Energy Unit Converter (J / kJ / cal / kcal / kWh / BTU / Wh)
Enter any energy value to see it instantly in joules (J), kilojoules (kJ), megajoules (MJ), watt-hours (Wh), kilowatt-hours (kWh), calories (cal), kilocalories (kcal), British thermal units (BTU) and foot-pounds (ft·lb) — every unit at once, so nutrition labels, electricity bills, AC capacity and mechanical work cross-reference in a single view.
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Live value in every unit
Tip: nutrition-label "Calories" (capital C) are kilocalories. 1 kcal = 4.184 kJ exactly. One unit of electricity (1 kWh) ≈ 860 kcal ≈ 3 412 BTU.
- kcal Nutrition labels (food energy)
- kWh Electricity bills / appliances
- BTU Air-conditioning / heating
- MJ Natural gas / fuel
- J Physics & mechanical work
- ft·lb US mechanical engineering
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Formula
1 kJ = 1 000 J 1 MJ = 1 000 000 J 1 Wh = 3 600 J 1 kWh = 3 600 000 J 1 cal (thermochemical) = 4.184 J (exact) 1 kcal = 4 184 J = 4.184 kJ 1 BTU (IT) = 1 055.05585262 J 1 ft·lb = 1.3558179483314004 J
- · Uses the thermochemical calorie (cal_th = 4.184 J exactly) — the convention used on nutrition labels and in food science. The International Table calorie (cal_IT = 4.1868 J) is reserved for steam-engineering work and differs by < 0.07 %.
- · "Calorie" with a capital C = 1 kilocalorie = 1 kcal. A "200-calorie" snack bar actually contains 200 kcal = 836.8 kJ.
- · One unit of electricity = 1 kWh = 3 600 000 J = 3 600 kJ ≈ 860 kcal ≈ 3 412 BTU. That is the "kWh" line on your electricity bill.
- · An air conditioner labelled "9 000 BTU" is using BTU/hour as shorthand — it dissipates 9 000 BTU of heat per hour, which is a power of ≈ 2.64 kW. This tool converts energy, not power, so divide your AC rating by 3 412 to get kW.
- · BTU has multiple historical definitions (IT, thermochemical, mean). This tool uses ISO 31-4 / NIST BTU(IT) = 1 055.05585262 J; the others differ by < 0.1 %.
- · Sources: NIST SP 811 "Guide for the Use of the International System of Units"; ISO 80000-5 "Quantities and units — Part 5: Thermodynamics".
Frequently asked
Nutrition labels show both "kcal" and "kJ" — which is the real number?
Both are the same energy in different units: 1 kcal = 4.184 kJ exactly. EU, Hong Kong and UK labels show both; US labels only show kcal (printed as "Calories"). A 330 mL can of cola ≈ 140 kcal = 586 kJ. Enter 140 in the kcal field and the kJ, Wh and BTU equivalents appear instantly.
My air conditioner is rated "12 000 BTU" — how many watts is that, and how many units of electricity?
A "12 000 BTU" rating is really 12 000 BTU/hour of heat removed. As a power: 12 000 × 1 055 J / 3 600 s ≈ 3 517 W ≈ 3.52 kW of cooling capacity. Electricity drawn is cooling ÷ EER (energy efficiency ratio); at EER ≈ 3.5 the unit consumes ≈ 1 kW, i.e. one kWh per hour of operation.
What is the difference between thermochemical, International Table and mean calories?
All three are historical answers to "the heat needed to raise 1 g of water by 1 °C". Thermochemical calorie (cal_th) = 4.184 J exactly, the nutrition / chemistry standard. International Table calorie (cal_IT) = 4.1868 J, used in steam-engineering tables. Mean calorie = 4.190 J, averaged over 0–100 °C. This tool uses cal_th because it matches nutrition labels and food science. The three differ by < 0.15 %, negligible for everyday use.
What does 1 kJ, 1 kcal and 1 kWh actually feel like?
About 1 kJ heats a cup (250 mL) of water by 1 °C, or lifts a 1 kg weight straight up by 100 m. 1 kcal = 4.184 kJ — roughly the energy in a single almond, or enough to heat that same cup by 4 °C. 1 kWh = 3 600 kJ ≈ 860 kcal — equivalent to running a 100 W bulb for 10 hours, or boiling water with a 1 kW kettle for an hour. Enter 1 in the kWh field to see all three numbers (and every other unit) side by side.
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