Gas Pipe Sizing Calculator (UK)

To calculate gas pipe size, you need to know three things: (1) the total gas demand in kW of all appliances on that pipe run, (2) the total equivalent pipe length in metres (measured route plus an allowance for fittings), and (3) the pipe material (copper or steel). Convert the kW demand to a volumetric flow rate (m³/h) by dividing by 10.73. Then look up the appropriate pipe diameter from a BS 6891 sizing table, or use a gas pipe sizing calculator like the one at the top of this page. The pipe you choose must carry the required flow with no more than 1 mbar pressure drop at 21 mbar supply pressure.

For most UK domestic combi boilers (24–35 kW) on pipe runs up to 15 m, 22 mm OD copper pipe is the standard recommendation. For runs longer than 20 m, or where multiple appliances share the same pipe section, 28 mm is usually required. A 15 mm pipe is generally too small for modern boilers on anything other than the very shortest spur connections (under 3 m). Always check against the boiler manufacturer's gas supply requirements in the installation manual.

Gas pipe sizing involves four steps: (1) list all appliances supplied by that pipe section and add their kW ratings together; (2) measure the pipe route carefully and add equivalent lengths for fittings (elbows, tees, valves); (3) convert the total kW to a flow rate in m³/h; (4) look up the minimum pipe diameter in the BS 6891 / Transco sizing tables that delivers that flow at your required equivalent length. The gas pipe sizing calculator on this page automates steps 3 and 4 for you.

Gas pipe size is determined by two main factors: the total gas demand (kW) that the pipe must supply, and the total equivalent pipe length. Longer runs and higher demands both require larger diameters to keep pressure drop within the 1 mbar limit specified by BS 6891. You use a gas pipe sizing calculator or published Transco sizing tables to find the minimum acceptable diameter. Always round up to the next standard pipe size — never down.

In the UK, copper gas pipe is measured and sold by outside diameter (OD). So "22 mm pipe" means the outside is 22 mm across. To measure an existing pipe, use a set of pipe ID/OD callipers or wrap a string around the pipe and measure the circumference, then divide by π (3.14159) to get the OD. Do not confuse OD with ID (internal diameter) — the internal diameter of 22 mm copper pipe is approximately 19.6 mm. Steel pipe is described differently using DN (nominal bore) sizes; DN20 steel pipe is broadly equivalent in capacity to 22 mm copper.

An undersized gas pipe creates excessive pressure drop. At the appliance end, the gas pressure falls below what the appliance needs to operate correctly. For a boiler, this typically causes: failure to ignite at full output, the boiler locking out on a low-gas-pressure fault, intermittent operation (especially when other appliances are also on), and incomplete combustion in severe cases. An undersized gas line can also cause a boiler to run at reduced output, raising energy bills and shortening appliance life.

Only on a very short spur of 3 m or less. At 10 m, 15 mm copper pipe can only carry around 14 kW — well below the 24 kW demand of the boiler. In practice, most Gas Safe engineers will specify 22 mm as the minimum for any combi boiler connection, even on short runs, to give adequate capacity margin and comply with manufacturer requirements. Using 15 mm pipe for a modern high-output boiler on a standard domestic pipe run will almost certainly cause pressure drop problems.

The most common standard gas pipe sizes in UK domestic properties are 15 mm, 22 mm and 28 mm OD copper tube (to EN 1057). 22 mm is the most widely used size, suitable for most combi boiler installations. 15 mm is used for short spurs to individual appliances. 28 mm is used for the main meter-to-distribution run in properties with multiple gas appliances or longer pipe routes. 35 mm and 42 mm are used in larger domestic or light commercial settings.

The gas pipe sizing formula is based on the Weymouth equation for compressible flow: ΔP = (C × L × Q²) ÷ D⁵, where ΔP is pressure drop in mbar, L is equivalent pipe length in metres, Q is flow rate in m³/h, D is internal pipe diameter in mm, and C is a constant that depends on gas type and pipe material. In practice, engineers use published BS 6891 sizing tables (based on this formula) or software tools rather than calculating this by hand. The target is always ΔP ≤ 1 mbar for domestic gas.

Yes. If a gas pipe is too small, the boiler cannot receive enough gas at the correct pressure to fire at full rated output. This means the boiler modulates down or locks out, reducing efficiency and increasing run times. On the other hand, correctly sized gas pipework ensures the boiler gets a stable gas supply at 20–21 mbar, allowing it to run at peak efficiency. This is particularly important for condensing boilers, which need stable operation to reach their rated seasonal efficiency.