Gas Rating Chart: kW from the Gas Meter
Gas rating tells you whether an appliance is actually burning at its rated input. Time the metric meter over a set volume, read the kW off the chart, and check it against the data plate before you sign the job off.
Gas rating is one of the quickest confidence checks on a gas appliance: with everything else off, you time how long the meter takes to pass a set volume of gas and convert that to a heat input in kilowatts. If the figure matches the appliance's rated input, the burner is drawing roughly the gas it should.
This guide gives you the method for a metric (m³) meter, a ready-reckoner chart for the common case of timing 0.01 m³, and the gross-versus-net distinction that trips people up. The arithmetic here assumes a gross calorific value of 39.5 MJ/m³ — always confirm the CV and working pressure on the actual job.
Key takeaways
- Gas rating confirms an appliance is burning at its rated input — not too little, not too much.
- On a metric meter, time a set volume (commonly 0.01 m³) with every other appliance off.
- Gross kW ≈ 395 ÷ seconds per 0.01 m³, from a gross CV of 39.5 MJ/m³; net kW is roughly 90% of gross.
- Data plates may quote gross or net input, so compare like with like before you judge the result.
- Confirm the actual calorific value and working pressure on the job rather than assuming the chart's figures.
What gas rating is and why you do it
Gas rating measures the rate at which an appliance is consuming gas and expresses it as a heat input in kilowatts. The point is to confirm the appliance is burning at the input the manufacturer rated it for — under-gassing leaves it short of output and prone to poor combustion, while over-gassing can overload the appliance and the flue.
It is a standard commissioning and fault-finding check. Read alongside the working gas pressure and a flue gas analysis, the rated input tells you whether the appliance is set up correctly before you hand it back to the customer.
The metric method
Turn off every other gas appliance in the property and any pilot lights so the meter only sees the appliance under test. Run that appliance at maximum rate, let it settle, then time how long the meter takes to pass a set volume — 0.01 m³ is the usual choice on a domestic metric meter because it gives a readable time.
Watch the test dial or the least-significant digits and start the stopwatch as a figure ticks over, stopping it exactly one 0.01 m³ later. Take the time in seconds and convert to kW using the chart below, then repeat the timing to confirm you get a consistent reading.
Gross vs net calorific value
The calorific value (CV) is how much heat you get from a volume of gas. Gross (higher) CV includes the heat that would be recovered by condensing the water vapour in the products of combustion; net (lower) CV does not. Natural gas is commonly around a gross CV of about 39.5 MJ/m³, and net is roughly 90% of gross.
This matters because a data plate may quote the rated input as gross or as net. If you calculate a gross input from the meter and compare it against a net figure on the plate (or the other way round), the numbers will look wrong when the appliance is fine. Always establish which basis each figure is on before you draw a conclusion.
Ready-reckoner: seconds to kW
The chart below converts the time to burn 0.01 m³ straight into kW. It uses Gross kW ≈ 395 ÷ (seconds per 0.01 m³), which comes from a gross CV of 39.5 MJ/m³ — one 0.01 m³ of gas holds about 0.1097 kWh, so burning it in a measured number of seconds fixes the rate. Net kW is taken as gross × 0.90.
As a worked check: 30 seconds gives 395 ÷ 30 ≈ 13.2 kW gross, and 20 seconds gives 395 ÷ 20 ≈ 19.8 kW gross. Use the chart to read off common times, and fall back to the formula for anything in between.
| Seconds per 0.01 m³ | Gross input (kW) | Approx net (kW) |
|---|---|---|
| 60 | 6.6 | 5.9 |
| 50 | 7.9 | 7.1 |
| 45 | 8.8 | 7.9 |
| 40 | 9.9 | 8.9 |
| 36 | 11.0 | 9.9 |
| 32 | 12.3 | 11.1 |
| 28 | 14.1 | 12.7 |
| 24 | 16.5 | 14.9 |
| 20 | 19.8 | 17.8 |
| 18 | 21.9 | 19.7 |
| 16 | 24.7 | 22.2 |
| 14 | 28.2 | 25.4 |
Cross-checking against the data plate
The rated input on the appliance data plate is your reference. Note whether it is quoted gross or net, then compare it with the matching figure from your timing. A close match confirms the appliance is gassed correctly; a meaningful shortfall or excess points you towards the gas supply, the working pressure, or the appliance's own gas valve setting.
Where you find a discrepancy, check the working pressure at the appliance and confirm the CV for the supply — a low pressure or an assumed CV that is off will move the numbers. Record the rated input, your measured figure and the working pressure against the job so the commissioning is evidenced.
Keep the rating on the job
Logging the timed input, the working pressure and the data-plate figure together in TradePlanr keeps the commissioning check with the certificate and the invoice, so it is there if the appliance is ever queried.
Frequently asked questions
How do I gas rate on a metric meter?
With every other appliance and pilot off, run the appliance at maximum, let it settle, and time how long the meter takes to pass a set volume — commonly 0.01 m³. Convert that time to kW using Gross kW ≈ 395 ÷ seconds, then compare it with the rated input on the data plate.
What is the difference between gross and net input?
Gross calorific value includes the heat recoverable by condensing the flue vapour; net does not, and is roughly 90% of gross. A data plate may quote either, so establish which basis the plate uses before comparing it with your calculated figure.
Why does the chart assume 39.5 MJ/m³?
That is a common gross calorific value for natural gas and gives a convenient factor of about 395 for a 0.01 m³ timing. The actual CV varies, so treat the chart as a ready-reckoner and confirm the CV and working pressure on the job.
What if my measured input does not match the data plate?
First make sure you are comparing gross with gross (or net with net). If it still differs, check the working gas pressure at the appliance and the CV for the supply, then investigate the gas valve setting — a real over- or under-gassing needs correcting before you sign off.
From guidance to action
Related guides
How to Gas Rate a Boiler
Gas rating by meter, converting to kW, and checking a boiler against its data plate.
Flue Gas Analysis and Combustion Ratios
Reading a flue gas analyser — CO/CO₂ ratio, acceptable combustion figures, and what fails.
The Benchmark Certificate and Boiler Warranties
What Benchmark is, how it protects the warranty, and completing it correctly at commissioning.
