Free tool · no sign-up
Gas pipe sizing calculator
Size a run so the appliance gets its full rate without exceeding the pressure drop you're allowed. Counts the fittings, not just the tape measure.
Gas pipe sizing
Size a run section by section against one pressure drop limit.
1 mbar is the working limit for natural gas from the meter to the appliance.
Everything downstream, added together.
Fittings on this section
Effective length 17.6 m including fittings.
Anything else with a known allowance.
Sizes are picked to keep the whole run inside the limit. Tap one to pin it and the rest of the run is sized around it.
Add a section wherever the pipe tees off or changes size. Each one carries only the load beyond it, and they all share the one pressure limit.
Minimum pipe size
22 mm
Pressure drop used
0.95 / 1 mbar
0.05 mbar of headroom left across the whole path from the meter.
Section by section
At the meter
30kW
Gas rate
2.99m³/h
first section
Minimum pipe size
22 mm
Pressure drop used
0.95 / 1 mbar
0.05 mbar of headroom left across the whole path from the meter.
Section by section
At the meter
30kW
Gas rate
2.99m³/h
first section
Guidance only. Follow Gas Safe procedures and the manufacturer's instructions. Gas work in the UK must be carried out by a Gas Safe registered engineer.
How it's worked out
Load becomes a gas rate, the run becomes an effective length, then each candidate size is checked against the pressure drop you've allowed.
- 01
The load, as a gas rate
Enter the appliance load in kilowatts and say whether that figure is net or gross. The calculator converts it to a volume flow in m³/h, which is what the pipe actually has to carry.
- 02
Straight length, plus the fittings
Every fitting behaves like extra pipe. You enter how many of each, and the calculator adds the effective length they contribute to the measured run.
90° elbows
Tees, taken through
Tees, off the branch
Service valves
There's also a free field for any additional length you want to allow — a chased section you can't measure properly, or a detour you know is coming.
- 03
Each size, checked
For the rate and the effective length, the calculator works out what each candidate size would actually do: internal diameter, resulting pressure drop across the run, and gas velocity. You set the permissible drop; it marks which sizes come in under it and recommends one.
Worked example
30 kW net · copper · natural gasA 30 kW net combi. Meter to appliance measures 12 m along the wall. On the way there are four 90° elbows, one tee taken off the branch, and a service valve at the appliance.
- Load as a gas rate30 kW net → ≈ 3.1 m³/h
- Measured straight run12.0 m
- Fittings, as effective length4 elbows + 1 branch tee + 1 valve
- Total effective lengthmaterially longer than 12 m
- Outputa recommended size, plus every candidate compared
The figures the calculator returns depend on the fittings you actually enter, which is the whole point — run the same 12 m with no fittings and a different size comes out as suitable. Enter the run in front of you rather than a typical one.
What the result means
You get four things back. The table is the useful part — it shows you the size above and below the recommendation, which is what you need when the ideal size isn't on the van.
- Gas rate in m³/hWhat the pipe has to carry, converted from the kW load.
- Total effective lengthThe measured run once the fittings are counted. Compare it against your tape measure — the gap is usually the surprise.
- A recommended sizeThe smallest candidate that comes in under the drop you allowed.
- The full tableEvery candidate size with its internal diameter, resulting pressure drop, gas velocity, and whether it's suitable for what you entered.
A surprising answer is nearly always the effective length. If the recommendation looks a size larger than you expected, look at the fittings count before you doubt the tool — and if it looks a size smaller, check you haven't left the fittings at zero.
The calculator reports figures. Whether the installation is acceptable is your judgement, against the manufacturer's instructions and Gas Safe procedures.
The mistake people make
Sizing on the straight run and ignoring the fittings
A run with several elbows and a branch tee is considerably longer in effective terms than the tape measure says. Each fitting behaves like extra pipe, and they add up faster than most people expect on a domestic run.
That's the difference between a pipe that passes and one that starves the appliance at full rate — which typically shows up as a fault on hot water demand, months later, on a job somebody else gets called to.
Count the fittings before you cut. It takes ten seconds in the calculator and it's the only input people routinely leave at zero.
Questions
How do you size a gas pipe?
Convert the appliance load to a gas rate in m³/h, work out the effective length of the run by adding what the fittings contribute to the measured length, then pick the smallest size whose resulting pressure drop stays inside what you're allowed for that installation.
What is effective length?
The measured length of pipe plus an allowance for every fitting, because a fitting resists flow the way a length of pipe does. It's what the sizing is actually based on, and it's always longer than the tape measure reading.
Do I enter the load as net or gross?
Either — the calculator asks which. Enter the figure the plate or the manufacturer's instructions give you and tell it the basis, rather than converting first.
Why does it show gas velocity as well as pressure drop?
Because a size can be inside the pressure drop and still be worth a second look on velocity. Both are shown per candidate size so you can see the whole picture rather than one number.
What if the recommended size isn't on the van?
That's what the table is for. It shows the sizes either side with their pressure drop and velocity, so you can see exactly what going up a size buys you and what going down would cost you.
Is this free?
Yes. No sign-up, no email, no limit, and nothing held back behind a signup.
Guidance only. Follow Gas Safe procedures and the manufacturer's instructions. Gas work in the UK must be carried out by a Gas Safe registered engineer.
This calculator is part of OnPoint, which does certificates, scheduling and invoicing for UK trades. £19 for the first seat, £9 for each after.