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.
Results
Fill in the details and select Calculate to see your figures.
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Get a free demoHow 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 to work it out yourself
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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Get a free demoFrequently asked questions
What size gas pipe do I need for a combi boiler?
It depends on both the boiler's kW and the length of the run. A typical 30kW combi draws about 2.8 m³/hr, which 15mm copper can carry only up to around 3m; beyond that you need 22mm to stay within the 1 mbar pressure-drop allowance. Always size on the actual run length, not just the boiler rating.
Why does pipe length matter so much?
The longer the pipe, the more the gas pressure drops along it for a given flow. BS 6891 allows a maximum 1 mbar drop across the whole installation, so the same pipe carries less gas over a longer run. Which is why a 15mm pipe fine at 3m becomes undersized at 9m for the same appliance.
How do I convert kW to m³/hr for gas?
Divide the appliance's kW rating by about 10.76 for natural gas. So a 24kW appliance needs roughly 2.2 m³/hr and a 40kW appliance about 3.7 m³/hr. That flow rate is what you look up against the pipe-size tables.
What is the 1 mbar pressure drop rule?
BS 6891 limits the total pressure drop between the meter and the appliance to 1 mbar for natural gas. This ensures the appliance gets enough pressure to burn correctly. The allowance covers the entire installation, so on a run with several sections you have to share the 1 mbar between them, not allow 1 mbar per section.
Do fittings and bends affect the size?
Yes. Elbows, tees and fittings add resistance equivalent to extra pipe length, so a run with many bends behaves like a longer straight run. Add an allowance for fittings when you measure the run, or step up a size if the run is close to the limit.
Can I size and install gas pipe myself?
No. In the UK it's a legal requirement that any gas work is carried out by a Gas Safe registered engineer. The Gas Pipe Sizing Calculator is a design aid for qualified engineers to sanity-check pipe sizing. It doesn't replace the full BS 6891 calculation or Gas Safe competence.
