Gas Pipe Sizing Calculator — What Size Gas Pipe Do I Need?

This calculator follows the BS 6891:2015 Annex A sizing method exactly. It converts the total gas load (kW) to a required gas rate (m³/h). Then, for each candidate pipe size, it reads the pressure loss per metre from the standard's pressure-loss tables, adds the diameter-dependent fitting allowances, and multiplies by the effective length. It returns the smallest pipe whose total pressure drop from the meter to the appliance stays within 1 mbar. The copper figures come from BS 6891:2015 Annex A Table A.1 and the medium-steel figures from Table A.2: the standard's own published pressure-loss tables (natural gas, relative density 0.6). Both have been re-derived from the standard's formula A.1 (efficiency factor 0.95 for copper, 0.86 for steel) and reproduce every published value to four decimal places.

The method

1. Convert load to gas rate:gas rate (m³/h) = load (kW) ÷ 10.80. BS 6891 Annex A uses a gross calorific value of 38.9 MJ/m³, so 38.9 ÷ 3.6 = 10.80 kW per m³/h. This is the divisor in the standard's own worked example and in the NICEIC Pocket Guide Gas 4A (14 kW ÷ 10.80 = 1.30 m³/h). It is a conversion factor, not a value tabulated in BS 6891.

2. Round the gas rate up to a tabulated flow:the Table A.1 / A.2 rows run in 0.25 m³/h steps; an untabulated rate is rounded up to the next row before reading the pressure loss (as the standard's worked example does).

3. Add fittings and check the drop: effective length = actual length + BS 6891 Table A.5 equivalent length for each elbow and tee at the size being tested. The pressure loss per metre (mbar/m) for that size and flow rate is multiplied by the effective length; the smallest size that keeps the total ≤ 1 mbar is selected.

Fittings equivalents (BS 6891 Table A.5)

Each elbow and tee adds resistance, so an equivalent pipe length is added to the measured run. These allowances are diameter-dependent in BS 6891:2015 Table A.5, so the fitting allowance grows as the pipe gets larger:

• 90° elbow: 0.40 / 0.60 / 0.80 / 1.00 m for 15 / 22 / 28 / 35 mm copper
• 90° bend: 0.20 / 0.30 / 0.40 / 0.50 m; 45° bend: 0.15 / 0.20 / 0.25 / 0.30 m
• Tee, flow exiting into branch: 1.20 / 1.80 / 2.30 / 3.00 m (the conservative default)
• Tee, flow entering from branch: 0.75 / 1.20 / 1.50 / 2.00 m

Pressure loss per metre: BS 6891 Table A.1 (copper, mbar/m)

BS 6891 tabulates pressure loss per metre against the gas flow rate, not a single "maximum capacity". Loss rises steeply with flow and falls fast with diameter, so the allowable length for a given size depends on the load. A few worked points for copper tube to BS EN 1057 (natural gas, relative density 0.6):

• 15mm copper: 0.0719 mbar/m at 1.0 m³/h (≈11 kW), 0.4577 at 3.0 m³/h (≈32 kW). So 1.0 m³/h is fine to ~13 m, but 3.0 m³/h only to ~2 m within 1 mbar
• 22mm copper: 0.0123 mbar/m at 1.0 m³/h, 0.2511 at 6.0 m³/h (≈65 kW)
• 28mm copper: 0.0035 mbar/m at 1.0 m³/h, 0.1657 at 10.0 m³/h (≈108 kW)
• 35mm copper: tabulated to 16 m³/h (≈173 kW), the upper size of the BS 6891 domestic scope; the steel Table A.2 runs to DN32

Standards

Gas pipe sizing must comply with BS 6891:2015 (Installation and maintenance of low pressure gas installation pipework of up to 35 mm on premises). Pipework above 35 mm (DN32), or runs/loads beyond the BS 6891 Annex A tables, fall under IGEM/UP/2. All gas work must be carried out by a Gas Safe registered engineer.