Energy Converter
Convert between joules, calories, kilocalories, watt-hours, and BTU.
Cómo funciona
This converter handles energy across scientific, nutritional, and everyday units — joules, calories, kilocalories, watt-hours, and BTU — converting your value to joules (the SI base unit for energy) first, then from joules into your target unit.
The joule is the standard scientific unit of energy, defined as the work done by one newton of force over one meter. It's the common thread linking every other unit here, since each has an exact, fixed relationship to it: 1 calorie = 4.184 joules, 1 watt-hour = 3,600 joules, and 1 BTU ≈ 1,055.06 joules.
Nutrition labels create a common point of confusion: the "Calories" listed on food packaging (capital C) are actually kilocalories (1,000 small calories), a convention carried over from older scientific usage. So "200 Calories" on a label means 200,000 small calories, or about 836,800 joules.
Watt-hours (and kilowatt-hours) measure energy as power sustained over time — a 100-watt lightbulb running for 10 hours uses 1,000 watt-hours (1 kWh) of energy. This is the standard unit for electricity billing, since it directly represents how utility companies measure and charge for energy consumption.
BTU (British Thermal Unit) is common in heating, cooling, and appliance specifications, particularly in the US — it's defined as the energy needed to raise one pound of water by one degree Fahrenheit, and air conditioner or furnace capacity is often rated in BTUs per hour.
For everyday reference: a typical adult's daily calorie intake is often 2,000-2,500 kcal (about 8.4-10.5 million joules), a household microwave uses roughly 1,000-1,500 watts of power, and a small residential air conditioner might be rated around 5,000-12,000 BTU/hour — useful benchmarks for sanity-checking a converted energy value.
Electricity bills typically use kilowatt-hours (kWh) rather than watt-hours, since a household's monthly usage in plain watt-hours would be an unwieldy number — 1 kWh is 1,000 watt-hours, or 3.6 million joules, and a typical US household uses on the order of 800-900 kWh per month depending on climate, home size, and appliance usage.
How many joules are in 1 kilocalorie?
1 kcal = 4184 joules.
Why does a food label say 'Calories' when it means kilocalories?
It's a historical labeling convention — the capitalized 'Calorie' on nutrition labels equals 1,000 small calories (1 kilocalorie), so '200 Calories' actually means 200,000 small calories.
How many joules are in a watt-hour?
1 watt-hour = 3,600 joules, since a watt-hour represents 1 watt of power sustained for one hour (3,600 seconds).
What is a BTU used for?
BTU (British Thermal Unit) is commonly used to rate heating, cooling, and appliance capacity, especially in the US — air conditioners and furnaces are often rated in BTUs per hour.
How many joules are in 1 BTU?
1 BTU ≈ 1,055.06 joules.
Why is the joule the base unit for this converter?
The joule is the SI (International System of Units) standard unit for energy, giving it an exact, well-defined relationship to every other energy unit, which makes it a natural common conversion point.
How many kilocalories does a typical adult need per day?
Roughly 2,000-2,500 kcal is a common daily reference range, though actual needs vary by age, sex, weight, and activity level.
What's the difference between a watt and a watt-hour?
A watt measures power (the rate of energy use at an instant), while a watt-hour measures energy (power sustained over time) — a 100-watt bulb running for 10 hours consumes 1,000 watt-hours of energy.
Can I convert between more than two energy units at once?
Yes — the "Quick reference" table below the main converter shows your entered value converted into every other supported energy unit simultaneously.
Why do electricity bills use kilowatt-hours instead of joules?
A kilowatt-hour (3.6 million joules) is a more practically sized unit for household electricity usage than the joule, which would produce unwieldy multi-million figures for typical monthly consumption.
How much electricity does a typical household use per month?
Roughly 800-900 kWh is common in the US, though actual usage varies widely based on climate, home size, and appliance efficiency.