Chemical Oxygen Demand (COD) Calculator

A Free Chemical Oxygen Demand (COD) Calculator

This calculator takes the numbers from a lab COD test, a treatment plant flow sheet, or a coursework problem and turns them into a clear, correct answer. Chemical Oxygen Demand — usually written COD — is one of the most widely used numbers in water and wastewater testing. It tells you how much oxygen would be needed to chemically break down all of the organic (and some inorganic) matter in a water sample, and it is one of the fastest, most repeatable ways to measure how polluted a sample is.

This tool covers three real situations people run into with COD. First, working out COD from a standard dichromate reflux titration test, the way it is actually measured in a lab. Second, turning a COD concentration into a mass load in kilograms per day for a wastewater treatment plant, along with removal efficiency and population equivalent. Third, comparing a BOD5 reading against COD to find the biodegradability index — a quick way to judge whether a wastewater is a good fit for biological treatment. Every result comes with a full step-by-step written solution, a water-quality classification, and a chart, so you can check your own work or double check a lab report.

What Is Chemical Oxygen Demand, in Plain Words?

When water contains organic matter — sewage, food waste, industrial waste, dyes, oils, or almost anything else that can be oxidized — that matter can be broken down chemically using a strong oxidizing agent. Chemical Oxygen Demand measures exactly how much oxygen would be needed to do that breakdown completely, expressed in milligrams of oxygen per litre of water (mg/L).

Unlike a biological test, COD does not rely on living bacteria at all. Instead, the lab uses a powerful chemical oxidant — almost always potassium dichromate in strong sulfuric acid — to oxidize essentially everything in the sample that can be oxidized, whether or not bacteria would ever be able to digest it. That is why COD numbers are almost always higher than BOD numbers for the same sample: COD captures the full chemically-oxidizable load, while BOD only captures the portion bacteria can actually break down.

Why Labs Use COD Instead of (or Alongside) BOD

A standard BOD5 test takes five full days to get a result, because it depends on bacteria actually consuming the organic matter over time. A COD test, by contrast, is normally finished in about two to three hours, because the strong chemical oxidant does the work directly instead of waiting on biology. That speed is exactly why COD is so popular for day-to-day plant monitoring — an operator can get same-day feedback on how a treatment process is performing, instead of waiting almost a week.

COD is also far more reproducible than BOD, since it does not depend on how many bacteria happen to be active in a particular bottle, whether the sample needed seeding, or subtle incubation temperature differences. Because of that consistency, many discharge permits and regulatory frameworks track COD directly, or track both COD and BOD together to get the full picture: COD for total chemical oxidation potential, and BOD for how well the biological treatment stage itself is actually performing.

The Dichromate Reflux Titration Formula

The standard method (Standard Methods for the Examination of Water and Wastewater, method 5220 B for open reflux or 5220 D for the closed reflux/colorimetric variant) works by refluxing the sample with a known, excess amount of potassium dichromate in strong sulfuric acid, using silver sulfate as a catalyst. The dichromate that is not consumed oxidizing the sample is then back-titrated with ferrous ammonium sulfate (FAS), using a ferroin indicator to spot the endpoint.

Because a blank (distilled water taken through the identical digestion) is titrated alongside the real sample, the difference between how much titrant the blank needed and how much the sample needed tells you exactly how much dichromate the sample's organic matter actually consumed. The formula is:

COD (mg/L) = (A - B) x N x 8000 / V

• A = volume of FAS titrant used for the blank, mL
• B = volume of FAS titrant used for the actual sample, mL
• N = normality of the FAS titrant, mol/L (equivalents per litre)
• V = volume of sample taken through the digestion, mL
• 8000 = the milliequivalent weight of oxygen (8 mg O2 per meq) scaled to a one-litre basis

A Worked Example: Reading a COD Titration

Say a 10 mL sample is refluxed with dichromate and back-titrated with 0.1 N FAS. The blank needed 12.5 mL of titrant to reach the ferroin endpoint, while the actual sample needed only 9.8 mL — meaning the sample's own organic matter consumed the dichromate that would otherwise have shown up as extra titrant on the blank.

The titrant difference is A - B = 12.5 - 9.8 = 2.7 mL. Plugging into the formula: COD = 2.7 x 0.1 x 8000 / 10 = 216 mg/L. That value sits in the 'Poor / Significantly Polluted' band on the surface-water scale, which is a reasonable reading for a lightly-treated or moderately contaminated wastewater stream rather than a clean river.

COD Mass Load, Removal Efficiency, and Population Equivalent

A concentration on its own — mg/L — does not tell a treatment plant operator how much total chemical oxygen demand is actually arriving each day; for that, you need the mass load. This calculator's load mode multiplies flow rate by COD concentration to get a daily mass in kilograms, which is exactly what plant sizing, permit compliance, and chemical/aeration dosing calculations are built on.

