- 1. IGEM/UP/1B EDITION 4
- 2. Gas Installation Volume Calculator & Tightness Test Pass/Fail Checker
- 3. Copper pipework
- 4. Steel pipework
- 5. Meter
- 6. Gas type
- 7. Tightness test parameters
- 8. Tightness Test Pass/Fail
- 9. Results
- 10. TRY GES FOR FREE
- 11. Start your 14-day free trial
- 12. How to use this calculator
- 13. What is installation volume?
- 14. Record the calculation in Gas Engineer Software
- 15. Make your job easier withGas Engineer Software
- 16. Frequently asked questions
- 1. IGEM/UP/1B EDITION 4
- 2. Gas Installation Volume Calculator & Tightness Test Pass/Fail Checker
- 3. Copper pipework
- 4. Steel pipework
- 5. Meter
- 6. Gas type
- 7. Tightness test parameters
- 8. Tightness Test Pass/Fail
- 9. Results
- 10. TRY GES FOR FREE
- 11. Start your 14-day free trial
- 12. How to use this calculator
- 13. What is installation volume?
- 14. Record the calculation in Gas Engineer Software
- 15. Make your job easier withGas Engineer Software
- 16. Frequently asked questions
IGEM/UP/1B EDITION 4
Installation Volume & Tightness Test Calculator
Use our free Installation Volume Calculator to work out the installation volume of any domestic gas system in seconds — then check whether the tightness test passes and how much gas you’d need to purge. For new permissible drops, see our full IGEM/UP/1B Edition 4 guide.
Gas Installation Volume Calculator & Tightness Test Pass/Fail Checker
Copper pipework
BS EN 1057 · Enter length in metres for each size used.
Steel pipework
BS 1387 medium grade · Nominal bore.
Meter
Gas type
Tightness test parameters
Tightness Test Pass/Fail
Results
Show breakdown
- No pipework entered.
For guidance only. Calculations follow the IGEM/UP/1B Edition 4 method for domestic natural gas (21 mbar), LPG (37 mbar) and LPG/air (21 mbar) tightness tests. Always verify the current standard, your installation conditions, and any local rules before relying on a result. Gas Engineer Software accepts no liability for tests carried out on the basis of this tool.
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How to use this calculator
- 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.
- 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.
- 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”.
- 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.
- 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.
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Frequently asked questions
Got a question we haven't covered? Talk to our team.
How do I calculate gas installation volume?
Installation volume (IV) is the total internal volume of the pipework, fittings and meter combined: IV = IVp + IVf + IVm. IVp is the sum of each pipe length multiplied by its internal volume per metre (e.g. 0.140 L/m for 15mm copper, 0.320 L/m for 22mm). IVf is the fittings allowance — a flat 10% of IVp under IGEM/UP/1B Edition 4. IVm is the meter volume: 8 L for a U6/G4, 2.4 L for an E6, or 25 L for a U16. Add them up and you have your total IV in litres.
How much drop is allowed on a tightness test?
Under IGEM/UP/1B Edition 4 the permissible drop depends on the gas type and your installation volume. For natural gas (21 mbar test): IV up to 5 L allows an 8 mbar drop, up to 10 L allows 4 mbar, up to 15 L allows 2.5 mbar, and up to 35 L allows 1 mbar. For LPG (37 mbar test): up to 2.5 L allows 2 mbar, up to 5 L allows 1 mbar, up to 10 L allows 0.5 mbar, and above 10 L there must be no perceptible movement. For LPG/air (21 mbar test): up to 25 L allows 1.5 mbar, up to 35 L allows 0.5 mbar. Installations over 35 L are outside the short-method scope.
What is the new tightness test procedure under Edition 4?
The Edition 4 procedure starts with a let-by test at 7-10 mbar for 1 minute to check the meter inlet valve. If that passes, raise to the working test pressure — 21 mbar for natural gas, 37 mbar for LPG, 21 mbar for LPG/air — and allow 1 minute for temperature stabilisation. Then time a 2-minute test and compare the observed pressure drop against the permissible drop for your IV and gas type. Edition 4 also introduced a pipework-only retest option: if the full installation fails, you can isolate the appliances and retest just the pipework against a tighter limit.
How many minutes is a tightness test?
The timed test itself is 2 minutes once the pressure has stabilised. The full procedure runs around 4 minutes of timed work — 1 minute for the let-by test, 1 minute for temperature stabilisation, then the 2-minute tightness test — plus the time to fit the gauge and get to working pressure.
How do I calculate purge volume?
Purge volume (PV) under IGEM/UP/1B Edition 4 is 1.5 × IV. So a 1.5 litre installation needs a 2.25 litre purge. The calculator displays PV alongside the installation volume.
What if my installation volume is over 35 litres?
The IGEM/UP/1B short method is only valid up to an IV of 35 L. Above that you're out of scope for the banded permissible-drop table and need to apply the proportional method from the full standard, or refer to IGEM/UP/1A for larger systems.