Gas rate calculator
The gas rate calculator turns a timed meter test into a gas rate in m³/h and a heat input in kW, gross and net: gas rate = 3600 × m³ used ÷ seconds. Add the manufacturer’s published rate and it checks the reading against the +5% to -10% tolerance that Worcester Bosch and Vaillant publish.
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 does the calculator work out gas rate?
It divides the gas used by the test time: gas rate in m³/h is 3600 × the gas used in m³ ÷ the test time in seconds. The rate × the calorific value ÷ 3.6 gives gross kW, and gross ÷ 1.11 gives net, the gross to net ratio Worcester Bosch’s TB 0105 b states for natural gas (DESNZ’s 2025 averages, 39.56 gross and 35.61 net, also work out at 1.11).
On an imperial meter GOV.UK says one revolution of the test dial is one cubic foot, which is 0.0283 m³, so time one revolution and enter 0.0283. The timing method step by step is in how to calculate gas rate.
What does a worked example look like?
Worcester Bosch’s own example in TB 0105 b, a Greenstar 30i on a metric E6 meter, uses 0.105 m³ in 122 seconds and comes to 33.33 kW gross and 30.03 kW net. The second row runs Worcester’s imperial formula for one revolution of a U6 meter’s test dial in 40 seconds.
| Example | Reading | Gas rate (m³/h) | Gross kW | Net kW |
|---|---|---|---|---|
| Greenstar 30i, E6 metric meter | 0.105 m³ in 122 s (120 s, then 2 s to the next digit) | 3600 × 0.105 ÷ 122 = 3.10 | 3600 × 0.105 × 10.76 ÷ 122 = 33.33 | 33.33 ÷ 1.11 = 30.03 |
| U6 imperial meter | 1 test dial revolution (1 ft³) in 40 s | 3600 × 0.0283 ÷ 40 = 2.55 | 3600 × 1040 ÷ 40 = 93,600 Btu/h; ÷ 3412 = 27.43 | 27.43 ÷ 1.11 = 24.71 |
Type the same readings into the calculator with Worcester’s 38.76 MJ/m³ and it gives 33.36 kW gross and 30.05 net for the 30i, because TB 0105 b rounds 38.76 ÷ 3.6 to 10.76, and 27.42 and 24.71 for the imperial test, where rounding in the conversion factors makes 0.01 of difference.
Is the gas rate within tolerance?
It is if the measured rate sits between 10% under and 5% over the manufacturer’s published gas rate, the band Worcester Bosch (TB 0105 b) and Vaillant (ecoTEC plus manual, section 9.13.5) both publish. Worcester’s manuals give rates for G20 test gas, whose gross calorific value of 37.75 MJ/m³ is lower than the 38.76 Worcester takes for mains gas, so TB 0105 b multiplies the published rate by 0.974 first; tick the Worcester box and the calculator does the same. Vaillant prints the band beside each rate, so its figure goes in as it is.
| Published rate | Judged against (m³/h) | -10% (m³/h) | +5% (m³/h) |
|---|---|---|---|
| Worcester Greenstar 30i, 3.24 m³/h for G20 test gas | 3.24 × 0.974 = 3.16 | 2.84 | 3.31 |
| Vaillant ecoTEC plus, 30.6 kW data plate, 3.24 m³/h | 3.24 | 2.92 | 3.40 |
| Vaillant ecoTEC plus, 24.5 kW data plate, 2.59 m³/h | 2.59 | 2.33 | 2.72 |
The 30i reading worked above, 3.10 m³/h, is 98.2% of 3.16 and inside the band. TB 0105 b prints the converted rate as 3.15; the calculator keeps the unrounded 3.156, and Vaillant’s printed band columns can differ from these by 0.01 m³/h for the same reason, so for a reading right on an edge, go by the manual.
One row of Vaillant’s table does not add up: it prints 2.95 m³/h against a 24.5 kW data plate, while its own band columns (2.33 and 2.72) and the manual’s technical data (2.59 m³/h for the VUW 25/32) both give 2.59, the figure used above. If in doubt, ask Vaillant. The ecoTEC plus outputs are on the Vaillant ecoTEC plus reference page, and every Greenstar range on Worcester Bosch boilers.
Which calorific value goes in the calculator?
The one on the property’s gas bill, which under the Gas (Calculation of Thermal Energy) Regulations 1996 is the average of the daily values for the gas supplied. Without a bill, DESNZ puts gas consumed in the UK in 2025 at 39.56 MJ/m³ gross and 35.61 net, and the calculator starts at 39.5. To reproduce Worcester’s figures, use its 38.76 MJ/m³ (1040 Btu/ft³).
The range depends on who you ask. GOV.UK’s billing guidance says gas transporters keep the calorific value within 38 to 41 MJ/m³; the Office for Product Safety and Standards’ gas types table, updated in January 2026, says no limit applies network wide and gives 36.9 to 42.3 as the normal range. The legal limit is on the Wobbe index, which Schedule 3 of the Gas Safety (Management) Regulations 1996 sets at 46.50 to 51.41 MJ/m³.
What if the rate is outside the band?
Check the test before you suspect the appliance: every other gas appliance off, the boiler held at maximum from start to finish, the right calorific value and the right published figure for the model and gas. Vaillant runs the ecoTEC plus in check programme P.01 with the front casing fitted and the room thermostat turned up, and waits at least five minutes before measuring at the meter; Worcester times 120 seconds and carries on to the next digit on the index.
If it is still out, Vaillant’s instructions say to check the boiler matches the gas group supplied, check the supply and watch the working pressure during the test, check the pipework, and put the boiler into operation only once the gas rate is right. Undersized gas pipe and low working pressure covers the supply side, and once the rate is right the flue gas ratio calculator checks the combustion.
Gas rating is gas work. Regulation 3 of the Gas Safety (Installation and Use) Regulations 1998 requires any employer whose employees work on gas fittings, and any self-employed person who does, to be a member of a class of persons approved by HSE, so only a Gas Safe registered engineer can size, install, service or certify a gas appliance. The calculator is a working aid; the manufacturer’s instructions come first.
Questions
Sources
Every figure on this page was read from these documents on 30 September 2026.
- Worcester Bosch, Technical Bulletin TB 0105 b (2016/03): Greenstar gas boilers, published gas rates
- Vaillant, ecoTEC plus installation and maintenance instructions 0020308118_06, section 9.13.5 and technical data
- GOV.UK (Office for Product Safety and Standards), Gas meter readings and bill calculation
- DESNZ, Digest of UK Energy Statistics: calorific values and density of fuels (tables A.1 and A.2)
- Office for Product Safety and Standards, Gas types and supply pressures in the UK
- The Gas Safety (Management) Regulations 1996, Schedule 3: content and characteristics of gas
- The Gas (Calculation of Thermal Energy) Regulations 1996, regulation 3: calorific value for billing
- The Gas Safety (Installation and Use) Regulations 1998, regulation 3: qualification and supervision