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The guide
Understanding the Energy Conversion
Convert joules, calories, kilowatt hours, BTU and foot-pounds in either direction.
What each input means
The calculator asks for 3 pieces of information. Every field is validated before the maths runs, so an impossible value is refused rather than quietly producing a wrong answer.
| Input | What it asks for | Example |
|---|---|---|
| Value to convert | A number. | 2000 |
| From | Choose one of 8 options. | Kilocalories (kcal) |
| To | Choose one of 8 options. | Kilojoules (kJ) |
A worked example using the default inputs
With value to convert of 2000, from of Kilocalories (kcal), to of Kilojoules (kJ), the calculator returns 8,368.00 kJ.
| Result line | Value |
|---|---|
| 2,000.00 kcal in kJ | 8,368.00 kJ |
| Conversion rate | 1 kcal = 4.184 kJ |
| Joules (J) | 8,368,000.00 J |
| Calories (cal) | 2,000,000.00 cal |
| Kilocalories (kcal) | 2,000.00 kcal |
| Watt hours (Wh) | 2,324.44 Wh |
The same arithmetic runs whatever you type into the form above, so you can reproduce any figure by matching the inputs listed here.
How this is calculated
Convert to joules (value × factor), then divide by the target factor. 1 kcal = 4,184 J; 1 kWh = 3.6 MJ.
Working values are kept at full precision and rounded only for display. Impossible dates, zero denominators, negative amounts where they make no sense, and out-of-range values are rejected before any result is shown.
How these conversions stay exact
Conversions are multiplications by fixed factors, so accuracy depends entirely on using the right factor and the right definition. Where a factor is defined exactly by international agreement it is used exactly; where a unit is a measurement convention, the agreed value is used and shown to a precision the input can support.
| Family | The approach | Where it is used |
|---|---|---|
| Exact defined factors | 1 inch = 25.4 mm, 1 pound = 0.45359237 kg | Length, weight |
| Base-unit chain | Convert into the SI base unit, then out to the target unit | Every converter in this section |
| Affine scales | F = C × 9/5 + 32; K = C + 273.15 | Temperature |
| US versus imperial | US gallon = 3.785411784 L; imperial gallon = 4.54609 L | Volume, fuel economy |
| Precision | Output rounded to the precision the input supports | Every converter in this section |
Temperature is the one family where the conversion is not a simple ratio. Because the scales start in different places, you multiply and then offset, which is why 20 °C is not twice as warm as 10 °C in any physical sense: it is 293 K against 283 K.
Recipe cups are the classic trap. A US customary cup is 236.6 mL, a metric cup is 250 mL and an Australian cup is 250 mL, so a cup of flour measured in different countries is a different weight before density is even considered.
Three worked examples from this section
Ten kilometres in miles
One mile is exactly 1.609344 km, so 10 ÷ 1.609344 = 6.2137 miles. For a quick check, multiply by 5 and divide by 8: 10 km is about 6.25 miles, which is close enough to sanity-check the number.
180 pounds in kilograms
One pound is exactly 0.45359237 kg, so 180 × 0.45359237 = 81.65 kg. In the other direction, a 70 kg adult is 70 ÷ 0.45359237 ≈ 154.3 lb.
- 180 lb
- 81.65 kg
- 70 kg
- 154.3 lb
- 1 stone
- 6.35 kg
100 °F in Celsius and kelvin
Subtract 32, multiply by 5 and divide by 9: (100 − 32) × 5 ÷ 9 = 37.8 °C. Adding 273.15 gives 310.9 K. The same steps reversed turn 37.0 °C into 98.6 °F.
Conversion factors and significant figures
Conversions use the exact defined factors where they exist (an inch is exactly 25.4 mm, a pound exactly 0.45359237 kg, a US gallon exactly 3.785411784 litres) and the internationally agreed values where measurement is involved.
- Results are shown to a precision that reflects the input. Multiplying a two-figure measurement by an exact factor still leaves you with a two-figure answer.
- US and imperial units differ: a US gallon is about 3.79 litres, an imperial gallon about 4.55. Choose the right one for the recipe or the vehicle.
- Temperature conversions are affine, not multiplicative; the offset is why doubling a Celsius temperature does not double a Fahrenheit one.
Sources and further reading
Reviewed 4 November 2026 · CalcFlow editorial
Common questions
Frequently asked
Why do food labels differ from physics calories?+
Labels use kilocalories but drop the “kilo”. So 250 calories on a packet is 250 kcal, or 1,046 kJ.
How much energy is a kWh?+
3.6 million joules, which is a 1,000 W heater running for one hour — the unit on your electricity bill.
Keep going
Related calculators
Temperature Converter
Move between Celsius, Fahrenheit and Kelvin, with the conversion steps spelled out underneath.
Length Converter
Convert between metres, kilometres, centimetres, feet, inches, yards and miles instantly.
Weight Converter
Switch between kilograms, grams, pounds, ounces and stones with the same number in every unit.
Often used together
Pairs well with
The next question people ask after this one usually sits in another section.