Joules ⇄ BTU Converter

The joules to BTU converter is a fast, exact energy-conversion tool that moves any value between joules (J), the SI unit of energy used in physics, mechanical and electrical engineering, thermodynamics, and combustion science, and British Thermal Units (BTU), the working unit of HVAC sizing, fuel-energy content, refrigeration, boiler and furnace rating, and North American combustion engineering, in both directions. The conversion rests on the exact International Table BTU definition adopted at the 5th International Conference on the Properties of Steam (London, 1956) and codified by NIST Special Publication 811: 1 BTU(IT) = 1 055.05585262 J exactly, with no rounding ambiguity and no measurement uncertainty. The same source defines the inverse, 1 J = 0.00094781712… BTU. Type a number into either field and the other updates live. The page shows bonus read-outs in kilojoules (kJ), kilowatt-hours (kWh), thermochemical calories (cal), dietary kilocalories (kcal), and foot-pounds-force (ft·lbf), so the same energy figure can be read in any unit your audience, an engineer, a homeowner, a chemistry student, a fuel supplier, or a clinical nutritionist, is comfortable with. The converter is built for everyday situations where an energy figure arrives in one unit and the answer needs to leave in another: an HVAC contractor's load calculation that quotes 60 000 BTU/h and a thermostat spec that reads in watts; a combustion-chemistry calculation that yields megajoules per cubic metre of natural gas and a fuel-supplier invoice that quotes BTU per cubic foot; a refrigeration-tonnage calculation that begins in kJ of latent heat and is checked against BTU/h of cooling capacity; a bomb-calorimeter result in joules per gram of fuel that needs to land in BTU per pound for a burner-spec sheet; a thermodynamics problem set that quotes enthalpy of combustion in kJ/mol and a homework grader who expects kcal/mol or BTU/lb-mol; or a personal-finance worksheet that converts an electricity bill in kWh into the "cooking-equivalent" energy of a natural-gas range in BTU. All math is exact where the input units are defined by international convention (kJ, kWh, kcal, BTU), and at NIST-traceable precision where the unit rests on a measured constant (ft·lbf).

How to use this converter

1. Type the numeric value you want to convert into the Joules (J) field or the BTU (IT) field. Decimal values are accepted, including sub-joule values such as 0.001 J (≈ 0.000000948 BTU) and sub-BTU values such as 0.001 BTU (≈ 1.055 J).
2. For the Joules to BTU Converter, J ⇄ BTU (Exact, +kJ/kWh/kcal), The opposite field updates automatically as you type. The update is recomputed on every keystroke, so the panel always reflects the latest value, including pasted numbers, scientific notation, and large figures.
3. Read the Conversion (from N J) result block to see the same energy quoted in BTU and kilojoules, with a one-line summary that also lists kilowatt-hours, calories, kilocalories, and foot-pounds-force.
4. Read the Joules (J), BTU (IT), Kilojoules (kJ), Kilowatt-hours (kWh), Calories (th), Kilocalories (kcal), Foot-pounds (ft·lbf), and Round-trip J from BTU tiles for eight independent read-outs that follow the current input state.
5. Use the presets row to load a domain-specific case with one click, for example "60 000 BTU/h residential furnace", "100 000 BTU (≈ 1 therm-hour)", or "1 kWh (3 600 000 J)".
6. The Round-trip J from BTU tile proves the converter is exact: whatever you type into BTU converts back to joules to within floating-point epsilon, because 1 BTU × 1 055.05585262 J/BTU = 1 055.05585262 J by definition.
7. Use this page for HVAC sizing (BTU/h of heating or cooling load per square foot of conditioned space), combustion energy content of fuels (BTU per cubic foot of natural gas, BTU per gallon of fuel oil, BTU per pound of coal or propane), refrigeration tonnage (1 ton of refrigeration = 12 000 BTU/h ≈ 3.516 kW), and chiller / heat-pump energy budgets.
8. the Joules to BTU Converter, J ⇄ BTU (Exact, +kJ/kWh/kcal) runs entirely in the browser. For the Joules to BTU Converter, J ⇄ BTU (Exact, +kJ/kWh/kcal), Inputs are not stored, not transmitted, and not associated with any account.

