BTU to Joule Converter
Convert energy between BTU, joules, kilojoules, calories, kilowatt-hours, therms, foot-pounds, and electronvolts with instant results.
BTU to Joule Converter
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Formula: Joules = BTU x 1055.06 | kWh = BTU / 3412.14 | cal = J / 4.184
Worked example โ 80,000 BTU = 84.4 MJ = 23.45 kWh
Formula
Joules = BTU x 1055.06 | kWh = BTU / 3412.14 | cal = J / 4.184
Energy conversion uses the joule as the base SI unit. One BTU equals approximately 1055.06 joules. All energy units relate through fixed conversion factors: one calorie is 4.184 joules, one watt-hour is 3600 joules, and one foot-pound force is about 1.356 joules. The conversion multiplies by the source factor and divides by the target factor.
Worked Examples
Example 1: Furnace Energy Output
Problem:A furnace is rated at 80,000 BTU/h. How many joules and kilowatt-hours of energy does it produce per hour?
Solution:Joules = 80,000 x 1055.06 = 84,404,800 J = 84.4 MJ kWh = 80,000 / 3412.14 = 23.45 kWh At $0.12/kWh equivalent, this equals $2.81/hour in energy output.
Result:80,000 BTU = 84.4 MJ = 23.45 kWh
Example 2: Natural Gas Bill Conversion
Problem:A gas bill shows 75 therms used. Convert to BTU, megajoules, and kilowatt-hours.
Solution:BTU = 75 x 100,000 = 7,500,000 BTU MJ = 75 x 105.506 = 7,912.9 MJ kWh = 7,500,000 / 3412.14 = 2,198.2 kWh This is typical monthly winter heating usage.
Result:75 therms = 7.5 million BTU = 7,913 MJ = 2,198 kWh
Frequently Asked Questions
What is a BTU and how does it relate to joules?
A British Thermal Unit (BTU) is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit at sea level. One BTU equals approximately 1055.06 joules. The BTU is still widely used in the US for HVAC systems, water heaters, and natural gas billing, while the rest of the world primarily uses joules and kilojoules. The joule is the SI unit of energy, defined as the work done by one newton of force over one meter.
Why are there different types of BTU?
Different BTU definitions exist because the specific heat of water varies slightly with temperature. The thermochemical BTU (1054.35 J) is based on thermochemical calorie definitions. The International Table BTU (1055.06 J) is the most commonly used and is based on the International Table calorie. The mean BTU (1055.87 J) averages over the 0-100 degree F range. For most practical purposes, the differences are less than 0.1% and the IT BTU is standard.
How do I convert BTU to kilowatt-hours?
To convert BTU to kilowatt-hours, divide by 3412.14 (since 1 kWh = 3412.14 BTU). To convert kWh to BTU, multiply by 3412.14. For example, a furnace rated at 100,000 BTU/h outputs about 29.3 kW of thermal energy. This conversion is essential for comparing the cost of heating with electricity versus natural gas, as electricity is billed in kWh and gas in therms (1 therm = 100,000 BTU).
What is a therm and how many BTU does it contain?
A therm is a unit of heat energy equal to exactly 100,000 BTU or approximately 105.5 megajoules. It is the standard billing unit for natural gas in the United States. One therm equals roughly 29.3 kilowatt-hours of energy. A typical US household uses about 50-100 therms of natural gas per month during winter for heating. The therm provides a convenient human-scale unit for measuring residential and commercial gas consumption.
What is the difference between BTU and BTU per hour on appliance labels?
BTU is a quantity of energy; BTU per hour is a rate, meaning power. An air conditioner advertised as 12,000 BTU actually means 12,000 BTU per hour of heat removal. Converting that to SI, 12,000 x 1055.06 joules gives 12.66 megajoules per hour, and dividing by 3,600 seconds yields about 3,517 watts, or 3.52 kW of cooling capacity. Mixing the two up by a factor of 3,600 is the most common error in heating and cooling calculations.
How do I convert joules to calories, and which calorie is meant?
The thermochemical calorie is defined as exactly 4.184 joules. The Calorie printed on food labels is a kilocalorie, so it equals 4,184 joules. A 250 kcal snack therefore holds 1,046,000 joules, which is 1,046 kJ or about 991 BTU. Always check whether a source means the small calorie or the food Calorie, because the two differ by a factor of a thousand and the capitalization convention that distinguishes them is applied inconsistently.
How do joules, watts and seconds fit together?
One watt is one joule per second, so energy equals power multiplied by time. A 1,500 watt heater running for two hours consumes 1,500 x 7,200, which is 10,800,000 joules, or 10.8 MJ. Dividing by 3,600,000 joules per kilowatt-hour gives exactly 3 kWh, and dividing by 1,055.06 gives about 10,236 BTU. This chain is how you move between an appliance nameplate in watts, a utility bill in kilowatt-hours, and a heating specification in BTU.
Why is natural gas billed in therms when the meter measures volume?
The meter records cubic feet or cubic meters of gas, but what you actually buy is energy, and gas composition varies from one supply to another. The utility multiplies your metered volume by a heating value, expressed in BTU per cubic foot or megajoules per cubic meter, to produce a therm or MMBtu figure. A therm is 100,000 BTU and an MMBtu is 1,000,000 BTU, where MM is the old Roman-numeral notation for a thousand thousand rather than mega.
What is the difference between higher and lower heating value?
Higher heating value counts the latent heat released when water vapor in the combustion products condenses back to liquid; lower heating value excludes it. For natural gas the higher value exceeds the lower by roughly ten percent. The distinction matters when comparing efficiency figures, because a condensing boiler that genuinely recovers the flue condensation can be quoted at over one hundred percent efficiency on a lower-heating-value basis while remaining well under one hundred percent on a higher-value basis.
How does an electronvolt compare with a joule?
One electronvolt is the energy an electron gains crossing a potential difference of one volt, and it equals 1.602176634 x 10 to the minus 19 joules, a value now exact because the elementary charge is fixed in the SI. It is the natural unit for atomic and particle physics, where joule-scale numbers would be absurdly small. For perspective, a visible photon with a wavelength of 500 nanometers carries about 2.48 eV, while a single BTU is roughly 6.6 x 10 to the 21st electronvolts.
References
Background & Theory
1 BTU (International Table) = 1,055.05585262 joules exactly, so 1 kJ = 0.947817120 BTU. At least three BTUs are in circulation and they differ by about 0.07 percent, so a source that says only "BTU" is slightly ambiguous.
Conversion reference table
| Conversion | Equivalent | Exact or rounded? |
|---|---|---|
| 1 BTU (International Table) | 1,055.05585262 joules | Exact by definition - the usual one |
| 1 BTU (thermochemical) | 1,054.350 joules | Exact by definition |
| 1 BTU (mean) | 1,055.87 joules | Defined, rounded |
| 1 kilojoule | 0.947817120 BTU (IT) | Derived, rounded |
| 1 therm (US) | 100,000 BTU | Exact by definition |
| 1 kilowatt-hour | 3,600,000 joules = 3,412.14163 BTU (IT) | Exact / derived |
| 1 BTU per hour | 0.293071070 watts | Derived, rounded |
The several BTUs exist because the original definition - the heat needed to raise one pound of water by one degree Fahrenheit - depends on which starting temperature you pick, since the specific heat of water varies slightly across its liquid range. The International Table value is the one used in HVAC catalogues, gas billing and refrigeration ratings, and is the value this page applies. The differences are around seven parts in ten thousand, so they never affect a furnace sizing calculation but will show up in a metrology comparison.
Reviewed for accuracy by Manoj Kumar, Mathematics Educator ยท Editorial policy
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