Gas pipe sizing calculator
Size a natural gas pipe from the meter to the appliance. Build the run section by section, count the fittings, and keep the whole path inside one pressure drop limit, in copper or steel.
- Works out
- The smallest pipe for each section
- Pipe
- Copper 15 mm to 42 mm, steel 1/2" to 1 1/2"
- Counts
- Elbows, tees and service valves
- Checks
- The whole run against one pressure drop
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.
- Your run stays on this device. It's kept as you type, and Save keeps it by name.
- The same calculator engineers use in OnPoint's Tools.
01What it works out
The smallest pipe that still feeds the appliance.
Gas loses pressure as it flows along a pipe and round every fitting. Too small a pipe, and the appliance is starved when it runs flat out. The calculator finds the smallest size that keeps the loss from the meter to the appliance inside the limit you set.
For each section you get the size, its effective length with the fittings counted, the pressure it loses and how fast the gas moves. For the whole run you get the total drop against your limit, and how much is left.
When you'd use it
- A new boilerSize the run from the meter before you fit it, so the boiler gets its full rate on hot water.
- A bigger applianceCheck the pipe that fed the old boiler can carry the new one, which often has a higher input.
- Adding an applianceTee a hob or a fire off an existing run, and see what the extra load does to the section it shares.
- Fault findingA boiler that struggles at full rate, or a working pressure that falls away, can point to an undersized pipe.
02The method
How it sizes a run.
Natural gas at low pressure, worked out from first principles rather than read off a chart. These are the exact steps and figures the calculator uses.
01The load, as a gas rate
gross kW = net kW × 1.11
gas rate (m³/h) = gross kW ÷ (39.2 × 1.02264 ÷ 3.6)
Each section carries the load of everything beyond it. Enter it net or gross, and it's turned into the flow the pipe has to carry: about 11.14 kW gross for every m³/h.
02Effective length
length = pipe + extra
+ 0.5 × elbows + 0.5 × tees through
+ 1 × tee branches + 0.3 × valves
Every fitting slows the gas like a bit more pipe, so each one adds metres. The extra box is for anything else with a known allowance.
03Pressure drop
velocity = gas rate ÷ 3600 ÷ bore area
drop (Pa) = f × (length ÷ bore) × density × velocity² ÷ 2
f = 0.25 ÷ [log₁₀(roughness ÷ 3.7 bore + 5.74 ÷ Re^0.9)]²
This is the Darcy method. The friction factor f comes from how rough the pipe is and the Reynolds number Re (density × velocity × bore ÷ viscosity). For very slow gas, below Re 2300, f is 64 ÷ Re. Divide pascals by 100 for mbar.
04The whole run
total drop = section 1 + section 2 + … ≤ your limit
Sections are one path in series, so their drops add up against one limit. Every section starts at the smallest pipe. Then the section losing the most pressure goes up a size, again and again, until the total fits. Tap a size to pin it and the rest is sized around it.
The figures it uses
- Calorific value39.2 MJ/m³UK natural gas.
- Correction factor1.02264Pressure and temperature.
- Gross to net1.11Used when you enter the load as net.
- Gas density0.78 kg/m³Natural gas.
- Gas viscosity1.1 × 10⁻⁵ Pa·sDynamic viscosity of natural gas.
- Pipe roughness0.0015 / 0.045 mmCopper, then steel. Steel's rougher bore loses more.
- Fittings0.5 / 0.5 / 1 / 0.3 mElbow, tee through, tee branch, service valve.
- Noise warning20 m/sFaster than this, pipework can whistle.
The bores it sizes with
Copper
- 15 mm13.6 mm
- 22 mm20.2 mm
- 28 mm26.2 mm
- 35 mm33.0 mm
- 42 mm40.0 mm
Steel
- 1/2"16.1 mm
- 3/4"21.7 mm
- 1"27.3 mm
- 1 1/4"36.0 mm
- 1 1/2"41.9 mm
Nominal size, then the internal bore the sums use.
03A worked example
A combi and a hob on one meter.
A 28 kW combi and a 7 kW hob, in copper. The pipe leaves the meter carrying both, and tees off to the hob on the way to the boiler. So the path to the boiler is two sections.
- Each section passes on its ownIn 22 mm, the section from the meter loses 0.88 mbar and the one to the boiler 0.21 mbar. Both are under 1 mbar. Together they lose 1.08 mbar, which is over it.