The formula

The conversion between joules and BTU is defined rather than measured. The International Table BTU is fixed by the 5th International Conference on the Properties of Steam (London, 1956), adopted by NIST SP 811 (Appendix B.9), the ASHRAE Handbook of Fundamentals (Chapter 1), and ISO 80000-5:2007. The defining relation is:

1 BTU(IT) = 1 055.05585262 J exactly
1 J      = 0.00094781712… BTU exactly

To go from joules to BTU, divide by 1 055.05585262:

BTU(IT) = J ÷ 1 055.05585262

To go from BTU to joules, multiply by 1 055.05585262:

J = BTU × 1 055.05585262

The auxiliary units on this page use the same NIST / CIPM constants:

1 kJ      = 1 000 J exactly                 (SI prefix)
1 kWh     = 3 600 000 J exactly             (1 kW × 1 h, NIST SP 811)
1 cal(th) = 4.184 J exactly                 (CIPM thermochemical calorie)
1 kcal    = 4 184 J exactly                 (1 000 × cal(th); dietary Calorie)
1 ft·lbf  = 1.3558179483314004 J            (NIST SP 811, foot-pound-force)

The kcal row is the dietary Calorie, the same unit printed on US nutrition labels and used in clinical-nutrition and exercise-physiology work, so the same number that tells you the energy content of a cubic metre of natural gas can be cross-checked against the energy content of an athlete's daily diet without any unit confusion.

Worked examples

Example 1, A 60 000 BTU/h residential furnace. A 2 000-square-foot home in climate zone 5 typically requires 30 BTU/h per square foot of heating capacity, giving 60 000 BTU/h of rated furnace output. Converting to SI:

P = 60 000 BTU/h × 1 055.05585262 J/BTU ÷ 3 600 s/h
  = 60 000 × 1 055.05585262 ÷ 3 600
  = 17 584.26 W = 17.584 kW

The same furnace output, expressed in kWh per hour, is 17.584 kWh/h, i.e. running the furnace for one hour consumes about 17.6 kWh of natural-gas energy, equivalent to leaving ten 100-W light bulbs on for 17.6 hours. ASHRAE sizing guidelines recommend sizing the furnace to the design heating load with no oversize factor; the BTU/h → kW conversion is the bridge between the field-standard sizing figure and the SI units that heat-pump and electric-resistance equipment carry on their spec sheets.

Example 2, Combustion energy content of natural gas. Pipeline-quality natural gas in the US has a gross (higher) heating value of about 1 050 BTU per standard cubic foot, varying from 950 to 1 100 BTU/SCF depending on composition (methane dominates, with ethane, propane, and nitrogen diluents):

E = 1 050 BTU/SCF × 1 055.05585262 J/BTU
  = 1 107 808.6 J/SCF
  = 1 107.81 kJ/SCF
  = 1.10781 MJ/SCF

At $1.20 per therm (100 000 BTU), the cost of the chemical energy in 1 cubic metre (about 35.31 SCF) of natural gas is $1.20 × 35.31/100 = $0.42 per cubic metre. The exact 1 055.05585262 J/BTU factor is what connects the BTU-keyed gas bill to the joule-keyed combustion calculation in any boiler or burner specification.

Example 3, A 5 000 mAh phone battery at 3.7 V. Consumer-electronics spec sheets quote capacity in mAh × nominal voltage; the underlying quantity is energy in joules:

E = 5 000 mAh × 3.7 V = 18 500 mWh = 18.5 Wh = 66 600 J
E = 66 600 J ÷ 1 055.05585262 = 63.124 BTU

A fully-charged phone battery holds about 63 BTU of chemical energy, equivalent to the heat released by burning roughly 0.18 millilitres of gasoline. This is the kind of figure that surfaces when someone asks "could a phone battery heat a cup of coffee?", the answer is "about 0.002 °C per cup", which is to say, not noticeably.

