Free tool

Gas rate calculator

Enter a timed meter test and the calorific value to get the gas rate and the gross and net heat input, ready to check against the appliance data plate.

Test time (seconds)

Gas used (m³)

Calorific value (MJ/m³)Gas used is the difference between your start and end meter readings. The calorific value is on the gas bill (UK natural gas is about 39.5, regulated to 38 to 41). Everything is worked out in your browser, nothing is saved.

A working aid for qualified engineers, not a pass/fail verdict. Always work to the appliance manufacturer’s commissioning data and the current gas procedures. Gas work must be carried out by an appropriately Gas Safe registered engineer.

How the calculation works

Run the appliance, time how long the meter takes to use a known volume of gas (the difference between your start and end readings), and the rate follows: gas rate in m³/h is 3600 times the gas used in m³, divided by the test time in seconds.

To turn that into heat input, multiply the rate by the calorific value and divide by 3.6 for the gross figure in kW, then divide by 1.11 for net. Appliance data plates are usually quoted net, so compare like with like.

This is a working aid, not a verdict. Always work to the manufacturer’s commissioning data and the current gas procedures, and use a calibrated analyser where required.

Questions

How do you calculate gas rate?

Time how long the meter takes to use a known volume of gas, then work out the rate: gas rate (m³/h) = 3600 × gas used (m³) ÷ test time (seconds). To get heat input, multiply the rate by the calorific value and divide by 3.6 for gross kW, then divide by 1.11 for net kW.

What's the difference between gross and net kW?

Gross heat input includes the latent heat in the water vapour produced by combustion; net leaves it out. The meter measures gas supplied, which gives a gross figure, but UK appliance data plates are usually quoted net, so divide the gross result by 1.11.

What calorific value should I use?

The value on the gas bill for the property. UK natural gas is around 39.5 MJ/m³ and is regulated to stay between 38.0 and 41.0, so 39.5 is a reasonable default if you don't have the exact figure.

Is my data stored?

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

Record the readings against the appliance, once.

Manifold keeps every appliance’s readings, photos and history joined up, so next year you walk in already knowing the boiler. In early access for UK gas engineers.

Get early access

Manifold

Loading workspace