- So the longest, busiest section goes upThe section from the meter is losing the most, so it goes up to 28 mm. The whole run now loses 0.47 mbar, with 0.53 mbar to spare.
Gas pipe sizing
Meter to combi, copper
35 kW at the meter · 1 mbar limit
- MeterEverything starts here: 35 kW for the combi and the hob.
Meter to tee
28 mm
35 kW net · 10 m of pipe · 4 elbows, 1 tee through
- Effective
- 12.5 m
- Drop
- 0.26 mbar
- Velocity
- 1.8 m/s
- TeeThe 7 kW hob branches off here. Size its branch as a run of its own.
Tee to boiler
22 mm
28 kW net · 3 m of pipe · 2 elbows, 1 service valve
- Effective
- 4.3 m
- Drop
- 0.21 mbar
- Velocity
- 2.42 m/s
- Combi boiler28 kW net, the last appliance on the path.
Pressure drop used, meter to boiler
0.47 mbar of 1 mbar
04Reading the result
What the answer tells you.
A size that comes out bigger than you expected is nearly always the fittings or a section you hadn't counted. One that comes out smaller usually means the fittings were left at zero.
Guidance only. The calculator sizes natural gas pipework at low pressure. Check the final sizing against the manufacturer's data and the standard for the installation, and follow Gas Safe procedures. In the UK, gas work must be done by a Gas Safe registered engineer.
- The pipe sizeThe smallest size for each section that keeps the whole run inside your limit. With more than one section, you get each size in order from the meter.
- Pressure drop usedHow much of the limit the run uses, and what's left. Little headroom leaves no room for a longer route than you measured.
- Fast enough to be heardGas moving faster than 20 m/s can make pipework whistle, even when the drop is fine. Going up a size slows it down.
- Over the limitYou've pinned sizes that don't leave enough for the rest of the run. Pin fewer, or allow a larger drop if the installation permits it.
- No size fitsEven the largest pipe can't carry the load within the limit. Shorten the run, or split the load.
More free calculators.
All free, with no account, and the same ones engineers use inside OnPoint.
OnPoint for your trade
- Gas rate calculatorHeat input from meter readings, checked against the data plate. Natural gas or LPG, metric or imperial, timed or one revolution.
- Gas ventilation calculatorThe permanent ventilation a domestic gas appliance needs, and how many airbricks that comes to.
- Heat loss calculatorRoom by room across a property, with radiators sized for the flow temperature you're fitting.
- Every calculator in OnPointThese four, plus the tightness test, IGEM/UP/1 testing and unsafe situations walk-throughs in the app.
Questions people ask.
Spotted something wrong in a sum? Ring +44 7884 697405 or email us.
How do you size a gas pipe?
Work out the gas rate the pipe has to carry from the load, add an allowance for each fitting to the length of pipe, then find the smallest size whose pressure drop stays inside the limit. With a tee in the run, size it as sections from the meter, each carrying only the load beyond it, and add their drops together.
What pressure drop is allowed?
The calculator starts at 1 mbar, the usual working limit for natural gas from the meter to the appliance, and offers 0.75 and 2.5 mbar or a figure of your own. Use the limit that applies to the installation in front of you.
What is effective length?
The length of pipe plus an allowance for every fitting, because a fitting slows the gas like extra pipe. The calculator adds 0.5 m for each 90° elbow and each tee the gas goes straight through, 1 m for each tee branch and 0.3 m for each service valve.
Why does it size the run in sections?
Because the limit applies to the whole path from the meter to the appliance, not to each length on its own. Two sections can each pass alone and still go over the limit together. Add a section wherever the pipe tees off or changes size.
Should I enter the load as net or gross?
Either. Enter the figure from the data plate or the manufacturer's instructions and say which it is. A net figure is multiplied by 1.11 to get gross before the gas rate is worked out.
Can it size steel pipe?
Yes. Choose steel for 1/2", 3/4", 1", 1 1/4", 1 1/2" pipe. Steel has a rougher bore than copper, so it loses more pressure at the same size, and the calculator allows for that.
Does it work for LPG?
No. It sizes natural gas pipework at low pressure. LPG is a different gas with different figures.
Is the gas pipe sizing calculator free?
Yes. There's no sign-up and no limit on sections. Your run is kept on your device as you type, and you can save it with a name. Nothing you enter is sent to OnPoint.
From the sizing to the signed certificate.
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