Energy Converter - Joules, Calories, kWh, BTU & More

Energy Converter for Joules, Calories, kWh, and BTU

The energy converter turns one energy value into every other unit at once, so you can switch between Joules, Calories, kilowatt-hours, and BTU without doing the math by hand. Type a number, pick the source unit, and the full list of conversions updates instantly.

It is built for anyone who meets mixed energy units in daily work: nutritionists comparing calories and kilojoules on food labels, homeowners reading electricity and gas bills, engineers and HVAC pros working in BTU or kWh, and students checking physics and chemistry calculations.

Private by design: every conversion runs in your browser. Your input values are never uploaded to a server, stored, or tracked.

How to Use the Energy Converter

Enter a value

Type your number into the input field. Large numbers are formatted with separators so figures like 3 600 000 stay easy to read and check.

Select your source unit

Open the unit dropdown and start typing to search — enter "kwh" or "calorie" to jump straight to the unit you need. Use Swap units to flip the direction.

Read the results

Every enabled unit updates in real time below. Use Customize Units to show only the unit groups you care about, and the Decimals selector to set 2, 4, 6, 8 places or Auto.

Copy what you need

Click the copy icon on any result row to grab a single value, or use Copy All Results for the whole list. Open Unit Reference if you want a quick explanation of any unit.

Handy equivalents: 1 kcal (food Calorie) = 4.184 kJ · 1 kWh = 3.6 MJ = 3 600 000 J · 1 BTU ≈ 1 055 J · 1 therm = 100 000 BTU ≈ 29.3 kWh.

Features

Instant Conversion

Results update in real time as you type — no button to press and no waiting, just enter a value and see every unit at once.

Comprehensive Unit Support

Convert across 23 energy units spanning metric (J, kJ, MJ, GJ), calories, electrical (Wh, kWh, MWh), thermal (BTU, therm, quad), atomic (eV), and mechanical units.

Smart Input Formatting

Large numbers are grouped with separators automatically, making long values like 3 600 000 easier to read and verify.

Searchable Unit Selection

Type a name or symbol in the unit dropdown to filter instantly, so picking the right unit takes one keystroke instead of scrolling.

Customizable Display

Toggle whole groups or individual units on and off so your results show only the energy units that matter to your work.

Precision Control

Choose 2, 4, 6, or 8 decimal places, or Auto — high precision for scientific work, fewer places for quick everyday reads.

One-Click Copy

Copy any single result or the entire list with Copy All Results, ready to paste straight into documents, spreadsheets, or reports.

Unit Reference Guide

A built-in reference panel explains each unit and where it is typically used — helpful for less familiar units like therm, electronvolt, or tonne of oil equivalent.

Works Everywhere

A fully responsive layout works on desktop, tablet, and phone, so you can convert energy units at home, in the lab, or on the job site.

Frequently Asked Questions

How many joules are in a calorie?

One small calorie (cal) equals 4.184 joules. A food Calorie — the capital "Calorie" or kilocalorie (kcal) on nutrition labels — is 1 000 small calories, so 1 kcal equals 4 184 joules. Enter your value in cal or kcal and read the Joule (J) result directly.

How do I convert kWh to joules?

Select kilowatt-hour (kWh) as the source unit and look at the Joule (J) row. For reference, 1 kWh = 3 600 000 J = 3.6 MJ. Kilowatt-hours measure electrical energy on your bill, while the joule is the SI base unit of energy.

How many joules are in 1 BTU?

One British Thermal Unit (BTU) is about 1 055 joules — the heat needed to raise one pound of water by 1°F. BTU is common in the US for heating and air-conditioning ratings. Enter a BTU value to see it in joules, kWh, and the other units at once.

What is the difference between a calorie and a Calorie?

A lowercase calorie (cal) is the small calorie — the energy to raise 1 gram of water by 1°C. A capital Calorie (Cal), also called a kilocalorie (kcal), equals 1 000 small calories and is what food labels use. "100 Calories" on a label means 100 kcal.

How do calories relate to kilojoules on food labels?

Many countries list kilojoules (kJ) instead of Calories. The conversion is 1 kcal = 4.184 kJ, so a food with 100 kcal contains about 418 kJ. Pick kcal as the source unit and read the kJ result to compare the two label styles.

What is an electronvolt (eV)?

An electronvolt is a tiny unit of energy used in particle physics and chemistry — the energy an electron gains moving through 1 volt of potential difference. 1 eV equals about 1.602 × 10⁻¹⁹ joules. The converter also handles keV, MeV, and GeV for larger atomic-scale energies.

What is a therm?

A therm is a unit of heat energy equal to 100 000 BTU, commonly used to bill natural gas in the United States. 1 therm is roughly 105.5 MJ or about 29.3 kWh. Convert therms here to see the equivalent in BTU, kWh, or joules.

How accurate are the conversions?

Conversions use internationally recognized conversion factors, so for everyday and most technical purposes the results are accurate to many decimal places. Use the Decimals selector to display as much precision as you need, up to 8 places or Auto.