Short answer
Installation volume is the sum of the internal volume of every pipe section, the meter and any significant fittings or appliance volumes. Multiply each pipe length by its internal volume per metre, add the meter's internal volume, and add an allowance for fittings. That figure drives the choice of standard, the test duration and the purge volume.
Pipe volumes
Volume per metre = π × (internal diameter ÷ 2)². As a guide, steel pipe internal volumes are roughly: 25 mm ≈ 0.00058 m³/m, 50 mm ≈ 0.0022 m³/m, 80 mm ≈ 0.0051 m³/m, 100 mm ≈ 0.0087 m³/m, 150 mm ≈ 0.019 m³/m. Use the actual internal diameter for the material and wall thickness installed.
Worked example
A kitchen has 18 m of 50 mm steel (18 × 0.0022 = 0.040 m³), 12 m of 28 mm copper (approximately 0.00053 m³/m = 0.0064 m³) and a U16 meter (check the manufacturer's figure; diaphragm meters of this size are typically around 0.008 m³). Adding 10% for fittings gives about 0.06 m³ — well inside UP/1A territory.
Why accuracy matters
Underestimating volume shortens the test duration and can let a real leak pass. Overestimating it can push a job into UP/1 unnecessarily. Record the calculation on the test certificate so it can be checked.
Figures are typical for UK natural gas. The current edition of the relevant IGEM standard and the manufacturer's instructions always take precedence on site.
Need this done on site?
Need a commercial gas engineer in London?
Gas Safe registered commercial engineers, 24 hours a day. Call now or book online and we'll confirm a time.