichromate reflux procedure used in environmental laboratories worldwide.
mL
mL
eq/L
mL
Significantly polluted wastewater
- 1
FAS volume difference
V_blank − V_sample = 15 − 8.2 = 6.8 - 2
Multiply by normality × 8000
6.8 × 0.1 × 8000 = 5,440 - 3
COD (mg O₂/L)
÷ V_s = ÷ 20 = 272COD = (V_blank − V_sample) × N × 8000 / V_s — factor 8000 is the O₂ equivalent weight (8 g/eq) × 1000 mL/L.
How does this calculator work?
COD (mg O₂/L) = (V_blank − V_sample) × N × 8000 / V_s, where V values are FAS titrant volumes (mL), N is FAS normality (eq/L), 8000 is the O₂ equivalent factor, and V_s is the sample volume in mL. Based on APHA Standard Method 5220C.
Formula
How this is calculated
The dichromate COD method (Standard Methods 5220C, APHA/AWWA/WEF) digests the water sample with excess potassium dichromate under acidic conditions at 150 °C for two hours. Dichromate oxidises virtually all organic matter and some inorganic reducing agents. After digestion, the unreacted dichromate is back-titrated with ferrous ammonium sulfate (FAS) solution using ferroin indicator. The blank — a reagent-only run without sample — undergoes the same procedure. The difference in FAS volume (V_blank − V_sample) represents the dichromate that was consumed by the sample.
The COD formula converts this difference to mg/L of equivalent oxygen. The factor 8000 derives from the equivalent weight of oxygen: O₂ has a molecular weight of 32 g/mol and accepts 4 electrons per molecule, giving an equivalent weight of 8 g/eq; multiplying by 1000 mL/L converts to the standard units. The FAS normality N is the concentration of reducing equivalents per litre.
Typical FAS normality is 0.025 N (for low-COD samples such as drinking water or clean river water) to 0.25 N (for concentrated wastewater). Results below 10 mg/L indicate very clean water; values above 250 mg/L generally require biological or chemical treatment before discharge. The method does not distinguish between biodegradable and non-biodegradable organic matter, nor does it account for chloride interference (which can be suppressed by adding mercuric sulfate to the digestion flask).
Frequently asked questions
Chemical Oxygen Demand (COD) measures the amount of oxygen needed to chemically oxidise all organic and inorganic matter in a water sample. It is a key water-quality indicator for regulatory compliance, effluent treatment design, and process monitoring in wastewater plants. Unlike BOD (Biochemical Oxygen Demand), COD results are available in hours rather than days and capture non-biodegradable organics too.
BOD (5-day biochemical oxygen demand) measures only the oxygen consumed by microbial decomposition of biodegradable matter over 5 days at 20 °C. COD includes all oxidisable substances — biodegradable, non-biodegradable organic, and some inorganic reducing agents — so COD ≥ BOD. A typical BOD:COD ratio for domestic sewage is 0.5–0.7; industrial effluents containing refractory compounds often have a much lower ratio.
If the sample FAS volume exceeds the blank, the result is negative — which is physically impossible and indicates an error. Common causes include a contaminated blank, mislabelled solutions, an incorrect FAS normality, or sample carryover. Check that the blank was prepared identically to the sample but with distilled water instead of sample, and that the FAS solution has been freshly standardised.
TG we-Calculate Editorial Team. (2026). Chemical Oxygen Demand (COD) Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/chemical-oxygen-demand-calculator
TG we-Calculate Editorial Team. "Chemical Oxygen Demand (COD) Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/chemical-oxygen-demand-calculator.
TG we-Calculate Editorial Team, "Chemical Oxygen Demand (COD) Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/chemical-oxygen-demand-calculator
@misc{wecalculate_chemical_oxygen_demand_calculator, title = {Chemical Oxygen Demand (COD) Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/chemical-oxygen-demand-calculator}}, year = {2026}, note = {TG we-Calculate} }
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