HVAC unit converter
Convert the units you switch between every day in design and exams: airflow, cooling capacity, static pressure, temperature and more. Every factor is computed from its definition; the table below lists each definition and its source.
Conversion factors
The "1 unit =" column gives each unit in the category's reference unit (shown in bold), to 12 significant digits. Where more than one convention exists, the one used here is stated.
| Unit | 1 unit = | Definition · basis |
|---|---|---|
| Temperature Temperature scales have different zeros: "a × x + b K" turns a value x in that unit into kelvin. | ||
| °C Celsius | 1 × x + 273.15 K | K = °C + 273.15 (definition) |
| °F Fahrenheit | 0.555555555556 × x + 255.372222222 K | °F = °C × 1.8 + 32, K = (°F + 459.67) × 5/9 (definition) |
| K Kelvin | Reference unit | SI base unit |
| °R Rankine | 0.555555555556 × x K | °R = K × 1.8: absolute zero, °F-sized degrees |
| Length | ||
| mm Millimetre | 0.001 m | 0.001 m |
| m Metre | Reference unit | SI base unit |
| in Inch | 0.0254 m | 25.4 mm (1959 agreement, exact) |
| ft Foot | 0.3048 m | 12 in = 0.3048 m (exact) |
| Area | ||
| m² Square metre | Reference unit | SI derived unit |
| ft² Square foot | 0.09290304 m² | (0.3048 m)², exact |
| Volume | ||
| m³ Cubic metre | Reference unit | SI derived unit |
| L Litre | 0.001 m³ | 1 dm³ (since the 1964 CGPM) |
| ft³ Cubic foot | 0.028316846592 m³ | (0.3048 m)³, exact; same constant as the calculator's airflow conversion |
| US gal US gallon | 0.003785411784 m³ | 231 in³ = 3.785411784 L (exact); not the imperial gallon (4.54609 L) |
| Airflow | ||
| m³/h CMH | Reference unit | m³/h, the usual airflow unit on Korean drawings |
| m³/min CMM | 60 m³/h | 1 m³/min = 60 m³/h |
| L/s Litres per second | 3.6 m³/h | 1 L/s = 3.6 m³/h |
| CFM CFM (ft³/min) | 1.69901079552 m³/h | 1 ft³/min = 0.028316846592 m³ × 60 (exact) |
| Liquid flow | ||
| L/min Litres per minute (LPM) | 0.06 m³/h | 1 L/min = 0.06 m³/h |
| m³/h Cubic metres per hour | Reference unit | m³/h |
| L/s Litres per second | 3.6 m³/h | 1 L/s = 3.6 m³/h |
| US GPM GPM (US gallons per minute) | 0.22712470704 m³/h | 1 US gal/min = 3.785411784 L/min (exact) |
| Pressure | ||
| Pa Pascal | Reference unit | SI derived unit, 1 N/m² |
| kPa Kilopascal | 1,000 Pa | 1000 Pa |
| MPa Megapascal | 1,000,000 Pa | 10⁶ Pa |
| bar Bar | 100,000 Pa | 10⁵ Pa (exact) |
| mmAq mm of water (mmH₂O) | 9.80665 Pa | Conventional: water at 1000 kg/m³ × standard gravity 9.80665 m/s² × 1 mm. Real water at 4 °C (999.97 kg/m³) gives 9.80638 Pa, 0.003 % less |
| inWG Inch of water (inH₂O) | 249.08891 Pa | Conventional: 25.4 × 9.80665 Pa (water at 1000 kg/m³, about 4 °C). Some references use 60 °F water, 248.84 Pa (0.1 % lower) |
| mmHg mm of mercury | 133.322387415 Pa | Conventional: mercury at 0 °C, 13595.1 kg/m³ × 9.80665 m/s² × 1 mm; practically equal to 1 Torr (101325/760 Pa) |
