What is COD and why does it matter?

Chemical Oxygen Demand (COD) measures the amount of oxygen required to chemically oxidize all organic matter in water. It is a key indicator of water quality and organic pollution load, widely used in wastewater treatment design and discharge permit compliance.

How is COD measured from absorbance?

COD is measured colorimetrically by digesting a sample with potassium dichromate at high temperature. The reduced chromium produces a color change read at 600 nm on a spectrophotometer. A standard curve (absorbance vs. known COD concentrations) is used to convert the reading to mg/L.

What is the typical COD:BOD5 ratio?

For domestic wastewater the COD:BOD5 ratio is typically 1.5–2.5. This tool uses a default ratio of 2.0 as a conservative estimate. Adjust the ratio in the advanced section if you have site-specific data.

How is organic load calculated?

Organic load (kg/day) = COD (mg/L) × Flow (m³/day) ÷ 1,000. This converts concentration and volume into a mass per day, which is used to size biological treatment units and clarifiers.

What dilution factor should I use?

If your sample was diluted before measurement (e.g., 1 mL sample in 10 mL total = 10× dilution), enter 10. If the sample was measured undiluted, enter 1. The calculator multiplies the result by the dilution factor to get the true COD.

What does oxygen demand output mean?

Theoretical oxygen demand (ThOD) for complete oxidation equals COD. For biological treatment sizing, the oxygen transfer rate required equals the daily COD load adjusted for treatment efficiency. This tool outputs the raw oxygen demand as kg O₂/day for initial design estimates.

Chemical Oxygen Demand (COD) Calculator

Enter absorbance and standard curve parameters to calculate COD concentration, organic load, BOD5 estimate, and daily oxygen demand.

Standard Curve Parameters

Slope (ΔAbs / mg·L⁻¹)

From your calibration curve regression

Intercept (Abs units)

Y-intercept of your standard curve

Sample Measurement

Sample Absorbance (600 nm)

Dilution Factor

Enter 1 if undiluted; 10 if 1:10 dilution

Organic Load (optional)

Wastewater Flow Rate (m³/day)

COD:BOD₅ Ratio

Typical domestic wastewater: 1.5–2.5. Default 2.0.

Enter your standard curve parameters and
sample absorbance, then click Calculate COD.

COD Concentration

—

mg/L (mg O₂ per liter)

Raw absorbance

—

Dilution factor

—

Undiluted COD

—

Equation used

—

BOD₅ Estimate

Estimated BOD₅

—

mg/L

COD:BOD₅ used

—

ratio

COD Pollution Level —

0 mg/L Clean Moderate High ≥2000 mg/L

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Summary

This COD calculator converts a spectrophotometer absorbance reading to COD concentration (mg/L) using your standard curve slope and intercept. It also estimates BOD5 using a configurable COD:BOD ratio, calculates daily organic load from flow rate, and outputs the theoretical oxygen demand — inputs commonly needed when sizing biological wastewater treatment systems.

How it works

Enter the slope and intercept from your COD standard curve (from a lab calibration).
Enter the absorbance reading measured at 600 nm for your sample.
Enter the dilution factor if the sample was diluted before measurement.
The calculator converts absorbance to COD concentration (mg/L) using the linear equation: COD = (Abs − intercept) / slope.
Enter flow rate to estimate daily organic load (kg COD/day) and an approximate BOD5 value.
Oxygen demand is calculated to help size aeration systems for treatment.

Use cases

Determine COD concentration from spectrophotometric absorbance readings in a lab.
Estimate BOD5 from COD for preliminary wastewater treatment design.
Calculate daily organic load for a municipal or industrial wastewater treatment plant.
Size aeration and oxygen supply systems for biological treatment.
Cross-check colorimetric COD test results during quality control.
Convert field absorbance data to COD for permit compliance reporting.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu