How do you calculate gas rate?
Run the appliance at maximum with every other gas appliance off, time the meter over a known volume, then: gas rate (m³/h) = 3600 × gas used (m³) ÷ seconds. Multiply by the calorific value and divide by 3.6 for gross kW, then by 1.11 for net. Worcester and Vaillant accept +5% to -10% of their published rate.
Reviewed by Jordan Valentine-Dunn, Gas Safe registered engineer · Portsmouth Gas Heating · Last reviewed September 2026
Gas rating proves an appliance burns at the input its maker designed. Undergassing points to low working pressure, a part-blocked filter or injector, or undersized pipework; overgassing risks incomplete combustion. Below are the formulas, two worked examples, the tolerance Worcester and Vaillant publish, and the calorific values to use.
What is the gas rate formula?
Every formula below is printed in Worcester Bosch's Technical Bulletin TB 0105 b; the unit conversions are GOV.UK's.
| To find | Formula | Notes |
|---|---|---|
| Gas rate, metric meter | 3600 × m³ used ÷ test time in seconds | Result in m³/h |
| Gross heat input, metric | 3600 × m³ used × (CV ÷ 3.6) ÷ seconds | At Worcester's 38.76 MJ/m³, CV ÷ 3.6 = 10.76 kW per m³ |
| Net heat input | kW gross ÷ 1.11 | Worcester: natural gas GCV to NCV is about 1.11:1 |
| Heat input, imperial meter | 3600 × 1040 ÷ seconds for one test dial revolution | Result in Btu/h; 1040 Btu/ft³ is Worcester's average gross CV |
| Btu/h to kW gross | Btu/h ÷ 3412 | Then ÷ 1.11 for net |
| Cubic feet to cubic metres | ft³ × 0.0283 | GOV.UK: 100 ft³ = 2.83 m³ |
How do you gas rate a boiler, step by step?
Hold the appliance at maximum
Turn off every other gas appliance. On a modulating boiler use the maker's test mode: Vaillant's ecoTEC plus manual says start check programme P.01, turn up the room thermostat and wait at least five minutes before measuring.
Time a known volume
Note the meter index, run for 120 seconds, then keep timing until the next digit appears, which is Worcester's method. The difference between the readings is the gas used in m³.
Work out the rate
3600 × gas used ÷ seconds. Using 0.073 m³ over 120 seconds: 3600 × 0.073 ÷ 120 = 2.19 m³/h.
Convert to kilowatts
Multiply by the calorific value from the bill and divide by 3.6: 2.19 × 39.5 ÷ 3.6 = 24.0 kW gross, and 24.0 ÷ 1.11 = 21.6 kW net.
Compare like with like
Check the rate against the maker's published gas rate and tolerance, and the net kW against the net heat input on the data label.
| Example | Reading | Gas rate | Gross kW | Net kW |
|---|---|---|---|---|
| Greenstar 30i, E6 metric meter | 0.105 m³ in 122 s | 3600 × 0.105 ÷ 122 = 3.10 m³/h | 3.10 × 10.76 = 33.33 | 33.33 ÷ 1.11 = 30.03 |
| U6 imperial meter | 1 test dial revolution in 36 s | 3600 × 0.0283 ÷ 36 = 2.83 m³/h | 3600 × 1040 ÷ 36 = 104,000 Btu/h ÷ 3412 = 30.48 | 30.48 ÷ 1.11 = 27.46 |
Free tool
What tolerance is allowed on a gas rate?
Worcester and Vaillant both allow +5% to -10% of the published gas rate. The pass mark is the manufacturer's, so read it from the appliance's own instructions. Worcester publishes its manual gas rates for G20 test gas, which is weaker than mains gas, so TB 0105 b says to multiply the published rate by 0.974 first: a Greenstar 30i's 3.24 m³/h becomes 3.15, and the 3.10 measured above sits inside the band of 2.84 to 3.31.
| Qnw on the data plate (kW) | Nominal (m³/h) | +5% (m³/h) | -10% (m³/h) |
|---|---|---|---|
| 20.4 | 2.16 | 2.27 | 1.94 |
| 24.5 | 2.95 as printed; 2.59 in the technical data | 2.72 | 2.33 |
| 26.5 | 2.80 | 2.94 | 2.52 |
| 30.6 | 3.24 | 3.40 | 2.91 |
| 32.6 | 3.45 | 3.62 | 3.10 |
| 35.5 | 3.76 | 3.94 | 3.38 |
| 36.7 | 3.88 | 4.08 | 3.50 |
| 40.8 | 4.32 | 4.53 | 3.89 |
Vaillant's manual disagrees with itself on one row: the check table prints 2.95 m³/h against 24.5 kW, but its own band columns fit 2.59, which is the figure in the technical data. Ask Vaillant if in doubt.
Which calorific value should you use?
Use the calorific value on the property's gas bill. Under the Gas (Calculation of Thermal Energy) Regulations 1996 it is the average of the daily values for the gas supplied. Without a bill, the averages below differ by source and year.
| Figure | Value | Source |
|---|---|---|
| Average gross CV, gas consumed in 2025 | 39.56 MJ/m³ (net 35.61) | DESNZ, DUKES A.1 and A.2 |
| Worcester's accepted average (gross) | 38.76 MJ/m³, 1040 Btu/ft³ | TB 0105 b (2016) |
| G20 test gas behind Worcester's manual rates | 37.75 MJ/m³ gross, 34.02 net | TB 0105 b |
| Range transporters maintain | 38 to 41 MJ/m³ | GOV.UK gas meter guidance |
| Wobbe index, the legal limit | 46.50 to 51.41 MJ/m³ | GS(M)R 1996, Schedule 3 |
Gross includes the latent heat in the water vapour combustion produces; net leaves it out. The meter gives a gross figure and data labels quote net, so dividing by 1.11 keeps the comparison fair. DESNZ's 2025 figures give 39.56 ÷ 35.61 = 1.11.
How does the imperial meter method work?
Imperial meters register in cubic feet, and GOV.UK says one complete revolution of the test dial is one cubic foot. Time one revolution with the appliance at maximum, then Btu/h = 3600 × 1040 ÷ seconds, and divide by 3412 for gross kW. The slower the dial turns, the lower the rate.
What if the rate is out of tolerance?
Re-check before you judge: every other appliance off, the boiler held at maximum throughout, the right calorific value, gross compared with gross. Then check standing and working pressure, the gas filter, the injectors and whether the pipework carries the load.
If the appliance still cannot reach its designed input, follow the manufacturer's instructions and the current Gas Industry Unsafe Situations Procedure. Record the result against the appliance either way, so next year's reading has something to compare with.
Gas rating is gas work. Only a Gas Safe registered engineer can size, install, service or certify a gas appliance: regulation 3 of the Gas Safety (Installation and Use) Regulations 1998 requires employers and self-employed people doing gas work to be members of the HSE-approved class of persons. This guide and the calculator are working aids; the manufacturer's instructions take precedence.
Manifold keeps each appliance's readings, photos and history together, so this year's gas rate lands beside last year's on the same record.
Frequently asked
Sources
- Worcester Bosch, Technical Bulletin TB 0105 b: Greenstar gas boilers, published gas rates
- Vaillant, ecoTEC plus installation and maintenance instructions (0020308118_06)
- GOV.UK, Gas meter readings and bill calculation
- DESNZ, Digest of UK Energy Statistics: calorific values and density of fuels (tables A.1 to A.3)
- The Gas Safety (Management) Regulations 1996, Schedule 3: content and characteristics of gas
- The Gas (Calculation of Thermal Energy) Regulations 1996, regulation 3
- The Gas Safety (Installation and Use) Regulations 1998, regulation 3
Related guides
Last reviewed September 2026. This guide is general information, not legal or safety advice, gas safety work must be carried out by an appropriately Gas Safe registered engineer. Rules can change, so check the linked official sources for the current position.