Example 4, Bomb-calorimeter combustion of gasoline. Isooctane (2,2,4-trimethylpentane, the primary reference fuel for octane rating) has a standard enthalpy of combustion of about 47.81 MJ/kg = 47 810 J/g:

E = 47 810 J/g ÷ 1 055.05585262 J/BTU = 45.30 BTU/g
E = 45.30 BTU/g × 453.592 g/lb = 20 545 BTU/lb

Real gasoline has a gross heating value close to this, about 20 400 to 20 700 BTU/lb, which is the headline number in any engineering reference for spark-ignition engines, fuel-economy calculations, and combustion-chemistry problems. The 1 055.05585262 J/BTU factor bridges the bomb-calorimeter reading (joules per gram) and the burner spec sheet (BTU per pound).

Example 5, One therm of natural gas. A therm is exactly 100 000 BTU by US convention, used on residential natural-gas bills:

E = 100 000 BTU × 1 055.05585262 J/BTU = 105 505 585.262 J
E = 105 505 585.262 J ÷ 1 000 = 105 505.585 kJ
E = 105 505 585.262 J ÷ 3 600 000 = 29.307107 kWh

One therm of natural gas therefore contains about 29.3 kWh of chemical energy. At 95% furnace efficiency, about 27.8 kWh of that energy reaches the home as useful heat, enough to raise the temperature of a 2 000-square-foot, 8-foot-ceiling home (about 14 960 cubic feet = 423.6 m³ of air) by about 32 °C, assuming no envelope losses. The 1 055.05585262 J/BTU factor is what bridges the bill (BTU or therms) to the engineering calculation (kWh or kJ).

Example 6, Round-trip identity. Any energy expressed in joules and converted to BTU and back, with no intermediate rounding, returns the original number to within floating-point epsilon:

E = 1 234 567 J → BTU = 1 170.2581… BTU → J = 1 234 567.00 J

The defining property of an exact conversion: there is no measurement uncertainty in either direction, only the precision of the floating-point representation on screen.

Where J ⇄ BTU shows up

HVAC sizing. Residential cooling loads in the US are sized in BTU/h ("tons of refrigeration", 1 ton = 12 000 BTU/h ≈ 3.516 kW). Furnace output is rated in BTU/h (typical residential 40 000 to 120 000 BTU/h). Heat-pump capacity at AHRI rating conditions (47 °F / 95 °F outdoor) is quoted in BTU/h even though the electrical input is in watts. The joule equivalent of any of these is what enables a side-by-side efficiency calculation with electric-resistance or solar-thermal equipment that uses SI units natively.

Combustion energy content. Natural gas (about 1 050 BTU/SCF, US average), propane (about 2 500 BTU/SCF = 91 500 BTU/gallon), fuel oil No. 2 (about 138 000 BTU/gallon), gasoline (about 114 000 BTU/gallon), diesel (about 138 000 BTU/gallon), and coal (8 000 to 15 000 BTU/lb depending on grade) are all quoted in BTU per unit volume or mass in commerce and contract specifications. Bomb-calorimeter measurements yield joules per gram; the 1 055.05585262 J/BTU factor bridges the two scales.

Refrigeration. One ton of refrigeration = 12 000 BTU/h ≈ 3.51685 kW ≈ 211 011 J/min of cooling capacity. Chiller tonnage ratings, ice-storage thermal-battery capacity, and cold-room load calculations all flow through BTU/h or BTU/day, often cross-checked against SI energy figures (kJ of latent heat of fusion, J of sensible heat).

Boilers and steam plants. Boiler fuel-input ratings use BTU/h for the heat released by combustion (input), while the steam output is rated in pounds per hour or kg/s. The 1 BTU = 1 055.05585262 J factor bridges the two, and the boiler efficiency = (steam enthalpy out) ÷ (fuel energy in) is a direct ratio of joules to joules.

Industrial process heat. Kilns, ovens, dryers, and curing furnaces are rated in BTU/h for natural-gas firing and kW for electric heating. Process engineers routinely need to compare the two, and the 1 BTU = 1 055.05585262 J = 0.00029307107 kWh conversion is the bridge.

