Conversion Runs locally Ready to use Built-in examples No tracking

Energy Converter

Convert energy units for food labels and electricity bills, all locally in your browser.

A food label prints kilojoules or kilocalories, an electricity meter reads kilowatt-hours, a physics problem uses joules — energy units span nutrition, electricity and physics. This tool converts between J, kJ, cal, kcal, Wh and kWh, with the anchors 1 kcal = 4184 J and 1 kWh = 3600000 J = 860 kcal.

On conventions: the "calorie" of everyday speech is really the kilocalorie, so a label given in kJ is divided by 4.184 to reach kcal, and the calories a fitness tracker reports are kcal as well. A 30-minute run burning roughly 300 kcal comes to about 0.35 kWh — a conversion that makes the scale of physical effort easy to grasp.

—

How to use

  1. Enter the energy value and pick its unit.
  2. Read the equivalents in the other units.
  3. Divide kilojoules by 4.184 when comparing against a kcal label.
  4. Use kWh for electricity and kcal for food, and do not mix them up.

How it works

The energy basis

Energy expresses how much work is done; the SI unit is the joule (J) = 1 watt-second. Based on the joule: 1 kJ = 1000 J, 1 cal = 4.184 J, 1 kcal = 4184 J, 1 Wh = 3600 J, 1 kWh = 3.6 MJ — a single multiply or divide.

Food-calorie conventions

The "large calorie," "kilocalorie" and "kcal" on food labels are the same unit, equal to 4184 joules; a snack labeled "energy 2000 kJ" is about 478 kcal. Note 1 kcal = 1000 cal, so cal and kcal differ 1000-fold.

Electricity: the kilowatt-hour

An electricity "unit" = 1 kWh = 1000 W x 3600 s = 3.6 MJ, about 860 kcal. So estimates like "boiling 1 liter of water from 20 C uses about 0.1 kWh" are essentially energy-unit conversion plus thermal efficiency.

Code example

JavaScript Joules as the hub 

const J = { J: 1, kJ: 1e3, cal: 4.184, kcal: 4184,
            Wh: 3600, kWh: 3.6e6 };

const convert = (v, from, to) => v * J[from] / J[to];

convert(200, "kcal", "kJ");    // 836.8 (a bowl of rice)
convert(1, "kWh", "kcal");     // 860.4 (one unit of electricity)

Python Quick maths from a food label 

KJ_PER_KCAL = 4.184

kj = 2300                       # the kJ figure on the pack
kcal = kj / KJ_PER_KCAL         # 549.7 kcal

# Electricity: 1 kWh = 3600 kJ
kwh = kj * 1000 / 3.6e6         # 0.639 kWh

FAQ

Are large calorie and kilocalorie the same?

Yes. "Large calorie," "kilocalorie" and "kcal" on labels mean the same unit, 4184 joules; "calorie (cal)" is one-thousandth of it. 1 kcal = 1000 cal — mixing them is off by 1000x.

How many joules in 1 kWh?

1 kWh = 1000 W x 3600 s = 3,600,000 J = 3.6 MJ, about 860 kcal. Electricity bills and water-heating time estimates all rest on this conversion.

Can "burning 500 kcal" be converted to electricity?

Yes: 500 kcal is about 2092 kJ, about 0.58 kWh, equivalent to a 2000 W appliance running about 17 minutes. But the human body's energy conversion efficiency is far lower than an appliance's, so exercise-machine figures are only a reference.

How do joules and watt-hours convert?

1 Wh = 3600 J (1 watt for 1 second = 1 joule). A power bank's Wh is energy; a phone battery's mAh must be multiplied by voltage to get energy: 3.7 V x 5000 mAh = 18.5 Wh, about 66.6 kJ.

Why do food-label energy values allow error?

Values are mostly calculated from the ingredient formula, and the same food varies with variety, ripeness and cooking, so usable energy differs. For strict counting, control the total across several days rather than fussing over a single decimal.

Can gasoline or natural gas be converted here?

This tool does pure energy-unit conversion only. Fuels require their own calorific values (gasoline roughly 44 MJ/kg, natural gas roughly 36 MJ/m3) and multiplication by combustion efficiency — not a direct unit conversion.

Is my data uploaded?

No. Conversion runs in your browser; the server only delivers the page code. Calculation history stays in local browser storage and can be cleared in one click. For no trace at all, use an incognito window — records vanish on close.