COD Dilution Calculator

A colorimetric COD reading that lands outside the calibration curve is worthless, and pushing a high-strength sample into a low-range kit is the most common way analysts produce a wrong number. This calculator works backwards from your standard curve: given an absorbance, it tells you the concentration, whether you diluted enough, and what to dilute to next time.

Concentration from your calibration curve

Curve slope Abs per mg/L

Curve intercept Abs units, usually 0.000–0.010

Sample absorbance at 600 nm, blanked

Dilution factor 1 = undiluted, 10 = 1:10

Why dilution is decided before the test, not after

A COD kit has a defined calibration range, typically 0–1000 mg/L for a standard kit. A wastewater sample at 3500 mg/L will saturate the absorbance reading and return a number that looks plausible but is wrong. The correct action is to dilute until the expected reading lands near the top of the range, not the middle — that is where relative error is smallest.

Work out the required dilution factor from an expected concentration before you pipette, not after you see a result you do not like.

Choosing the dilution factor

Expected COD (mg/L)	Recommended dilution	Target absorbance	Typical use
50 – 200	none (1×)	0.2 – 0.6	Diluted effluent, surface water
200 – 800	2×	0.3 – 0.7	Secondary treatment effluent
800 – 2000	5×	0.3 – 0.7	Primary effluent
2000 – 5000	10× to 20×	0.3 – 0.7	Raw industrial wastewater
5000 – 20000	25× to 100×	0.3 – 0.7	High-strength process effluent
> 20000	sample pretreatment	—	Use a high-range kit or membrane method

What this calculator does and does not do

It performs arithmetic on a standard curve you supply and reports a screening estimate. It is a cross-check on your bench work, useful for catching a mis-pipetted dilution or a curve that has drifted.

It is not a compliance method. Regulatory COD is determined by the dichromate digestion method (EPA 410.4 / SM 5220 B), and a permit will name the method. Report the method result, and use this only to plan dilutions and to sanity-check the laboratory.

Standard curve quality decides the answer

The slope and intercept come from your own calibration regression. If the curve was built on a stale reagent lot, or the blank was not properly zeroed, every value derived from it is biased — and the calculator will faithfully reproduce that bias. Rebuild the curve whenever reagents change, and record the R².

A curve with R² below 0.995 should not be used for reporting. A visibly curved relationship means the absorbance is beyond the linear region, and the sample needs more dilution, not a polynomial fit.

Instruments for this measurement

BOD206

BOD analyser, mercury-free differential pressure

0–4000 mg/L · 0.01 mg/L resolution

Spectrophotometer workflow

Bench COD / colourimetric analysis

Used with a 600 nm visible spectrophotometer

CLO201

Free chlorine colourimetric analyser

Same 445 nm visible-light principle as COD

Models shown are representative families, not a complete catalogue. Send us your application, water analysis and installation constraints and we will match instruments to it.

Frequently asked

How do I calculate the dilution factor for a COD test?

Divide the expected concentration by the top of your kit’s calibration range, then round up to a practical factor. A 3500 mg/L sample tested against a 1000 mg/L kit needs at least 4×, so 5× is the sensible choice. Aim for the expected reading to land in the upper half of the curve.

Does a higher dilution give a more accurate result?

More accurate, up to a point. Dilution reduces the sample matrix that interferes with the colour reaction, so accuracy usually improves. Beyond about 10×, pipetting error starts to dominate, because each 0.1 mL error becomes larger in absolute terms.

Why does my COD result change when I re-run the same sample?

Usually the curve, not the sample. Rebuild the calibration whenever the reagent lot changes, keep the blank consistent, and check that the R² is still above 0.995. Temperature and digestion time also matter — both must be held to the method’s specification.

Can this replace a laboratory COD result?

No. It is an arithmetic aid for planning dilutions and cross-checking bench work. For any value you report, use the method named in your permit — typically EPA 410.4 or Standard Method 5220 B.

What is the difference between COD and BOD₅?

COD measures chemically oxidisable demand using a strong oxidant and completes in about two hours, capturing everything oxidisable including non-biodegradable material. BOD₅ measures the oxygen microorganisms consume over five days, capturing only the biodegradable fraction. COD is always the larger number, typically two to three times BOD₅ for domestic sewage.

This page provides calculation guidance and a screening cross-check. It is not a substitute for the measurement method named in your permit, and values shown are typical ranges rather than guarantees for a particular instrument.

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