Reference

HVAC Unit Conversions

The conversion factors behind every metric and imperial result on this site, from tons and kilowatts to CFM, pressure and water flow.

Last updated

Heat and power

Unit Equals
1 ton of cooling 12,000 BTU/hr = 3.517 kW
1 kW 3,412.14 BTU/hr = 0.284 tons
1 W 3.412 BTU/hr
1 MBH 1,000 BTU/hr
1 boiler horsepower 33,475 BTU/hr = 9.81 kW
1 BTU/hr 0.252 kcal/h = 0.293 W
1 therm 100,000 BTU = 29.31 kWh
1 kWh 3,412.14 BTU

Common sizes

Tons BTU/hr kW Tons BTU/hr kW
1 12,000 3.5 3.5 42,000 12.3
1.5 18,000 5.3 4 48,000 14.1
2 24,000 7.0 5 60,000 17.6
2.5 30,000 8.8 7.5 90,000 26.4
3 36,000 10.6 10 120,000 35.2

US model numbers usually encode capacity in thousands of BTU/hr: “24” is 2 tons, “36” is 3 tons, “60” is 5 tons. A 100,000 BTU/hr furnace input is 29.3 kW of fuel and burns one therm per hour.

Airflow, pressure and velocity

Unit Equals
1 CFM 1.699 m³/h = 0.472 L/s
1 m³/h 0.589 CFM
1 FPM 0.00508 m/s
1 in wc (inch of water column) 249.09 Pa
0.1 in wc 24.9 Pa
1 Pa 0.004 in wc
velocity (FPM) = CFM × 144 ÷ area (sq in) sensible heat (BTU/hr) = 1.08 × CFM × temperature difference (°F) water heat (BTU/hr) = 500 × GPM × temperature rise (°F)

Water and volume

Unit Equals
1 US gallon 3.785 L = 8.34 lb of water
1 GPM 3.785 L/min
1 pint (US) 0.473 L
1 cubic foot 28.32 L = 7.48 US gallons
1 lb of air at room conditions about 13.3 cu ft
1 grain of moisture 1/7,000 lb

Temperature and area

°C = (°F − 32) ÷ 1.8 °F = °C × 1.8 + 32 a temperature difference of 1 °C = 1.8 °F
Unit Equals
1 sq ft 0.0929 m²
1 m² 10.764 sq ft
1 ft 0.3048 m
1 in 25.4 mm
1 BTU/hr per sq ft 3.155 W/m²

Calculators that use these conversions

The guide to furnace input and output uses these conversions on a real nameplate.