| inHg Inch of mercury | 3,386.38864034 Pa | Conventional: 25.4 mmHg (mercury at 0 °C). Some references use mercury at 60 °F, 3376.85 Pa (0.3 % lower) |
| psi psi (lbf/in²) | 6,894.75729317 Pa | 1 lbf/in² = 0.45359237 kg × 9.80665 m/s² ÷ (0.0254 m)² (exact) |
| kgf/cm² Kilogram-force per cm² | 98,066.5 Pa | 9.80665 N/cm² (exact), the technical atmosphere |
| atm Standard atmosphere | 101,325 Pa | 101325 Pa (exact, standard atmosphere) |
| Heat rate · power | ||
| kW Kilowatt | 1,000 W | 1000 W |
| W Watt | Reference unit | SI derived unit, 1 J/s |
| RT Refrigeration ton (US) | 3,516.85284207 W | 12000 Btu/h: melting 2000 lb of 0 °C ice in 24 h (144 Btu/lb), about 3024 kcal/h. "RT" on this site means this ton |
| JRT Japanese refrigeration ton | 3,861.16 W | 3320 kcal/h: freezing 1 t of 0 °C water in 24 h (79.68 kcal/kg), conventional. Older Korean and Japanese texts may mean this by "RT" |
| kcal/h Kilocalories per hour | 1.163 W | 1 kcal(IT)/h = 4186.8 J ÷ 3600 s = 1.163 W (exact) |
| Btu/h Btu per hour | 0.293071070172 W | 1 Btu(IT)/h = 1055.05585262 J ÷ 3600 s |
| MBH MBH (1000 Btu/h) | 293.071070172 W | 1000 Btu/h (M is the Roman thousand, not a million) |
| HP Horsepower (mechanical) | 745.699871582 W | Mechanical horsepower, 550 ft·lbf/s; not metric horsepower (PS, 735.49875 W) |
| Enthalpy | ||
| kJ/kg Kilojoules per kg | Reference unit | SI unit, the calculator's enthalpy unit |
| kcal/kg Kilocalories per kg | 4.1868 kJ/kg | 4.1868 kJ/kg (IT calorie, exact) |
| Btu/lb Btu per pound | 2.326 kJ/kg | 2.326 kJ/kg (IT Btu per lb, exact) |
| Energy | ||
| kWh Kilowatt-hour | 3,600 kJ | 3.6 MJ |
| MJ Megajoule | 1,000 kJ | 1000 kJ |
| kJ Kilojoule | Reference unit | SI derived unit |
| kcal Kilocalorie | 4.1868 kJ | IT calorie, 4.1868 kJ (exact); the thermochemical calorie (4.184 kJ) is 0.07 % smaller |
| Btu British thermal unit | 1.05505585262 kJ | IT Btu, 1055.05585262 J (exact) |
| Humidity ratio | ||
| g/kg g/kg (dry air) | Reference unit | Grams of water vapour per kg of dry air, the calculator's unit |
| kg/kg kg/kg (dry air) | 1,000 g/kg | 1000 g/kg |
| gr/lb Grains per pound | 0.142857142857 g/kg | 1 gr = 64.79891 mg and 1 lb = 7000 gr, so 1 gr/lb = 1/7 g/kg |
| Velocity | ||
| m/s Metres per second | Reference unit | SI derived unit |
| ft/min Feet per minute (FPM) | 0.00508 m/s | 0.3048 m ÷ 60 s = 0.00508 m/s (exact) |
| km/h Kilometres per hour | 0.277777777778 m/s | 1000 m ÷ 3600 s |
Sources
- NIST Special Publication 811 (2008), Appendix B, conversion factors
- BIPM, The International System of Units (SI), 9th edition (2019)
- International yard and pound agreement, 1959 (1 in = 25.4 mm, 1 lb = 0.45359237 kg)
- Fifth International Conference on the Properties of Steam, 1956 (IT calorie 4.1868 J, IT Btu 1055.05585262 J)
- ASHRAE Handbook, Fundamentals (SI), chapter "Units and Conversions"
Last updated 2026-10-07.