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CO/CO₂ Ratio Calculator

Calculate the ratio between the carbon monoxide and carbon dioxide readings from a flue-gas analyser.

CO/CO₂ Ratio

Calculate the ratio from CO and CO₂ readings.

CO (ppm)CO₂ (%)

CO/CO₂ ratio0.0005

AssessmentCompare all readings with appliance instructions

Method: CO/CO₂ concentration ratio

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.

The calculation does not decide whether an appliance passes. Use the instructions for the model, fuel and operating condition to assess the ratio, absolute CO and every other required check. A low ratio cannot cancel a failed check, and a strict "below" limit does not include equality.

Worked example

Readings of 40 ppm CO and 8% CO₂: (40 ÷ 10,000) ÷ 8 = 0.004 ÷ 8 = 0.0005. This is the calculated ratio. Assess it with the CO reading, appliance instructions and remaining checks.

Interpret flue gas analyser readings after the calculation

Record the original CO and CO₂ readings as well as the calculated ratio. Different pairs of readings can produce the same ratio, so reducing the result to one number loses information. The CPA1 revision guide places the arithmetic within analyser preparation, sampling and appliance-specific interpretation.

For a revision scenario, explain which evidence supports the next action and which checks are still required. Use the unsafe-situations guide for that separate decision process; the ratio calculator does not classify an installation.

CO/CO₂ ratio questions

What is an acceptable CO/CO₂ ratio?

Use the limit and test conditions stated for the appliance. There is no universal pass decision from a ratio alone: absolute CO, the other combustion readings and all required safety checks also matter. Do not substitute a generic 0.004 threshold for the model instructions.

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 calculated ratio a percentage?

The calculator displays a dimensionless ratio after converting both gas concentrations to the same units. Enter CO in ppm and CO₂ in percent: for 8% CO₂, enter 8. Keep that input convention throughout the calculation.