What is the difference between a calorie (cal) and a Calorie (kcal)?

A small calorie (cal) is the energy needed to raise 1 gram of water by 1 °C. A large Calorie (kcal), often written with a capital C, equals 1,000 small calories and is the unit used on food nutrition labels.

How many joules are in one calorie?

One small calorie (cal) equals exactly 4.184 joules (J) using the thermochemical definition. One kilocalorie (kcal) equals 4,184 joules or 4.184 kilojoules (kJ).

Why does the food industry use kcal instead of joules?

Kilocalories were the standard before the SI system was adopted. Many countries still label food in kcal for historical reasons, though EU regulations also require kJ to appear alongside kcal on packaging.

Is the 4.184 factor exact?

The thermochemical calorie is defined as exactly 4.184 J. The International Table calorie is 4.1868 J — a small difference. This tool uses the more common thermochemical definition (4.184 J/cal).

Can I convert joules back to calories with this tool?

Yes. Click the swap button to reverse the conversion direction, then enter a value in joules or kilojoules to get the calorie equivalent.

Calories to Joules Converter

Convert small calories (cal) or large Calories (kcal) to joules (J) and kilojoules (kJ) instantly, with a quick-reference conversion table.

Energy Converter

Input unit

cal (small) kcal (large)

Value

Joules (J)

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Kilojoules (kJ)

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Quick Reference Table

cal (small)	Joules (J)	kcal (large)	kJ

Joules (J)	cal (small)	kJ	kcal (large)

1 cal = 4.184 J (thermochemical definition) • 1 kcal = 4,184 J = 4.184 kJ

Summary

This tool converts energy values expressed in small calories (cal) or large Calories (kcal) into joules (J) and kilojoules (kJ). One small calorie equals exactly 4.184 joules (the thermochemical definition), and one kilocalorie equals 4,184 joules or 4.184 kJ. The reference table at the bottom covers the most common values so you can look up conversions at a glance without entering a number.

How it works

Select whether your input is in small calories (cal) or large Calories / kilocalories (kcal).
Type a number into the input field.
The converter instantly displays the result in both joules and kilojoules.
Use the swap button to reverse the direction (joules to calories).
Scroll down to the reference table for instant look-up of common values.

Use cases

Convert food energy values from kcal to kJ for nutrition labels.
Check chemistry homework that mixes calorie and joule units.
Understand energy expenditure data from fitness trackers.
Convert scientific measurements between SI (joules) and legacy (calories) units.
Quickly verify calorimeter results against joule equivalents.
Explain energy unit differences to students learning thermodynamics.

Frequently Asked Questions

Last updated: 2026-05-23 · Reviewed by Nham Vu