Free tool
CO/CO₂ ratio calculator
Enter the two readings off the analyser and get the ratio, with a straight pass or fail against the 0.004 maximum.
Take both from the same analyser sample, with the appliance settled at the rate you are testing.
What the ratio tells you
The ratio compares how much carbon monoxide the appliance is producing against how much carbon dioxide. It is a measure of how completely the gas is burning, and unlike a raw CO figure it does not move just because the sample is diluted. That is why it is the number used as the pass or fail.
The maximum for a domestic gas appliance is 0.004. It is one upper limit, not a min and max pair.
The calculation
ratio = (CO ppm ÷ 10000) ÷ CO₂ %
100 ppm against 9.5% CO₂ gives 0.01 ÷ 9.5 = 0.0011, comfortably inside the limit. 500 ppm against the same 9.5% gives 0.0053, which fails.
A ratio inside the limit is not the whole test
The ratio says the combustion is clean. It does not say the appliance is set correctly. The CO₂ percentage still has to fall inside the band the manufacturer specifies for that exact model, at maximum and at minimum rate, and those figures vary more than people expect between models in the same family.
A Greenstar 12 Ri ErP wants 9.65% CO₂ at maximum rate where the 15, 18 and 24 Ri want 9.8%. The badge on the front will not tell you which you are stood at.
The other calculators
The app
The combustion figures for that exact model
Flint lifts the commissioning figures out of the manufacturer manual and lays them out as readouts: CO₂ at maximum and minimum, the CO limits, the ratio to check against, and the service-mode sequence for the appliance in front of you. Every value carries the manual page it came from.
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Common questions
What is the maximum CO/CO₂ ratio for a gas boiler?
0.004 is the maximum for a domestic gas appliance. It is a single upper limit rather than a range, so anything at or below 0.004 passes and anything above it fails.
How do I calculate the CO/CO₂ ratio?
Divide the CO reading in ppm by 10,000 to convert it to a percentage, then divide that by the CO₂ reading in percent. A reading of 100 ppm CO against 9.5% CO₂ gives a ratio of 0.0011.
What do I do if the ratio fails?
Do not leave the appliance in service on a failed ratio. Investigate the cause, and treat the installation under the Gas Industry Unsafe Situations Procedure while you do. A high ratio points at incomplete combustion, so the usual suspects are the burner, the heat exchanger, the air supply and the flue.
Does the ratio replace checking CO₂ against the manual?
No. The ratio is a combustion-quality check on top of the manufacturer's own commissioning figures. The CO₂ percentage still has to sit inside the range that model's manual specifies at both maximum and minimum rate.
Flint is a reference tool for competent, Gas Safe registered engineers. Everything here informs your judgement and never replaces it. Always work to the manufacturer's instructions and the current standards, and check every figure against the appliance data badge and manual.