Free Gas Pipe Sizing Calculator
Find the minimum copper gas pipe diameter for an appliance load and pipe run, to keep the pressure drop within 1 mbar (BS 6891).
Undersized gas pipework starves an appliance and fails a tightness/working-pressure check. Enter the total appliance load in kW and the length of the pipe run, and this calculator converts the load to a gas flow rate and returns the minimum copper pipe size (15, 22 or 28mm) that keeps the pressure drop within the 1 mbar allowance from BS 6891.
Gas Pipe Sizing Calculator results
Fill in the details and select Calculate to see your figures.
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Get startedHow to use the Gas Pipe Sizing Calculator
- 1Enter the total gas load in kW of the appliance(s) the pipe will feed: add them up if the run supplies more than one.
- 2Enter the length of the pipe run from the meter to the appliance. Allow extra for bends and fittings, which add resistance.
- 3Read the gas flow in m³/hr and the minimum copper pipe size that keeps the drop within 1 mbar.
- 4Remember the 1 mbar allowance is for the WHOLE installation. A long run in a smaller pipe may need stepping up, or splitting the allowance across sections.
How the Gas Pipe Sizing Calculator works
Sizing gas pipe is a two-step job: convert the load to a flow rate, then look up the smallest pipe that can carry it over your run length without exceeding the 1 mbar drop.
- 30 kW combi ÷ 10.76 = 2.79 m³/hr.
- At a 6m run, BS 6891 gives 15mm = 1.9 m³/hr (too small) and 22mm = 5.8 m³/hr (ample).
- So use 22mm copper for a 30kW combi on a 6m run.
- At a 3m run, 15mm carries 2.9 m³/hr, so a short run could take 15mm. Length is decisive.
Guide values based on BS 6891 for copper at a 1 mbar drop and relative density 0.6; verify against the current BS 6891 standard for the specific installation. Uses kW ÷ 10.76 (net CV basis). Gas work must be carried out by a Gas Safe registered engineer.
Typical UK benchmarks
| Pressure drop allowance | ≤1 mbar total across the installation: BS 6891 |
| 15mm copper at 6m | max 1.9 m³/hr at 1 mbar: BS 6891 Table 2 |
| 22mm copper at 6m | max 5.8 m³/hr at 1 mbar: BS 6891 Table 2 |
| kW → m³/hr | divide kW by 10.76 (natural gas) |
Disclaimer
This tool gives an estimate for general guidance only, based on typical UK figures and the details you enter. It is not a formal quote, survey, or professional advice. Always confirm with a qualified tradesperson before relying on any figure, as regional prices, site conditions, and current standards vary.
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