Cp value is taken from the converter above. Enter a Cp value first, then mass flow and ΔT.
Conversion Factors
1 kJ/kg·K = 0.23885 BTU/lb·°F = 0.23885 kcal/kg·°C
Note: kJ/kg·K = kJ/kg·°C = J/g·K (same degree size for K and °C)
Sensible heat: Q (kW) = ṁ (kg/s) x Cp (kJ/kg·K) x ΔT (K or °C)
Specific Heat Reference -- HVAC Fluids and Materials
| Substance | Cp (kJ/kg·K) | Cp (BTU/lb·°F) | Conditions / Notes |
|---|---|---|---|
| Water (liquid) | 4.187 | 1.000 | At 20°C; reference value for most HVAC hydronic calcs |
| Dry air | 1.006 | 0.240 | Constant pressure (Cp); standard HVAC conditions |
| Moist air (50% RH, 20°C) | 1.012 | 0.242 | Slightly higher than dry air; use psychrometrics for accuracy |
| Ethylene glycol 30% (vol.) | 3.60 | 0.860 | Freeze to -15°C; common Canadian hydronic antifreeze |
| Ethylene glycol 50% (vol.) | 3.30 | 0.788 | Freeze to -36°C; Prairie province hydronic systems |
| Propylene glycol 30% (vol.) | 3.75 | 0.896 | Food-safe antifreeze; CFIA-accepted in food facilities |
| Propylene glycol 50% (vol.) | 3.45 | 0.824 | Freeze to -33°C; food-safe for cold-climate use |
| R-410A (liquid, 20°C) | 1.686 | 0.403 | Refrigerant liquid specific heat for subcooling calcs |
| R-32 (liquid, 20°C) | 1.726 | 0.412 | R-410A replacement refrigerant |
| R-134a (liquid, 20°C) | 1.459 | 0.349 | Chiller and automotive A/C refrigerant |
| Refrigerant oil (mineral) | 1.674 | 0.400 | Compressor lubricant; varies by oil type |
| Steel / carbon steel | 0.490 | 0.117 | Piping, ductwork, equipment casings |
| Copper | 0.385 | 0.092 | ACR tubing, coil fins, heat exchangers |
| Aluminium | 0.897 | 0.214 | Coil fins, condenser coils |
| Concrete (normal weight) | 0.88 | 0.210 | Thermal mass in buildings |
| Gypsum board (drywall) | 1.09 | 0.260 | Interior wall and ceiling finish |
| Wood (softwood) | 1.70 | 0.406 | Framing lumber; varies with moisture content |
How to Use the Specific Heat Converter
Type a Cp value in BTU/lb·°F, kJ/kg·K, kcal/kg·°C, or J/g·K. All other units update instantly. Use the quick presets to load common HVAC fluid values: water, dry air, 30% or 50% ethylene glycol, R-410A liquid, or concrete thermal mass.
With a Cp value entered, fill in mass flow rate (kg/s) and temperature difference (ΔT in K or °C) in the Sensible Heat Calculator. The result shows heat transfer in both kW and BTU/hr -- ready for coil selection, pump sizing, or pipe flow calculations.
The result panel identifies what class of material your Cp value matches: low-Cp metals, medium-Cp refrigerants and building materials, or high-Cp fluids like water and glycol. This helps catch unit entry errors -- a Cp of 41.87 kJ/kg·K is 10x too high for water and flags immediately as likely a decimal error.
The reference table covers 17 common HVAC fluids and materials from refrigerants and glycol mixes to metals and building materials. Use it to find the correct Cp before entering it into heat transfer calculations. For glycol system design, pay attention to concentration: a 50% ethylene glycol mix has Cp = 3.3 kJ/kg·K vs. pure water's 4.187 -- using water's Cp for a glycol system overstates heat transfer capacity by 27%. For complete hydronic system design, use the Hydronic Flow Calculator.
Specific Heat Capacity in HVAC -- Complete Guide
Specific heat capacity (Cp) is the amount of energy required to raise the temperature of one kilogram of a substance by one degree Kelvin (or Celsius). It appears in every sensible heat transfer calculation in HVAC: coil sizing, hydronic system flow calculations, thermal mass analysis, and refrigerant subcooling and superheating work, with reference values published in the ASHRAE Handbook -- Fundamentals.
The Core Conversion: BTU/lb·°F to kJ/kg·K
The conversion factor is 4.1868: multiply BTU/lb·°F by 4.1868 to get kJ/kg·K, a relationship set out in the NIST Guide to the SI, Appendix B. This factor also equals the specific heat of water in SI units, which isn't a coincidence -- the BTU was historically defined using water's specific heat as the reference point. So water's Cp of exactly 1.0 BTU/lb·°F = 4.187 kJ/kg·K. Air's Cp of 0.240 BTU/lb·°F = 1.006 kJ/kg·K. Note that kJ/kg·K and kJ/kg·°C are identical in magnitude since a degree Celsius and a Kelvin are the same size; only the zero point differs, which doesn't affect Cp. J/g·K also equals kJ/kg·K numerically.
