Energy Converter Free Online
Convert between energy units instantly. Supports joule, kilojoule, megajoule, calorie, kilocalorie, watt-hour, kilowatt-hour, BTU, electronvolt, and foot-pound. Useful for physics, electrical bills, nutrition, and HVAC.
All Conversions
Pro — bulk batch conversion, API access, history & favorites
API access · Priority queue · Team workspace
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How It Works
Enter a Value
Type the number you want to convert into the From input. The conversion runs as you type — no Convert button needed. Decimals and very small or very large numbers are supported via scientific notation.
Pick Source & Target Units
Use the two dropdowns to choose source (From) and destination (To) units. The result updates automatically. Use the swap button to flip the conversion direction in one click.
See All Conversions
Below the main result, the All Conversions panel shows your input value converted into every supported unit at once — perfect for comparing metric vs imperial side by side.
Energy Converter Features
SI Joule
Joule is the SI energy unit (1 J = 1 N·m = 1 W·s). All other units derive from it. Kilojoule and megajoule cover everyday to industrial scales.
Electrical kWh
Watt-hour and kilowatt-hour are the standard for electrical bills. 1 kWh = 3.6 MJ. A typical UK home uses about 8–12 kWh per day.
Heat & Food
Calorie (4.184 J) is the small physics calorie. Kilocalorie (kcal = 4184 J) is the "Calorie" on food labels. BTU (1055 J) is standard for HVAC and gas appliances.
Physics Units
Electronvolt (eV = 1.6022 × 10⁻¹⁹ J) is the standard energy unit in particle physics, atomic physics, and semiconductors.
Live Conversion
Result updates on every keystroke. Quickly check kWh → J for billing math or kcal → kJ for nutrition labels.
100% Private
All math runs in your browser. Safe for confidential utility, lab, or industrial data.
Free vs Pro
| Feature | Free | Pro |
|---|---|---|
| All 10 energy units | ||
| Live conversion | ||
| All-units result table | ||
| Bulk CSV / Excel conversion | — | |
| REST API access | — | |
| Custom precision settings | — |
Frequently Asked Questions
1 watt = 1 joule per second. 1 watt-hour = 1 J/s × 3600 s = 3600 J. So 1 kWh = 1000 W × 3600 s = 3.6 MJ exactly. This is why electrical energy uses kWh instead of joules — it gives more readable numbers on a bill.
The small calorie (cal) is 4.184 J. The food Calorie (capital C) is actually 1000 small calories = 1 kilocalorie = 4184 J. Food labels often write "Calories" but mean kcal.
British Thermal Unit = the energy needed to raise 1 pound of water by 1 °F. 1 BTU = 1055.06 J. Standard for HVAC, furnaces, and gas-appliance ratings (e.g. a 24,000 BTU/hr air conditioner).
1 eV = 1.602 × 10⁻¹⁹ J — the energy gained by an electron crossing 1 volt. Used because joule is too large for atomic-scale energies. Visible light photons are about 2 eV; X-rays are keV; gamma rays are MeV.
1 kWh = 3.6 MJ = 860 kcal = 3412 BTU = 2.65 million ft·lb. Enough to lift a 100 kg weight to 3.7 km, or run a 100 W bulb for 10 hours.