Electricity generation. 1 kWh of electricity delivered to the grid requires about 10 500 BTU (≈ 3 000 BTU/kWh for combined-cycle gas turbine, 9 000 to 11 000 BTU/kWh for older coal plants) of thermal energy input at the plant. US energy statistics are still published in BTU alongside TWh and quads, and the joule-BTU conversion is implicit in every conversion between metric and customary energy accounting.

Chemistry and calorimetry. Enthalpy of combustion, heat of formation, latent heat, and heat capacity are tabulated in J/mol, kJ/mol, or J/g in SI sources and in BTU/lb-mol, BTU/lb, or BTU/(lb·°F) in older US engineering references. The 1 055.05585262 J/BTU factor lets a thermodynamic table move between scales without rounding error.

Food and nutrition. The "calorie" on a US food label is actually a kilocalorie (1 kcal = 4 184 J exactly, = 3.96567 BTU). A 2 000 kcal/day diet is 8 368 kJ/day = 7 920 BTU/day of food energy. Comparing a person's daily food intake to the daily heating of a small home (a 60 000 BTU/h furnace running 8 hours uses 480 000 BTU = 121 100 kcal) puts the two on the same energy scale.

Common mistakes when converting joules and BTU

Mistake 1: Confusing the International Table BTU with the "thermochemical" or older BTU variants. The International Table BTU (1 BTU = 1 055.05585262 J exactly, this converter) is the modern standard in NIST SP 811, ASHRAE, and ISO 80000-5. The older "thermochemical BTU" was 1 054.35026449 J and the "mean BTU" was about 1 055.87 J; both predate the 1956 International Table definition and survive only in legacy literature. A 1950s US engineering reference using the older factor will be off by about 0.07% from the modern figure, which accumulates to 700 BTU on a 1 000 000-BTU boiler rating.

Mistake 2: Confusing energy (BTU) with power (BTU/h). A 60 000-BTU furnace is shorthand for 60 000 BTU/h, i.e. energy per hour, a power rating. A 60 000-BTU battery or thermal-storage tank is energy. The "BTU per hour" is implicit in most residential furnace contexts but not in industrial steam-plant or thermal-storage contexts. The joule/second (W) and joule (J) are unambiguous; the BTU/BTU-h confusion is unique to the customary system.

Mistake 3: Treating "1 BTU ≈ 1 055 J" or "1 BTU ≈ 1 kJ" as exact. The International Table BTU is defined as 1 055.05585262 J, which is about 5.06% larger than 1 kJ. A 100 000-BTU gas bill at "1 kJ per BTU" reads 100 000 kJ = 100 MJ, but the correct figure is 105.5 MJ. The 5% error compounds at the megajoule scale and is significant in NIST-traceable work and in contract specifications.

Mistake 4: Using "therms" without converting to BTU first. A therm is exactly 100 000 BTU, a dekatherm (10 therms) is exactly 1 000 000 BTU, and a quad is 10¹⁵ BTU. Converting therms directly to joules without the 100 000 BTU/therm factor drops three orders of magnitude. Use the converter: enter "100000" in the BTU field for one therm, read 105 505.585 kJ, and verify against the energy-equivalent column on any utility bill.

Mistake 5: Mixing calories and BTU without going through joules. A food energy figure in kcal (say, 2 000 kcal/day = 8 368 kJ/day = 7 920 BTU/day) and a household heating figure in BTU/day (say, 60 000 BTU/h × 8 h = 480 000 BTU/day) are not directly comparable unless both go through joules first. The 4 184 J/kcal and 1 055.05585262 J/BTU factors are the bridge, and the converter handles them all simultaneously.

Mistake 6: Treating "1 BTU raises 1 lb of water by 1 °F" as the primary definition. It is approximately right, but the original 19th-century BTU was the heat required by 1 lb of water at its maximum-density temperature (~39 °F). The IT BTU is fixed by the 1956 steam-conference definition (1 BTU_IT = 1 055.05585262 J) and the older "water-heating" definition survives only as a memory. The numerical coincidence is real but the definition is now exact and steam-table based, not calorimetric.

