Energy conversion shows up when a nutrition label lists kilocalories, a lab notebook uses joules, or a utility bill is written in kilowatt-hours. Those units all describe the same physical idea, how much work or heat is stored or transferred, but they grew from different fields. This guide explains everyday energy conversion with plain formulas, tables, and worked examples, then links the free Energy Converter.
Convert energy now: Open the free Energy Converter, joules, calories, kilocalories, BTU, kilowatt-hours, and more. No sign-up required.
What energy conversion is (and when you need it)
Energy is the capacity to do work or transfer heat. Converting energy rewrites that quantity in another unit, joules to kilocalories, BTU to kilowatt-hours, without changing the physical amount. Food science, HVAC, and physics homework all use different “house units,” so conversion is how you compare a snack label to a SI calculation or a furnace rating to a utility meter.
Power (watts) is energy per time; this page stays on energy totals. For rates of energy use, see the power conversion guide after you finish here.
Plain-language energy formulas
- Calories (thermochemical) ↔ joules: 1 cal ≈ 4.184 J. Multiply cal by 4.184 for joules.
- Food Calories (kcal) ↔ joules: 1 kcal ≈ 4,184 J (same as 1 kilocalorie). Nutrition “Calories” with a capital C are usually kilocalories.
- Kilojoules ↔ kilocalories: divide kJ by ≈ 4.184 for kcal.
- BTU ↔ joules: 1 BTU ≈ 1,055.06 J.
- Kilowatt-hours ↔ joules: 1 kWh = 3,600,000 J exactly (3.6 MJ).
When a label says “Calories,” assume kilocalories unless the context is clearly chemistry-scale calories.
Energy conversion tables
Kilocalories to kilojoules (nutrition)
| kcal | kJ (approx.) |
|---|---|
| 50 | 209 |
| 100 | 418 |
| 250 | 1,046 |
| 500 | 2,092 |
| 2,000 | 8,368 |
Kilowatt-hours to megajoules (utility)
| kWh | MJ |
|---|---|
| 0.5 | 1.8 |
| 1 | 3.6 |
| 5 | 18 |
| 10 | 36 |
| 100 | 360 |
Worked examples
Example 1, snack label to SI
A bar lists 220 kcal. In kilojoules: 220 × 4.184 ≈ 920 kJ. In joules that is about 9.20×10⁵ J, useful when a homework set insists on SI.
Example 2, small appliance energy use
A device draws energy totaling 0.35 kWh over a day. In megajoules: 0.35 × 3.6 = 1.26 MJ. Compare that to a battery pack rated in Wh by converting Wh ÷ 1,000 = kWh first.
Example 3, HVAC BTU to joules
A heater output of 12,000 BTU (as an energy quantity for a given period) is 12,000 × 1,055.06 ≈ 1.266×10⁷ J ≈ 12.66 MJ. Keep the time window explicit when you later turn this into power.
Common energy conversion mistakes
- Mixing food Calories with small calories. A capital-C Calorie on a label is usually 1 kcal = 1,000 cal.
- Confusing energy with power. Joules and kWh are energy; watts are power (energy per second).
- Forgetting kWh = 3.6 MJ. Utility math is exact; do not approximate as “about 3” for billing checks.
- Using the wrong BTU definition in precision work. International Table and thermochemical BTU differ slightly, match your standard.
Energy converter tools
Use the multi-unit Energy Converter for any pair on this page:
Related reading
- Unit conversion chart & calculator guide, full category map, including power when you need rates