Chemical Oxygen Demand Calculator (COD in mg/L from dichromate titration)

Understanding the chemical oxygen demand (COD) calculator — concept and principle

Chemical oxygen demand (COD) measures how much oxygen is needed to chemically oxidise the organic matter in a water sample. It is a standard indicator of pollution in wastewater. This calculator works out COD from a dichromate reflux titration.

The sample used up 2.325 meq of dichromate, worth 18.6 mg of oxygen in 50 mL: 372 mg O₂/L.

How it is calculated

The sample is boiled with a known excess of potassium dichromate in sulfuric acid; organics reduce part of it.
The remaining dichromate is titrated with ferrous ammonium sulfate (FAS) and compared with a blank.
(A − B) mL × M = milli-equivalents of oxidant used, where A is the blank titre and B the sample titre.
One equivalent of oxygen is 8 g, so COD (mg O₂/L) = (A − B) × M × 8,000 ÷ sample mL, times any dilution factor.

The principle

Organic matter reduces dichromate just as it would consume oxygen. The blank shows how much FAS is needed when no sample is present; the sample needs less, because part of the dichromate was used up. That difference, in equivalents, is converted to the mass of oxygen that would do the same oxidation. COD captures most organic matter, including material bacteria cannot easily break down, so it is usually higher than BOD.

Worked example

Default inputs: blank 24.5 mL, sample 15.2 mL, FAS 0.25 M, sample 50 mL, dilution 1.

A − B = 24.5 − 15.2 = 9.3 mL.
Dichromate consumed = 9.3 × 0.25 = 2.325 meq.
Oxygen equivalent in the flask = 2.325 × 8 = 18.6 mg O₂.
COD = 18.6 mg ÷ 0.050 L = 372 mg O₂/L = 0.372 kg O₂/m³.
Result is within the method range (≥ 50 mg/L).

Each point is the calculator re-run with only “Sample volume digested (mL)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Sample volume digested (mL)	Chemical oxygen demand (mg O₂/L)
12.5	1,488
25	744
37.5	496
50	372
62.5	297.6
75	248
100	186

How to read the result

Compare the result with the discharge limit in your permit or local regulation, which varies by country and type of discharge. Typical untreated domestic wastewater has COD in the hundreds of mg/L, while well-treated effluent is much lower. The COD ÷ BOD ratio indicates how biodegradable the wastewater is.

Common mistakes

Swapping blank and sample titres, giving a negative result.
Forgetting the dilution factor for strong samples.
Ignoring chloride interference; chloride must be masked with mercuric sulfate.
Using the macro method for very low COD, where a low-range method is needed.

Keep in mind: COD analysis uses toxic and corrosive reagents (dichromate, mercury, concentrated acid). Follow the standard method and laboratory safety rules. Results for compliance must come from accredited labs.

Frequently asked questions

What is the difference between COD and BOD?

COD measures oxygen needed to oxidise organics chemically; BOD measures oxygen used by microorganisms over five days.

Why is 8,000 in the formula?

One equivalent of oxygen weighs 8 g = 8,000 mg, converting milli-equivalents to mg of O₂.

What is a typical COD for domestic wastewater?

Usually several hundred mg/L before treatment, depending on water use.

Sources: APHA, AWWA & WEF, Standard Methods for the Examination of Water and Wastewater, Method 5220 B (open reflux) · ISO 6060:1989, Water quality — Determination of the chemical oxygen demand

Checked 9 Nov 2026. COD (mg O₂/L) = (A − B) × M × 8000 ÷ mL sample: King Fahd University of Petroleum & Minerals, CE 370 laboratory sheet “COD Experiment”; method scope (reflux with dichromate, FAS titration with ferroin, range ≥ 50 mg/L): US EPA Method 410.1 summary, National Environmental Methods Index (NEMI).

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