Load (kg/day) = Flow (m3/day) x COD (mg/L) / 1000. If you also know the effluent (treated) COD, the calculator works out removal efficiency — the percentage of chemical oxygen load the treatment process actually removed — and a population equivalent, which converts the load into 'how many people's worth' of typical domestic COD output it represents, using a common design figure of about 0.12 kg COD per person per day.

The BOD5/COD Ratio and Biodegradability

Knowing COD on its own tells you the total chemically-oxidizable load, but it does not tell you how much of that load a biological treatment plant can actually remove. That is where the biodegradability index, BI = BOD5 / COD, comes in — it directly compares the biologically-degradable oxygen demand against the total chemical oxygen demand.

A high BI (roughly above 0.5) means most of the pollution load is readily biodegradable, and a conventional biological process such as activated sludge should handle it well. A low BI (roughly below 0.2) means most of the COD resists biological breakdown — common with some industrial, chemical, or dye-house wastewaters — and biological treatment alone is unlikely to reach the discharge target without pretreatment or a different treatment strategy such as advanced oxidation.

How to Use This Calculator

Pick the mode that matches your situation. For a standard lab COD test, use the titration mode and enter the blank and sample titrant volumes, the FAS normality, and the sample volume used in the digestion. For a treatment plant flow, use the load mode with your flow rate and COD concentrations. For judging how treatable a wastewater is biologically, use the ratio mode with a BOD5 and COD pair.

Every mode shows the full step-by-step written solution below the results, a chart specific to that calculation, and an instant water-quality reading, so you always see exactly how the final number was built and what it actually means.

COD Water Quality Classification

For surface water — rivers, lakes, and streams — COD is a widely used, fast-turnaround pollution indicator, following commonly taught reference bands:

• 0-10 mg/L — Excellent: essentially unpolluted, low chemical oxygen demand
• 10-20 mg/L — Good: clean water with only light organic loading
• 20-40 mg/L — Fair: moderate organic pollution, often seen downstream of towns or well-treated discharges
• 40-100 mg/L — Poor: significant organic pollution; the water is carrying a real chemical oxygen load
• Above 100 mg/L — Very Poor: heavy organic pollution, overlapping with partially treated or raw wastewater

COD vs BOD: What's the Difference?

COD and Biological Oxygen Demand (BOD) both measure oxygen demand from pollution, but they get there differently. COD uses a strong chemical oxidant to break down essentially everything oxidizable in a sample — including material bacteria cannot digest — in a matter of hours. BOD relies on living bacteria to break down only the organic matter they can actually digest, over 5 full days.

Because of this, COD is almost always a higher number than BOD for the same sample, and it runs much faster. Treatment plant operators commonly track both: COD gives a same-day snapshot of total chemical oxygen demand, while BOD tells you specifically how well the biological part of the treatment process is performing.

Common Mistakes When Working With COD

The most common mistake is mixing up which titrant volume belongs to the blank and which belongs to the sample. The blank should always need more titrant than the sample, since none of its dichromate was consumed by organic matter — if your sample number is larger than your blank number, double-check which reading is which.

A second common mistake is forgetting that the 8000 constant already bakes in both the meq weight of oxygen and the mL-to-L conversion, and mistakenly applying an extra unit conversion on top of it. A third mistake is comparing COD directly against a BOD-based classification scale — the two use different units in the same sense (mg/L) but very different pollution thresholds, since COD always reads higher for the same water.

Why COD Matters

COD is one of the fastest, most repeatable water quality tests in the world, and it plays a central role in keeping treatment plants running correctly and keeping receiving waters healthy. Because it does not depend on bacteria or multi-day incubation, it gives operators, regulators, and industries a same-day read on organic pollution levels.

• Environmental monitoring — regulators and researchers use COD alongside BOD to track river, lake, and coastal water health over time, and to spot pollution sources early.
• Wastewater treatment plant operation — COD load and removal efficiency are core, fast-turnaround numbers used for day-to-day process control, chemical dosing, and permit compliance checks.
• Industrial wastewater management — factories, textile mills, food processors, and chemical plants routinely test their own wastewater's COD before it goes to a treatment plant or a receiving water, since COD limits are commonly written into industrial discharge permits.
• Treatability screening — the BOD5/COD ratio is a quick, practical first check on whether a wastewater is a good match for conventional biological treatment, or whether it needs a different treatment strategy entirely.

Chemical Oxygen Demand Calculator: Quick Reference Summary

COD (mg/L) = (A - B) x N x 8000 / V, where A and B are the blank and sample FAS titrant volumes, N is the FAS normality, and V is the sample volume in mL. COD load (kg/day) = Flow (m3/day) x COD (mg/L) / 1000. Removal efficiency (%) = (COD in - COD out) / COD in x 100. Biodegradability index: BI = BOD5 / COD.

This free calculator is built to support learning, lab work, and everyday treatment plant planning. For anything tied to a regulatory discharge permit or a certified lab report, always confirm results against your accredited laboratory's own method and your local environmental authority's requirements.