Straight-run discharge table with 0.5m allowance per fitting. Verify the full installation against IGE/UP/2. Guidance only — always verify against manufacturer instructions and current Gas Safe / IGEM standards.
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This free gas pipe sizing calculator gives you the minimum copper pipe size for a natural gas load over a given run, based on a maximum 1 mbar pressure drop. It is built for UK Gas Safe registered engineers checking whether an existing run can support a new appliance, or sizing pipework for a boiler swap or new installation.
Enter the appliance's gross input in kW, the pipe run in metres and the number of elbows or tees on the run. The calculator converts the load to a required gas flow in m³/h, adds an equivalent-length allowance of 0.5 m per fitting, then checks the result against an IGE/UP/2 style discharge table for 10 mm, 15 mm, 22 mm and 28 mm copper.
The result is the smallest standard copper size that can carry the required flow over the effective length without exceeding a 1 mbar drop. Runs over 15 m effective length, or loads beyond 28 mm copper, fall outside the built-in table and should be sized directly from IGE/UP/2.
required flow (m³/h) = gross kW × 3.6 ÷ 39.3; effective length (m) = pipe run + (fittings × 0.5)
Gross kW × 3.6 converts the appliance's heat input to MJ/h, and dividing by the gross calorific value of natural gas (39.3 MJ/m³) gives the gas flow the pipe must carry in m³/h. Each elbow or tee adds resistance roughly equivalent to 0.5 m of straight pipe, so the effective length is the measured run plus 0.5 m per fitting. The calculator then finds the smallest copper size whose tabulated discharge at that length band meets or exceeds the required flow.
Enter the gross heat input in kW from the appliance data badge. If the badge quotes net input, convert to gross first (net × 1.11 for natural gas) so the flow calculation is correct.
Enter the length of the run in metres from the meter (or the point you are sizing from) to the appliance. Measure along the actual pipe route, not the straight-line distance.
Enter the number of elbows and tees on the run. Each one adds 0.5 m to the effective length, which can push a borderline run into the next size up.
The calculator shows the required flow in m³/h, the effective length, and the minimum copper size (10 mm, 15 mm, 22 mm or 28 mm) that carries that flow within a 1 mbar drop. A warning appears if the run or load is beyond the built-in table.
UK practice limits the pressure drop across installation pipework to 1 mbar between the meter and any appliance. This calculator uses a straight-run copper discharge table in the style of IGE/UP/2 — the same approach taught in Essential Gas Safety — with a flat 0.5 m equivalent-length allowance per elbow or tee.
The table covers single straight runs in copper up to 28 mm and 15 m effective length, carrying natural gas only. Branched installations with several appliances drawing through shared sections, steel pipework, LPG, or longer runs all need to be sized section by section from the full IGE/UP/2 tables rather than this simplified check.
Remember the whole installation matters: an adequately sized final run can still starve an appliance if an upstream shared section is undersized. Verify the complete installation against IGE/UP/2, and confirm the result on site with a working pressure check at the appliance. Guidance only — always verify against manufacturer instructions and current Gas Safe / IGEM standards.
Straight-run discharge table with 0.5m allowance per fitting. Verify the full installation against IGE/UP/2. Guidance only — always verify against manufacturer instructions and current Gas Safe / IGEM standards.
A 30 kW gross input needs about 2.75 m³/h of natural gas (30 × 3.6 ÷ 39.3). On a short run that is within 22 mm copper's capacity, but as the effective length increases — including 0.5 m per elbow or tee — you may need 28 mm. Enter your actual run and fittings above to get the minimum size for your installation.
This calculator sizes copper pipework to a maximum 1 mbar pressure drop across the run, which is the figure used for UK domestic natural gas installations between the meter and the appliance. Confirm the drop on site with a working pressure test at the appliance with it running at full rate.
Every fitting adds resistance to flow. This calculator allows 0.5 m of equivalent straight length per elbow or tee, so a 7 m run with four elbows is treated as 9 m effective length. On a borderline run, a few fittings can be the difference between 22 mm and 28 mm, so count them honestly.
No. The discharge table and the 39.3 MJ/m³ calorific value are for natural gas in copper pipe only. LPG has a much higher calorific value and different design pressure drops, so LPG pipework must be sized from the appropriate LPG tables and standards.
The built-in table covers effective lengths up to 15 m. Beyond that the calculator flags the run and you should size it directly from the IGE/UP/2 tables, which cover longer runs and larger pipe sizes. The same applies if the required flow exceeds what 28 mm copper can carry.
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