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Calculate gas appliance heat input (kW) from test dial measurements on imperial (cubic feet) gas meters. Essential for Gas Safe registered engineers working with older meter installations.
Supports test dial sizes: 0.5, 1, 2, and 5 cubic feet
For gas meters measuring in cubic feet (cu ft)
Check the dial face on your meter for the size marking
Select your test dial size and enter the time to calculate
Volume (cu ft) = Test dial size
Volume (m³) = cu ft × 0.0283168
Flow Rate (m³/h) = Volume × (3600 / seconds)
Gross kW = Flow Rate × 39.5 ÷ 3.6
Net kW = Gross kW ÷ 1.11
| Appliance Type | Approx. kW | 1 cu ft dial | 2 cu ft dial |
|---|---|---|---|
| Pilot Light | 0.2-0.5 kW | 3-6 mins | 6-12 mins |
| Gas Hob (one ring) | 2-3 kW | 40-60 secs | 80-120 secs |
| Combi Boiler (DHW) | 24-30 kW | 10-15 secs | 20-30 secs |
| System Boiler | 15-24 kW | 12-20 secs | 24-40 secs |
Times are approximate. Always compare your calculated kW to the appliance data plate rating.
An imperial gas meter measures gas consumption in cubic feet (cu ft) rather than cubic metres. These are older-style meters still found in many UK properties, particularly those built before metrication. They have a test dial that shows smaller increments of gas usage for accurate testing.
Imperial meters display readings in cubic feet (cu ft or ft³). The meter face will show 'cubic feet' or 'cu ft'. The test dial (small dial on the face) will be marked in fractions of a cubic foot, typically 0.5, 1, 2, or 5 cu ft per revolution.
1) Turn off all other gas appliances. 2) Identify your test dial size (usually marked on the dial). 3) Turn on the appliance at full rate. 4) Time one complete revolution of the test dial using a stopwatch. 5) Enter the dial size and time into this calculator to get the heat input in kW.
Use the test dial fitted to your meter - this is usually marked on the dial face. Most domestic imperial meters have a 1 cu ft or 2 cu ft test dial. Smaller 0.5 cu ft dials are used for low-output appliances. The dial size doesn't affect the calculation - just ensure you time one complete revolution.
The time depends on the appliance output. A typical 24kW boiler on a 1 cu ft dial takes approximately 12-15 seconds per revolution. Lower output appliances take longer. If the dial is spinning very fast (under 5 seconds), consider using a larger test dial if available for more accuracy.
Both methods should give similar results when done correctly. Small differences (±5%) are normal due to timing accuracy and calorific value variations. Larger differences may indicate a timing error, incorrect test dial size selection, or an issue with the appliance.
Official register of gas engineers. Verify registrations and find qualified engineers in your area.
Institution of Gas Engineers and Managers - technical standards and guidance for the gas industry.
If your meter displays readings in cubic metres (m³), use our standard gas rate calculator instead.
Metric Gas Rate Calculator