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Calculate gas appliance heat input in kW from meter readings or test dial measurements. Essential tool for commissioning, servicing, and fault-finding.
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Enter your meter readings and time to calculate
How to calculate gas rate for boilers and appliances — the essential formulas used by UK Gas Safe engineers.
CV = Calorific Value (typically 39.5 MJ/m³)
1.02264 = Volume correction factor
0.0283168 = Conversion factor (cu ft to m³)
Then apply metric formula for kW
Metric Meter Reading:
Common Calorific Values:
Calculate gas rates for LPG (Liquefied Petroleum Gas) appliances — propane and butane cylinders or bulk tanks.
Calorific Value:
93.1 MJ/m³ (25.9 kWh/m³)
Typical Cylinder Sizes:
Common for heating, hot water, cooking in caravans and off-grid homes
Calorific Value:
121.8 MJ/m³ (33.8 kWh/m³)
Typical Cylinder Sizes:
Better for indoor use, portable heaters, doesn't work below -2°C
1. For Bulk Tanks: Use the metric mode with meter readings, but adjust the calorific value in your calculations (Propane: 93.1 MJ/m³, Butane: 121.8 MJ/m³ instead of 39.5 MJ/m³ for natural gas).
2. For Cylinders: Time the appliance run duration and calculate consumption rate. A 4kW heater uses approximately 0.29 kg/hour of propane or 0.31 kg/hour of butane.
3. Commercial LPG: For commercial LPG installations, the gas rate calculation is essential for sizing pipework, regulators, and ensuring adequate supply capacity.
Read the meter in cubic metres (m³) before and after the test
Time one complete revolution of the test dial (cu ft)
Volume (m³) = End Reading - Start Reading
Flow Rate (m³/h) = Volume × (3600 / seconds)
Gross kW = Flow Rate × 39.5 ÷ 3.6
Net kW = Gross kW ÷ 1.11
This calculator uses 39.5 MJ/m³ as the standard calorific value for UK natural gas. The actual CV supplied to your area may vary between 37.5-43.0 MJ/m³. For precise work, check the current CV with your gas transporter or use the value from recent gas bills.
A gas rate calculator is a tool used by Gas Safe registered engineers to calculate the heat input (in kilowatts) of gas appliances. It uses meter readings or test dial measurements to determine if an appliance is operating within its rated capacity.
Gross kW is the total heat input including the latent heat in water vapour. Net kW (gross ÷ 1.11) is the usable heat output. Most appliance data plates show net kW, so compare your calculated net kW to the appliance rating.
Use metric mode if your gas meter displays readings in cubic metres (m³). Use imperial mode if you're using a test dial gauge that measures in cubic feet (cu ft). Most modern UK meters are metric.
For metric mode, run the test for at least 2 minutes (120 seconds) to get an accurate reading. For imperial mode with a test dial, time one complete revolution of the dial - faster-burning appliances will complete a revolution quicker.
This calculator uses 39.5 MJ/m³, which is the typical calorific value for UK natural gas. The actual CV can vary slightly (typically 37.5-43.0 MJ/m³), but 39.5 is the standard used for most calculations.
Small variations (±5%) are normal due to gas pressure, calorific value variations, and measurement timing. Larger differences may indicate a fault, incorrect burner pressure, or a need for servicing. Always follow Gas Safe procedures.
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This gas rate calculator is designed to assist Gas Safe registered engineers. Always follow official Gas Safe procedures and regulations.