1
Unit Conversions
Energy Converter — Calories, Joules, kWh, BTU & More
From food labels to electric bills — turn calories into joules, kWh and BTU with exact factors
Energy shows up under a dozen different names depending on who's measuring it. A nutrition label counts Calories, a science class works in joules, your electric bill bills you in kilowatt-hours (kWh), an air conditioner is rated in BTU, a physics problem uses foot-pounds, and a particle physicist measures in electronvolts (eV). They're all the same physical quantity — energy — just expressed in different units. This converter moves any value between the nine units you actually run into.
The method is exact: every unit is first converted to a single base — the joule (J) — and then back out to the unit you want. The joule is the SI unit of energy: one joule is the work done by a force of one newton acting over one meter.
The factors this tool uses (joules per unit, exact or standard):
- 1 kilojoule (kJ) = 1,000 J
- 1 calorie (small, thermochemical, cal) = 4.184 J
- 1 kilocalorie / food Calorie (kcal, Cal) = 4,184 J
- 1 watt-hour (Wh) = 3,600 J
- 1 kilowatt-hour (kWh) = 3,600,000 J
- 1 BTU = 1,055.05585262 J
- 1 foot-pound (ft·lb) = 1.3558179483 J
- 1 electronvolt (eV) = 1.602176634 × 10⁻¹⁹ J
Formula: joules = value × (factor of the FROM unit), then result = joules ÷ (factor of the TO unit).
Worked example — convert 100 food Calories to joules. A 100-Calorie snack is 100 kcal: 100 × 4,184 = 418,400 J (about 418 kJ). The same snack is 418,400 ÷ 3,600,000 = 0.116 kWh of energy.
The single biggest trap is the calorie. The lowercase "calorie" of chemistry (4.184 J) is 1,000 times smaller than the capital-C food Calorie you see on a candy bar, which is really a kilocalorie. So "250 Calories" on a label means 250 kcal = 1,046,000 J, not 250 cal. This converter keeps the two separate so you never lose or gain three orders of magnitude by accident. Pick a value, a from unit and a to unit, and you'll also get a quick table showing the same amount of energy in every unit at once.
Calculator
Fill in the fields and click "Calculate" for instant results.
📰 Formula
• Convert to joules: J = value × (factor of the FROM unit) • Convert out: result = J ÷ (factor of the TO unit) • J = 1 • kJ = 1000 • cal (small) = 4.184 • kcal / food Calorie = 4184 • Wh = 3600 • kWh = 3,600,000 • BTU = 1055.05585262 • ft·lb = 1.3558179483 • eV = 1.602176634 × 10⁻¹⁹
📰 Formula
• Convert to joules: J = value × (factor of the FROM unit) • Convert out: result = J ÷ (factor of the TO unit) • J = 1 • kJ = 1000 • cal (small) = 4.184 • kcal / food Calorie = 4184 • Wh = 3600 • kWh = 3,600,000 • BTU = 1055.05585262 • ft·lb = 1.3558179483 • eV = 1.602176634 × 10⁻¹⁹
🧪 Worked examples
2
Example 2
3
Example 3
4
Example 4
⚠️ Common mistakes
- Confusing the small calorie (4.184 J) with the food Calorie / kcal (4,184 J) — they differ by 1,000×.
- Reading a European label's kJ as Calories — 2,000 kJ is about 478 Calories, not 2,000.
- Mixing watt-hours and kilowatt-hours — there are 1,000 Wh in 1 kWh.
- Using a rounded BTU factor like 1,055 instead of the exact 1,055.05585262 J.
- Treating power (watts) and energy (joules or kWh) as the same thing — energy is power × time.
💡 Tips
- On a food label, every "Calorie" with a capital C is a kilocalorie — use the kcal row in this tool.
- Convert through joules as a single base; it removes guesswork between any two energy units.
- Quick gut checks: 1 kcal ≈ 4.2 kJ, 1 kWh ≈ 3,412 BTU, and 1 BTU ≈ 1,055 J.
- To turn kWh into joules, multiply by 3.6 million; to go back, divide by 3.6 million.
- For 'how many Calories did I burn,' divide joules by 4,184 to get food Calories (kcal).
Embed this calculator on your site
Copy the code below and paste it into the HTML of your site or blog.
<iframe src="https://www.calcnimbus.com/embed/energy-converter" width="100%" height="500" frameborder="0" style="border:1px solid #eee;border-radius:12px"></iframe>
❓ Frequently asked questions
How many joules are in a calorie?
One small (thermochemical) calorie equals exactly 4.184 joules. The food Calorie you see on labels is a kilocalorie, which equals 4,184 joules — 1,000 times larger.
What is the difference between a calorie and a Calorie?
A lowercase "calorie" (cal) is the heat needed to raise 1 gram of water by 1 °C and equals 4.184 J. A capital-C "Calorie" (Cal) used on nutrition labels is a kilocalorie — 1,000 calories, or 4,184 J. So 250 food Calories is 250 kcal.
How do I convert food Calories to joules?
Multiply the food Calories (kcal) by 4,184. Example: a 200-Calorie snack is 200 × 4,184 = 836,800 J, or about 837 kJ. To convert to kilojoules instead, multiply by 4.184.
How many joules are in a kilowatt-hour?
Exactly 3,600,000 joules (3.6 megajoules). A kilowatt-hour is 1,000 watts running for one hour: 1,000 W × 3,600 s = 3,600,000 J. To go back, divide joules by 3.6 million.
How do I convert kWh to BTU?
1 kWh = 3,600,000 J, and 1 BTU = 1,055.05585262 J, so 1 kWh = 3,600,000 ÷ 1,055.05585262 ≈ 3,412.14 BTU. This is the factor used when comparing electric heaters to gas appliances.
What is a BTU and how many joules is it?
A British thermal unit (BTU) is the energy to raise one pound of water by 1 °F. The International Table BTU used here equals 1,055.05585262 J. Air conditioners and furnaces are commonly rated in BTU per hour.
How do I convert kilojoules to Calories?
Divide kilojoules by 4.184 to get food Calories (kcal). Example: a label showing 1,200 kJ is 1,200 ÷ 4.184 ≈ 287 Calories. European labels list kJ, US labels list Calories.
What is an electronvolt and why is it so small?
An electronvolt (eV) is the energy one electron gains crossing a one-volt potential, equal to 1.602176634 × 10⁻¹⁹ J exactly. It's tiny because it describes single particles, so it's the natural unit in atomic and particle physics.
Are these conversion factors exact or approximate?
Most are exact by definition: 1 kJ = 1,000 J, 1 cal = 4.184 J, 1 kWh = 3,600,000 J, and 1 eV = 1.602176634 × 10⁻¹⁹ J. The BTU (1,055.05585262 J) and foot-pound (1.3558179483 J) are the standard International Table values, so this converter matches official references.