The UK gas tightness test

In Britain the test that proves a domestic installation is sound is a tightness test, run to IGEM/UP/1B. It is a different animal from the American pressure test: about a tenth of the pressure, four minutes on the clock instead of ten, and the gas itself in the pipe. Here is the procedure as it stands — and the change that lands on 1 October 2026.

Let-by: 7–10 mbar · 1 minute, no rise
Test pressure: 20–21 mbar · natural gas; 18 mbar floor
Stabilise: 1 min · before timing starts
Test: 2 min · no perceptible movement

What changes on 1 October 2026

IGEM/UP/1B Edition 4 was published in March 2026. Editions 3 and 4 have run side by side since; Edition 3 is withdrawn on 30 September 2026, and from 1 October every tightness test is to be carried out to Edition 4. The procedure is broadly the same. What changed is how a drop on the gauge is judged:

Permissible limits now follow installation volume, not meter size. Where the gauge moves on a system with existing appliances, you calculate the installation volume (IV) of the system — or of the part under test — and read the drop against the table for the fuel gas.
A drop within limits is followed by a pipework-only retest. Isolate the appliances and test again; on pipework alone there can be no perceptible movement. That establishes the drop is not in the pipes.
A floor for the test pressure. Where the minimum tightness test pressure cannot be reached, the installation is isolated and the low pressure investigated first. For natural gas, a test that cannot reach 20 mbar may be carried out at no less than 18 mbar.

"Updated tightness testing standard: what you need to know" and "Industry Standard Update 133 (Edition 2)", Registered Gas Engineer, 2026

The procedure, natural gas

Make readyAppliances off, pilots out, no open ends on the pipework. Connect the gauge to the test point.
Let-by: 7–10 mbar for 1 minuteWith the emergency control valve off, the gauge must not rise. A rise means the valve is passing gas, which would hide a leak — stop, and deal with the valve before anything else.
Raise to the test pressure20–21 mbar for natural gas; no less than 18 mbar where 20 cannot be reached, once the low pressure has been looked into.
Stabilise for 1 minuteGas that has just been let in is changing temperature, and a cooling charge reads as a drop. Do not start the clock until it has settled.
Test for 2 minutesWatch the gauge. On pipework only, or with only new appliances connected, there can be no perceptible movement.
If it moved, with existing appliances connectedWork out the installation volume and compare the drop with the permissible-limits table in Edition 4. Within limits: isolate the appliances and retest the pipework alone, which must not move at all. Outside limits: trace and repair the escape, or make the installation safe.

Smell of gas overrides the gauge. Permissible limits only apply where there are existing appliances and no smell of gas has been reported. A pass with a smell of gas is not a pass.

The permissible-drop figures themselves are in IGEM/UP/1B Edition 4 and are not reproduced here. Read them from the standard, not from a table on somebody's website — including this one.

UK tightness test and US pressure test, side by side

The two are often searched for as if they were the same thing. They are not built for the same job: the UK test checks a domestic installation at close to the pressure it runs at, with the gas in it; the US test proves new piping before any gas is allowed in.

	UK tightness test	US pressure test
Pressure	20–21 mbar≈ 0.3 psi, natural gas	≥ 3 psi≈ 207 mbar, or 1.5× working
In the pipe	The gas supply, through the meter	Air, nitrogen or CO₂ — never the fuel gasIFGC 2021 §406.2
Timing	1 + 1 + 2 minlet-by, stabilise, test	10 mina house; ½ h per 500 ft³ otherwise
Who	A Gas Safe registered engineer — required by the Gas Safety (Installation and Use) Regulations 1998	A licensed plumber or gas fitter, under state and local rules

The US side in detail: gas pressure test requirements, and what US utilities ask for on paper: pressure test forms.

Where GasTest Log fits — and where it does not

GasTest Log is built around the American test: its presets, its hold rules and the code sections printed on its certificate are IFGC, IRC and UPC. It does not run the IGEM/UP/1B sequence and its certificate is not a UK gas safety record, so in Britain use the paperwork your Gas Safe registration requires. If you work on both sides of the Atlantic, the US pressure test is the one it is for.

What the app does on a US job

A summary of published guidance on IGEM/UP/1B, not a copy of the standard or a substitute for it. Gas work in Great Britain must be carried out by a Gas Safe registered engineer; work to the standard itself and to your training.

Last reviewed 22 September 2026