#How to Gas Rate a Boiler: Metric and Imperial Formulas (with Examples)

Gas rating is the timed meter test that tells you whether an appliance is actually burning gas at the rate the manufacturer intended. It takes five minutes on site, and it catches problems that a flue gas reading alone will miss — undersized pipework, a struggling gas valve, a regulator on its way out. This guide covers the metric and imperial methods, the formulas behind them, gross vs net, and a worked example for a 30 kW boiler.

#What gas rating is and why it's done

When you gas rate an appliance, you measure how much gas it consumes over a fixed period at full rate, convert that to a flow in m³/h, and then to a heat input in kW. You compare the result against the data badge.

It's a standard part of:

• Commissioning — proving the new boiler is set up and supplied correctly before you sign the Benchmark checklist
• Servicing — confirming the appliance still burns at the rated input year on year
• Fault-finding — a rate well below badge points at the supply (pipework, valve, regulator) rather than the appliance itself

A rate significantly below the badge value suggests a supply problem: undersized or restricted pipework, a partially closed valve, low working pressure or a faulty regulator. A rate above badge value also needs investigating before the appliance is left in service.

#Before you start

Two things make or break the accuracy of the test:

1. Turn off all other gas appliances. The meter must only be registering the appliance under test — a pilot light on an old fire or a hob left simmering will skew the result.
2. Run the appliance at maximum rate and let it stabilise. Fire the boiler at full rate (combis usually via hot water at full flow, or the manufacturer's max-rate mode) and give it a minute or two to settle before you start timing.

A longer test averages out meter resolution: 2 minutes is the common choice, and 3 minutes gives a more accurate result on small-input appliances.

#The metric method (meters reading in m³)

Note the meter reading, time a fixed period — 60, 120 or 180 seconds — then note the reading again. Two formulas do the rest:

Step 1 — flow rate:

m³/h = (final reading − initial reading) × 3600 ÷ seconds

Step 2 — heat input:

gross kW = m³/h × CV ÷ 3.6

CV is the gas's gross calorific value in MJ/m³ — 39.3 for natural gas (a typical UK figure; actual CV varies slightly by region and day to day).

Quick example: initial reading 12345.678, final reading 12345.780 after 120 seconds.

• Volume used = 0.102 m³
• m³/h = 0.102 × 3600 ÷ 120 = 3.06 m³/h
• gross kW = 3.06 × 39.3 ÷ 3.6 = 33.4 kW gross
• net kW = 33.4 ÷ 1.11 = 30.1 kW net

Our free gas rate calculator has a built-in countdown for the 60/120/180-second test and does all three conversions live as you type the readings.

#The imperial method (meters reading in ft³)

Older imperial meters have a small test dial registering a fixed volume per revolution — typically 1 ft³. Instead of timing a fixed period, you time one complete revolution of the test dial with the appliance at full rate:

• ft³/h = (dial volume × 3600) ÷ seconds per revolution
• Convert to metric: m³/h = ft³/h × 0.0283
• Then as before: gross kW = m³/h × 39.3 ÷ 3.6

Example: one revolution of a 1 ft³ test dial takes 33 seconds.

• ft³/h = 3600 ÷ 33 = 109.1
• m³/h = 109.1 × 0.0283 = 3.09
• gross kW = 3.09 × 39.3 ÷ 3.6 = 33.7 kW gross → 30.4 kW net

You'll also see the imperial formula written with a 1.02264 correction factor — gross kW = (ft³/h × 1.02264 × 39.3 × 0.0283) ÷ 3.6 — which corrects the metered volume to standard temperature and pressure. It shifts the result by about 2%, well inside normal tolerance; the simpler form above is what our calculator uses.

The gas rate calculator has a stopwatch mode for imperial dials — start it as the dial pointer passes a mark, stop it one revolution later, and read off gross and net kW.

#Gross vs net — which figure does the badge quote?

Gross heat input includes the latent heat in the water vapour produced by combustion; net input excludes it. The conversion is fixed:

net kW = gross kW ÷ 1.11 (natural gas) or ÷ 1.09 (LPG)

Most modern appliance data badges quote net input, so check which figure the manufacturer states before you compare. Rating a boiler and comparing your gross result against a net badge figure makes a healthy appliance look 11% over-firing — a classic way to chase a fault that isn't there.

#Worked example: gas rating a 30 kW boiler

Say the badge states 30 kW net input on natural gas. What should the meter show?

• Expected gross input = 30 × 1.11 = 33.3 kW
• Expected flow = 33.3 × 3.6 ÷ 39.3 = 3.05 m³/h
• Over a 120-second test = 3.05 ÷ 30 = about 0.102 m³ on the meter

So if you time 2 minutes and the meter has moved roughly 0.10 m³, the boiler is rating correctly. If it's only moved 0.08 m³ (≈2.4 m³/h, ≈26 kW gross), the appliance is under-firing by around 20% and the supply needs investigating before it leaves your hands.

#Quick reference: common boiler sizes vs expected gas rate

Figures below assume natural gas at 39.3 MJ/m³ gross CV and a net badge figure (gross = net × 1.11).

Treat these as sanity-check figures, not pass/fail limits — the badge and the manufacturer's instructions are always the reference.

#What tolerance is acceptable?

Manufacturers typically state an acceptable tolerance in the installation or servicing instructions — commonly in the region of ±5–10% of the rated input. Two things to keep in mind:

• Small differences are normal. The actual calorific value of the gas varies slightly by region and from day to day, so don't chase a 2–3% discrepancy.
• Outside tolerance means investigate, not adjust and walk away. Check the test was done properly (full rate, everything else off, appliance stabilised), then look at working pressure at the appliance, pipework sizing and the regulator. Under-firing that traces back to undersized pipework won't be fixed at the gas valve.

#Gas rating on LPG

The method is identical — only the constants change. LPG has a much higher calorific value than natural gas: 93.1 MJ/m³ gross for propane and 121.8 MJ/m³ for butane, with a gross-to-net factor of 1.09. That means an LPG appliance moves far less volume through the meter for the same kW, so a longer test duration pays off. Where there's no meter (cylinder installations), gas rating isn't possible and you're checking burner pressure and combustion to the manufacturer's data instead.

A note on safety: this guide is guidance only. Always follow the manufacturer's instructions, the Gas Safety (Installation and Use) Regulations and current Gas Safe / IGEM standards — and remember that work on gas appliances in the UK must only be carried out by Gas Safe registered engineers.

#Do it faster with TradePlanr

TradePlanr's free gas rate calculator handles metric and imperial meters, natural gas and LPG, with a built-in countdown and stopwatch and live gross/net kW — no sign-up needed. When the numbers check out, record them where they count: complete the Benchmark commissioning checklist or a gas service and maintenance record on your phone, sign on screen and send the customer a PDF before you leave site. The free plan covers your first certificates, and the paid plan is a flat £9.99/month — no per-certificate fees.