Free tool

Flue gas ratio calculator

Enter your analyser’s CO and CO₂ readings to get the combustion ratio and check it against the BS 7967 action level of 0.0040.

CO (ppm)

CO₂ (%)

Enter the measured carbon monoxide (ppm) and carbon dioxide (%) from your flue gas analyser. The ratio is worked out in your browser, so nothing is sent or saved.

A screening figure for qualified engineers, not a pass/fail verdict. Always work to the appliance manufacturer’s commissioning data and the current Gas Safe / BS 7967 procedure. Gas work must be carried out by an appropriately Gas Safe registered engineer.

What the ratio tells you

The combustion ratio, carbon monoxide divided by carbon dioxide, is a sensitive indicator of how completely an appliance is burning its fuel. As a burner drifts out of adjustment or a heat exchanger fouls, CO climbs relative to CO₂ and the ratio rises.

BS 7967 sets the action level at 0.0040. A ratio at or below that is within the accepted limit for a correctly operating appliance; above it, investigate against the manufacturer’s data and the current standard. It’s a screening figure, not the whole story.

For a fuller refresher, read flue gas analysis explained.

Questions

How is the combustion ratio calculated?

It's the measured carbon monoxide divided by the measured carbon dioxide, on the same basis. With CO in ppm and CO₂ as a percentage, the ratio is CO ÷ (10000 × CO₂). This tool does that for you.

What is an acceptable CO/CO₂ ratio?

BS 7967 sets an action level of 0.0040. At or below that, combustion is within the accepted limit for a correctly operating appliance; above it, the appliance needs investigation following the manufacturer's instructions and the standard.

Is this a substitute for the standard?

No. It's a quick screening and refresher figure for qualified engineers. Always work to the appliance manufacturer's commissioning data and the current Gas Safe / BS 7967 procedure, and use a calibrated analyser.

Is my data stored?

No. The calculation runs entirely in your browser. The values you enter are never sent to a server or saved.

Stop copying readings into the van.

Manifold’s FlueScan reads CO, CO₂ and the ratio straight off your analyser into the appliance check for you to confirm. In early access for UK gas engineers.

Get early access

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