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CO/CO₂ Ratio Calculator

Check combustion performance by ratioing the carbon monoxide and carbon dioxide your flue-gas analyser reads.

CO/CO₂ Ratio

Combustion ratio and pass/fail vs the 0.004 limit.

CO (ppm)CO₂ (%)

CO/CO₂ ratio0.0005

ResultWithin limit (≤ 0.004)

Method: BS 7967 (combustion performance)

How it works

A flue-gas analyser reports CO in parts per million (ppm) and CO₂ as a percentage. The combustion ratio puts them on the same basis: ratio = (CO ppm ÷ 10,000) ÷ CO₂ %. Dividing CO by 10,000 converts ppm to a percentage so the two figures compare directly.

A correctly burning appliance gives a low ratio. The accepted limit is 0.004 — at or below is within limit; above means incomplete combustion to investigate (sooting, air supply, heat exchanger or burner).

Worked example

Readings of 40 ppm CO and 8% CO₂: (40 ÷ 10,000) ÷ 8 = 0.004 ÷ 8 = 0.0005 — well within the 0.004 limit.

CO/CO₂ ratio questions

What is an acceptable CO/CO₂ ratio?

A ratio at or below 0.004 is the common revision limit, with measurement practice sitting under BS 7967. A low ratio does not override other unsafe evidence such as spillage, damage or high absolute CO.

What causes a high CO/CO₂ ratio?

Incomplete combustion — typically poor air supply, burner or injector condition, heat-exchanger restriction, incorrect set-up or flueing problems. The reading is the prompt to investigate, not the diagnosis itself.

Is the 0.004 limit a percentage?

No. It is a dimensionless ratio of CO to CO₂ once both are on the same percentage basis. Writing it as 0.4% is a common revision mistake.