Installation Volume Calculator

How to use this calculator

1. Pick your gas type at the top of the right panel — natural gas, LPG, or LPG/air. The initial test pressure auto-fills to the right default (21 mbar for natural gas and LPG/air, 37 mbar for LPG). Override it if your job runs on a non-standard pressure.
2. Enter pipework lengths in metres for each size of copper and steel you’ve used. You can mix copper and steel freely — the calculator adds them all into IVp. Leave any size you haven’t used blank or zero.
3. Select the meter you’re testing through. If you’re testing a section of pipework with no meter included (e.g. before the meter is fitted), choose “No meter”.
4. Enter your observed pressure drop from the tightness test. The calculator immediately tells you whether the test passes or fails against the permissible drop band. See our step-by-step tightness test guide.
5. Hit “Copy result” to put a plain-text record of the calculation on your clipboard — pipework breakdown, IVp + IVf + IVm components, IV total, purge volume, and pass/fail. Paste straight into a job sheet, customer email, or your job record in Gas Engineer Software.

Results are aligned to IGEM/UP/1B Edition 4. Built for UK gas engineers; works for natural gas, LPG, and LPG/air installations.

What is installation volume?

Installation volume — usually written as IV or IVt — is the total internal volume of all the pipework, fittings, and the meter on a gas installation. It’s the figure that determines the permissible pressure drop on a tightness test and the amount of gas needed to purge the system safely after work is finished.

The bigger the installation, the more air the engineer is testing through, and the more allowance there is for small natural pressure changes during the test. That’s why permissible drop is banded by installation volume rather than fixed at a single number.

You need to calculate IV any time you:

• Carry out a tightness test under IGEM/UP/1B Edition 4
• Purge a new or altered installation before recommissioning
• Test an existing installation after replacing a meter or part of the pipework
• Need to record the calculation on a CP12 or job record

Record the calculation in Gas Engineer Software

Calculating IV is the easy part. Getting it onto a certificate, a job sheet, and into the customer record without re-typing the numbers three times is where Gas Engineer Software helps.

The app generates CP12s, CP14s, service records, and tightness test certificates from your phone or tablet, attaches them to the customer, and emails them straight to the homeowner or letting agent. It works offline, syncs when you’re back in signal, and stores every certificate against the property so you can find them again in seconds.

Try Gas Engineer Software free for 14 days — no card required.