Heat & Energy Calculators
Boiler Efficiency Calculator
Calculate boiler efficiency using the direct method from heat output and fuel input.
BTU Calculator
Calculate BTU/hr heating load from water flow rate and temperature rise, the standard formula for boiler and heat exchanger sizing.
Calorific Value Calculator
Calculate a fuel's calorific value from bomb calorimeter test measurements.
CHP Efficiency Calculator
Calculate the overall efficiency of a combined heat and power (cogeneration) system from electrical output, useful heat output, and fuel input.
Combustion Efficiency Calculator
Calculate combustion efficiency from flue gas temperature, ambient temperature, and CO2 percentage using the Siegert method.
Degree Days Calculator
Calculate heating degree days (HDD) or cooling degree days (CDD) from average outdoor temperature and a base temperature.
Energy Content Calculator
Calculate total energy content of a fuel quantity from its mass and heating value.
Energy Conversion Calculator
Convert between energy units used in combustion engineering: kWh, MJ, BTU, kcal, and therm.
Fourier Conduction Calculator
Calculate steady-state heat conduction through a solid material using Fourier's Law from thermal conductivity, area, temperature difference, and thickness.
Furnace Efficiency Calculator
Calculate furnace efficiency from heat utilized in a process versus total heat supplied.
Heat Input Calculator
Calculate total heat input to a boiler or furnace from fuel flow rate and heating value.
Heat Loss Calculator
Calculate heat lost from combustion equipment based on heat input and efficiency.
Heat Output Calculator
Calculate useful heat output from heat input and equipment efficiency.
Heat Release Rate Calculator
Calculate the rate of heat release inside a combustion chamber from fuel flow rate and heating value.
Higher Heating Value (HHV) Calculator
Calculate a fuel's higher heating value (HHV) from its ultimate analysis using Dulong's formula.
Latent Heat Calculator
Calculate the heat energy absorbed or released during a phase change (melting, freezing, boiling, condensing) from mass and latent heat value.
LMTD Calculator
Calculate the Log Mean Temperature Difference (LMTD) for heat exchanger design from temperature differences at each end.
Lower Heating Value (LHV) Calculator
Calculate lower heating value (LHV) from a fuel's HHV and hydrogen content, accounting for water vapor latent heat.
Specific Heat Capacity Calculator
Calculate the heat energy required to change a substance's temperature using mass, specific heat capacity, and temperature change.
Thermal Efficiency Calculator
Calculate the theoretical maximum thermal efficiency (Carnot efficiency) from hot and cold reservoir temperatures.
Not All Heat Is Created Equal
Fuel energy content, combustion efficiency, and heat transfer all get measured differently depending on what question you are answering — how much energy is in the fuel versus how much of it actually became useful heat are two very different numbers. These fourteen calculators cover that full range of heat and energy math.
Why HHV and LHV Give Different Answers for the Same Fuel
HHV includes the latent heat released when combustion's water vapor byproduct condenses back to liquid; LHV assumes that vapor stays as vapor and leaves with the exhaust. Standard furnaces are rated against LHV since they vent that vapor, while condensing furnaces recover part of that extra HHV energy — which is exactly why condensing units can report efficiency ratings that look unusually high. Compare both with the Higher Heating Value (HHV) Calculator and Lower Heating Value (LHV) Calculator.
Popular Heat & Energy Calculators
- BTU Calculator — converts between BTUs and other common energy units.
- Thermal Efficiency Calculator — calculates what fraction of input energy converts to useful output work or heat.
- Heat Loss Calculator — estimates heat lost through a building envelope or system, given insulation and temperature differential.
Related: HVAC Calculators for building-scale heating and cooling, Boiler & Furnace Calculators.