Short answer
Gas flow (m³/h) = gross heat input (kW) ÷ calorific value (kWh/m³). UK natural gas has a gross calorific value of around 39 MJ/m³, which is about 10.8 kWh/m³. If an appliance is rated on net input — as most boilers are — multiply by about 1.11 to get gross before dividing.
Worked example
A 500 kW (net input) boiler: 500 × 1.11 = 555 kW gross. 555 ÷ 10.8 ≈ 51 m³/h at full fire. Two such boilers need roughly 103 m³/h, which drives both pipe size and meter selection.
Input vs output
Always use heat input, not heat output. A boiler with 500 kW output at 95% efficiency has an input of about 526 kW net. Sizing on output underestimates gas demand.
Checking at the meter
Measure actual gas rate by timing the meter with only the appliance running: flow (m³/h) = volume passed (m³) ÷ time (s) × 3,600. Multiply by the calorific value for gross kW. Compare with the data plate to confirm the appliance is correctly set.
Figures are typical for UK natural gas. The current edition of the relevant IGEM standard and the manufacturer's instructions always take precedence on site.
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