Mistake 7: Round-trip rounding error. Converting 1 000 000 J to BTU gives 947.81712… BTU, and converting back to joules gives 1 000 000 J exactly because 1 055.05585262 is exact. But rounding the intermediate BTU figure to "948 BTU" before converting back gives 1 000 140 J, a 140 J error from a three-significant-figure rounding choice. Use the full stored value at every step.

Frequently Asked Questions

What is the exact relationship between joules and BTU? The International Table BTU is defined by the 5th International Conference on the Properties of Steam (London, 1956) as exactly 1 055.05585262 joules. NIST Special Publication 811 ("Guide for the Use of the SI Units"), the BIPM SI Brochure (9th edition, §2.3.3), ISO 80000-5:2007 (Thermodynamics), and the ASHRAE Handbook of Fundamentals all cite the same constant. Therefore 1 BTU(IT) = 1 055.05585262 J is a definition rather than a measurement, and the reciprocal 1 J = 0.00094781712… BTU is exact.

How many BTU does it take to heat a 2 000-square-foot home? An ASHRAE-style cooling load for a 2 000-square-foot home in US climate zone 5 is about 30 BTU/h per square foot of conditioned floor area, giving 60 000 BTU/h of cooling capacity. Heating loads depend on the climate zone and the envelope (insulation, air-sealing, window U-factor); climate zone 5 typically needs 50 to 60 BTU/h per square foot of heating capacity. A 60 000 BTU/h furnace is 17.58 kW and runs about 8 hours at full output on a design day, consuming about 480 000 BTU = 506 426 kJ = 140.7 kWh of natural-gas energy. The exact 1 055.05585262 J/BTU factor is what bridges the field-standard BTU/h to the SI kW and kWh figures on heat-pump and solar-thermal spec sheets.

How many BTU are in a gallon of propane, gasoline, fuel oil, and natural gas? Propane has about 91 500 BTU per US gallon (≈ 25.4 MJ/litre, gross heating value). Gasoline has about 114 000 to 122 000 BTU per gallon depending on seasonal blend and ethanol content. Diesel and fuel oil No. 2 have about 138 000 BTU per gallon. Natural gas is 950 to 1 100 BTU per standard cubic foot (US average ≈ 1 050 BTU/SCF), or about 1 030 000 BTU per standard cubic metre. All of these figures convert to SI units through 1 BTU = 1 055.05585262 J, so 1 gallon of propane = 96.6 MJ, 1 gallon of gasoline = 121 to 129 MJ, 1 gallon of diesel = 145.6 MJ, and 1 m³ of natural gas = 1.085 GJ.

How many BTU is 1 kWh? 1 kWh = 3 600 000 J exactly (NIST SP 811, BIPM SI Brochure 9th ed.; since 1 kW × 1 h = 1 000 W × 3 600 s = 3 600 000 J). Dividing by the exact 1 055.05585262 J/BTU gives 3 412.14163… BTU per kWh. To convert kWh to BTU, multiply by 3 412.14163. To convert BTU to kWh, divide by 3 412.14163. A typical US household uses about 30 kWh per day of electricity (≈ 102 364 BTU/day of electrical energy) and about 1 therm of natural gas per day for heating in winter (100 000 BTU/day of thermal energy).

How do I convert kcal (dietary Calories) to BTU? 1 kcal = 4 184 J exactly (since 1 kcal = 1 000 × 4.184 J), and 1 BTU = 1 055.05585262 J exactly. So 1 kcal = 4 184 ÷ 1 055.05585262 = 3.96567… BTU. To convert kcal to BTU, multiply by 3.96567. A 2 000 kcal/day diet is 7 920 BTU/day of food energy, the same order of magnitude as the heat released by burning 0.013 gallon of gasoline. A Tour de France rider sustaining 8 000 kcal/day during a three-week grand tour is ingesting about 31 700 BTU/day, the daily heating equivalent of a small apartment.