Air Specific Heat in Sensible Load Calculations
The standard HVAC sensible heat equation for air in metric is Q (kW) = 1.2 x V (m³/s) x DeltaT (°C), where 1.2 kg/m³ is standard air density and Cp is embedded in the coefficient. Explicitly: Q = rho x V x Cp x DeltaT = 1.2 x V x 1.006 x DeltaT = 1.207 x V x DeltaT, rounded to 1.2. In Imperial, the equivalent is Q (BTU/hr) = 1.08 x CFM x DeltaT (°F). Use the Airflow Converter to convert between CFM and m³/s before applying these formulas, or the Sensible Heat Calculator to work through a full sensible load directly.
Glycol Specific Heat -- Critical for Canadian Hydronic Systems
Antifreeze is essential in outdoor hydronic equipment exposed to Canadian winters -- heat pump ground loops, solar collectors, snowmelt systems, and exposed piping. Glycol-water mixes have lower Cp than pure water, which reduces heat transfer capacity and increases required flow rates. A 30% ethylene glycol mix (freeze protection to -15°C, common in BC and Ontario) has Cp = 3.60 kJ/kg·K -- 14% lower than water. A 50% mix (freeze to -36°C, needed in the Prairies) has Cp = 3.30 kJ/kg·K -- 21% lower. Always recalculate pump flow rates, coil capacities, and pipe sizes when switching from water to glycol, using the Glycol System Calculator to work through the derating. For food-safe applications under CFIA Safe Food for Canadians regulations, use propylene glycol rather than ethylene glycol.
Specific Heat in Refrigerant Work
Refrigerant liquid specific heat is used to calculate subcooling heat removal in condensers and subcooling coils. Subcooling of X degrees Celsius requires removal of m x Cp_liquid x X kJ per kilogram of refrigerant. R-410A liquid Cp is 1.686 kJ/kg·K; R-32 liquid Cp is 1.726 kJ/kg·K. These values appear in detailed refrigeration cycle analysis and condenser design calculations. For refrigerant pressure and P-T data, use the Refrigerant Unit Converter.
Frequently Asked Questions
Multiply BTU/lb·°F by 4.1868 to get kJ/kg·K. So water's Cp of 1.0 BTU/lb·°F equals 4.187 kJ/kg·K, and air's Cp of 0.240 BTU/lb·°F equals 1.006 kJ/kg·K. To go the other way, multiply kJ/kg·K by 0.23885. This conversion is needed whenever heat transfer calculations require switching between Imperial HVAC formulas and SI engineering calculations. Note that kJ/kg·K and kJ/kg·°C are numerically identical since a Kelvin and a degree Celsius are the same size.
Dry air Cp at constant pressure is 1.006 kJ/kg·K (0.240 BTU/lb·°F) at standard HVAC conditions. This value is embedded in the standard sensible heat formulas: Q (BTU/hr) = 1.08 x CFM x DeltaT in Imperial, and Q (kW) = 1.2 x flow (m³/s) x DeltaT in metric. Moist air has a slightly higher effective Cp (about 1.01-1.02 kJ/kg·K at typical indoor humidity), but the difference is small enough that 1.006 is used for most HVAC sensible calculations. Use the Airflow Converter to convert airflow units before applying these formulas.
Ethylene glycol-water mixtures have lower Cp than pure water. A 30% ethylene glycol mix (freeze protection to -15°C) has Cp = 3.60 kJ/kg·K (0.860 BTU/lb·°F). A 50% mix (freeze to -36°C, common in Prairie province systems) has Cp = 3.30 kJ/kg·K (0.788 BTU/lb·°F). Using water's Cp of 4.187 for a 50% glycol system overstates heat transfer capacity by 27% -- a significant error that leads to undersized pumps and insufficient heating or cooling. Always use the glycol concentration-specific Cp from manufacturer data or the table above.
Sensible heat transfer Q = mass flow rate x Cp x DeltaT. In metric: Q (kW) = ṁ (kg/s) x Cp (kJ/kg·K) x DeltaT (K). For a chilled water coil delivering 100 kW at DeltaT = 8°C with water as the fluid: flow = 100 / (4.187 x 8) = 2.98 kg/s (about 2.98 L/s). Switch that to 30% ethylene glycol: flow = 100 / (3.60 x 8) = 3.47 kg/s -- 16% more flow needed for the same heat transfer. Enter any Cp value in the converter above, then use the Sensible Heat Calculator to compute kW and BTU/hr directly.