Free tool · Edition 4
Gas tightness test calculator
The whole test in the order you do it, worked to IGEM/UP/1B Edition 4, mandatory from 1 October 2026: check the installation, prove the ECV is not letting by, test, prove any movement is not in the pipework, then purge.
Test pressure: 20 to 21 mbar
1 · Before you start
A test is only as good as what you checked first.
2 · Let-by
ECV closed, gauge on the test nipple. Raise to 7 to 10 mbar and hold for a minute. You are watching for a rise, and a small one is allowed: only a rise past the permitted figure means the valve is letting gas by.
Allowed rise on this gauge: 0.25 mbar
3 · The test
Stabilise for a minute, then test for two. Natural gas timings: LPG stabilisation is not stated here because only one source gives it.
4 · Installation volume
Only needed if the gauge moved and you have to check it against what is permitted, or for the purge. A clean test needs none of it.
Per-metre volumes include the 10% fittings allowance. Estimate hidden runs from what you can see and use the largest size you cannot.
5 · Retest the pipework
Isolate every appliance and test again. There must be no perceptible movement. An appliance that cannot be isolated stays connected, and the no-movement rule then applies to the whole test.
Purge
How Edition 4 changes the test
Under Edition 3 the permissible drop came from the meter. Under Edition 4 it comes from the installation volume, so two properties with the same U6 can have different permitted drops depending on how much pipework is in them. That is why the volume has to be worked out first.
The second change is what a small drop means. A drop inside the permitted figure is not a pass on its own. Isolate every appliance and retest, and at that point there must be no perceptible movement. The allowance exists to account for appliances, not to give pipework a pass.
Where these figures come from
The per-metre volumes, the meter volumes and the permitted drops here follow Gas Safe Register's Industry Standard Update 133 and IGEM's own UP/1B FAQ, both published openly. The standard itself is copyright and is not reproduced here. This tool works out an answer for the numbers you enter. If you are testing to Edition 4 you should be working from your own copy of the standard, and you remain responsible for the result.
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Common questions
When does IGEM/UP/1B Edition 4 come into force?
Edition 4 is mandatory from 1 October 2026. Edition 3 is withdrawn on 30 September 2026.
How do I work out installation volume?
Add the meter volume to the volume of the pipework in the section under test. This calculator uses per-metre copper volumes that already include the 10% fittings allowance applied where actual component volumes are not known. Steel and PE are not built in, so enter those as a volume from manufacturer data.
What is the permissible drop under Edition 4?
It depends on the installation volume and the gas, not on the meter size as it did under Edition 3. Enter your figures above and the calculator gives the permitted drop for that installation.
Is a drop within the permitted figure a pass?
No. Any drop has to be proved not to be in the pipework. Isolate every appliance and retest, and there must be no perceptible movement. An appliance that cannot be isolated stays connected, and the no-movement rule then applies to the whole test.
How much rise is allowed on the let-by test?
0.25 mbar over the minute on a water gauge or a standard electronic gauge, and 0.2 mbar on an electronic gauge reading to one decimal place. A rise inside that is a pass. Only a rise past it means the ECV is letting gas by.
Can I purge to air?
On natural gas, where the purge volume is 0.02 m³ or less. Above that, purge to ignition. LPG and LPG/Air purge to ignition. The purge volume is 1.5 times the installation volume.
What counts as no perceptible movement?
0.25 mbar or less on a water gauge or a standard electronic gauge, and 0.20 mbar or less on an electronic gauge reading to one decimal place.
When is the installation outside UP/1B?
Above 0.035 m³ installation volume you are working to IGEM/UP/1A instead, which is a different procedure. The calculator says so when your figures go over.
Flint is a reference tool for competent, Gas Safe registered engineers. Everything here informs your judgement and never replaces it. Always work to the manufacturer's instructions and the current standards, and check every figure against the appliance data badge and manual.