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.
Combustion Efficiency Calculator
Calculate combustion efficiency from flue gas temperature, ambient temperature, and CO2 percentage using the Siegert method.
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.
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.
Lower Heating Value (LHV) Calculator
Calculate lower heating value (LHV) from a fuel's HHV and hydrogen content, accounting for water vapor latent heat.
Thermal Efficiency Calculator
Calculate the theoretical maximum thermal efficiency (Carnot efficiency) from hot and cold reservoir temperatures.
The Numbers That Define How Efficiently Fuel Becomes Useful Heat
Combustion efficiency, boiler efficiency, and heating value all answer variations of the same question: how much of a fuel's chemical energy actually ends up doing useful work, versus disappearing up the stack or through equipment walls. Fourteen calculators here cover the thermal efficiency and energy content math used across boilers, furnaces, and combustion equipment.
Popular Heat & Energy Calculators
Highlights across efficiency and heating value:
- Combustion Efficiency Calculator — calculates how much of a fuel's energy is converted to useful heat versus lost in flue gas.
- Boiler Efficiency Calculator — estimates overall boiler efficiency from fuel input and steam or hot water output.
- Higher Heating Value (HHV) Calculator — calculates gross heating value including the energy in water vapor condensation.
- Lower Heating Value (LHV) Calculator — calculates net heating value excluding water vapor condensation energy.
- Heat Loss Calculator — estimates heat lost through equipment surfaces, flue gas, and incomplete combustion.
Why HHV and LHV Give Two Different "Right" Answers
Higher heating value assumes any water vapor produced by combustion condenses back to liquid, releasing its latent heat, while lower heating value assumes that water vapor stays as vapor and exits with the flue gas — real combustion equipment almost always operates in LHV territory since flue gas is normally too hot for that water vapor to condense. This is exactly why boiler efficiency figures can look dramatically different (sometimes over 100% on an LHV basis for high-efficiency condensing boilers) depending on which heating value basis a manufacturer quotes, which is worth checking carefully when comparing equipment.
Frequently Asked Questions
Why can a condensing boiler have "over 100%" efficiency?
Only when efficiency is calculated against LHV — a condensing boiler recovers some of the latent heat LHV assumes is lost, so its output can exceed 100% of the LHV input even though it's always below 100% of the HHV input.
Explore More
Need fuel quantities to match this heat output? See the Fuel Consumption Calculators, or check flue gas losses directly in the Flue Gas Calculators.