How many BTU does a typical air conditioner produce? "BTU" on a residential air conditioner refers to cooling capacity per hour (BTU/h), not total energy. Window units are typically 5 000 to 12 000 BTU/h, medium room units 12 000 to 24 000 BTU/h, and central systems 24 000 to 60 000 BTU/h. A 2-ton central system is 24 000 BTU/h = 7.03 kW of cooling capacity, drawing about 2.5 kW of electrical input at a seasonal energy-efficiency ratio (SEER) of 14. The "ton" rating (12 000 BTU/h) originates from the heat needed to melt one ton (2 000 lb) of ice in 24 hours. Converting 24 000 BTU/h to watts gives 24 000 × 0.29307107 = 7 033.7 W, consistent with the standard ton-of-refrigeration figure.

What is the energy content of common fuels in BTU per pound? Bituminous coal: about 12 000 to 15 000 BTU/lb (24 to 31 MJ/kg). Anthracite coal: about 14 000 to 16 000 BTU/lb. Sub-bituminous coal (Powder River Basin): about 8 500 to 9 500 BTU/lb. Lignite: about 4 000 to 8 300 BTU/lb. Gasoline (isooctane reference): about 20 500 BTU/lb (45.3 MJ/kg). Diesel (cetane reference): about 20 800 BTU/lb. Natural gas (methane): about 23 800 BTU/lb (55.5 MJ/kg). Hydrogen: about 51 600 BTU/lb (120 MJ/kg, gross heating value). All figures convert to SI units through 1 BTU = 1 055.05585262 J and 1 lb = 453.59237 g.

Is 1 BTU the same as 1 dietary Calorie? No, this is the most common unit confusion at the food/fuel interface. 1 BTU = 1 055.05585262 J exactly. 1 dietary Calorie (1 kcal, capital C) = 4 184 J exactly. So 1 dietary Calorie = 4 184 ÷ 1 055.05585262 = 3.96567… BTU. A "100 Cal" snack-bar label is 100 kcal = 418 400 J = 396.567 BTU, not 100 BTU. A typical US adult needs about 2 000 kcal/day = 8 368 kJ/day = 7 920 BTU/day of food energy, while a typical US home uses about 480 000 BTU/day of heating energy in winter, about 60× more, which is why food-energy costs are tiny relative to home-heating costs.

How do I convert BTU per hour (BTU/h) to watts? 1 BTU/h = 0.29307107 W exactly (NIST SP 811, ASHRAE Handbook of Fundamentals; since 1 BTU = 1 055.05585262 J and 1 h = 3 600 s, 1 BTU/h = 1 055.05585262 ÷ 3 600 = 0.29307107 J/s = 0.29307107 W). To convert BTU/h to watts, multiply by 0.29307107. To convert watts to BTU/h, divide by 0.29307107. A 60 000 BTU/h furnace is 60 000 × 0.29307107 = 17 584.26 W ≈ 17.58 kW. The "BTU per hour" is a power rating (energy per unit time), distinct from the "BTU" energy unit on this converter; the two are linked by the 3 600-second hour.

Also try these free tools:

• Joule to Calorie Converter, exact 4.184 J/cal with kcal, kWh, BTU bonus
• BTU Calculator, HVAC load sizing in BTU/h
• BTU to Watts Converter, power-rate conversion
• Energy Converter, multi-unit energy panel
• Watts to BTU per Hour, reverse HVAC power conversion

References

NIST Special Publication 811 ("Guide for the Use of the SI Units"), 2008 edition, Appendix B.9 (recommended conversion factors for energy units); BIPM SI Brochure (9th edition, 2019, §2.3.3); 5th International Conference on the Properties of Steam (London, 1956, adopted the International Table BTU definition now codified as 1 BTU_IT = 1 055.05585262 J exactly); ASHRAE Handbook of Fundamentals (Chapter 1, "Thermodynamic and Physical Properties"); ISO 80000-5:2007 ("Quantities and units, Part 5: Thermodynamics"); CIPM (Comité International des Poids et Mesures) thermochemical-calorie definition (1954); UK Weights and Measures Act 1985 (Schedule 1); CODATA 2018 recommended values of the fundamental physical constants.