Energy shows up in wildly different units depending on where you’re looking: joules in physics, calories on a nutrition label, kilowatt-hours on your electricity bill, and BTU on an air conditioner’s rating plate. This energy conversion calculator lets you convert between 11 common energy units in either direction, so you can move from one context to another without hunting down conversion factors or doing the math by hand.
How to use this energy conversion calculator
- Enter a value in the “From” field.
- Select the unit you’re converting from in the “From” dropdown.
- Select the unit you want to convert to in the “To” dropdown.
- Your converted result appears automatically in the “To” field as you type or change either unit, no separate calculate button needed.
- Use the swap button (the up/down arrow icon between the two fields) to instantly reverse the From and To units.
- Check the “Quick Reference” list below your result to see your input value converted into several other units at once.
- Open “About Energy Units” if you want a quick refresher on what each unit measures and where it’s commonly used.
- Use “Save This Calculation” to keep a record in your history, or export the result as an Image or PDF.
What this energy conversion calculator does
Rather than requiring you to look up a specific conversion factor for whichever two units you happen to need, this calculator handles all 11 supported energy units as a single connected system. Enter a value once, pick your From and To units, and the conversion happens instantly and updates live as you adjust either field. The Quick Reference panel goes a step further, showing your input value already converted into several other units at once, which is useful when you’re not sure which unit you’ll actually need next.
The energy units this calculator supports
| Unit |
Symbol |
Typical use |
| Joule |
J |
The SI (International System) base unit of energy, used throughout physics and engineering |
| Kilojoule |
kJ |
1,000 joules; common on nutrition labels outside the US and in engineering contexts |
| Calorie |
cal |
The energy needed to raise 1 gram of water by 1°C; a small unit used mainly in chemistry |
| Kilocalorie (food Calorie) |
kcal |
1,000 calories; what’s actually meant by “Calories” on US nutrition labels |
| Watt-hour |
Wh |
Energy delivered by 1 watt of power over 1 hour; used for small appliances and batteries |
| Kilowatt-hour |
kWh |
1,000 watt-hours; the standard unit for electricity billing |
| British Thermal Unit |
BTU |
Common in HVAC, heating, and cooling equipment ratings |
| Foot-pound |
ft·lb |
A unit of work/energy common in US mechanical and engineering contexts |
| Electronvolt |
eV |
An extremely small unit used in physics and chemistry for atomic and particle-scale energy |
| Erg |
erg |
A small CGS (centimeter-gram-second) unit of energy, mostly seen in older scientific literature |
| Therm |
thm |
A larger unit used for natural gas billing, roughly equal to 100,000 BTU |
Key conversion factors
The joule anchors every conversion in this calculator, since it’s the SI base unit of energy. Every other unit converts through a fixed relationship to the joule, which is what allows the calculator to convert between any two of the 11 supported units directly, not just to and from joules.
1 calorie = 4.184 J
1 kilocalorie (food Calorie) = 4,184 J
1 watt-hour = 3,600 J
1 kilowatt-hour = 3,600,000 J = 3,600 kJ = 3,412.14 BTU
1 BTU ≈ 1,055.06 J
1 foot-pound ≈ 1.3558 J
1 electronvolt ≈ 1.602 × 10⁻¹⁹ J
1 erg = 1 × 10⁻⁷ J
1 therm ≈ 105,505,600 J ≈ 100,000 BTU
Why the food Calorie causes confusion
The “Calorie” on a nutrition label, written with a capital C, is actually a kilocalorie, 1,000 of the small calories used in chemistry. This calculator keeps kilocalorie as its own selectable unit specifically because of how often this distinction gets lost in casual conversation, letting you convert food energy values into joules, kilowatt-hours, or any other unit without needing to remember the 1,000x difference yourself.
Why kWh and BTU matter for everyday bills
Electricity bills are measured in kilowatt-hours, while gas heating and furnace or air conditioner capacity are frequently rated in BTU. Because these two units come from completely different measurement traditions (kWh from electrical engineering, BTU from imperial heat measurement), converting between them by hand means remembering that 1 kWh equals roughly 3,412.14 BTU, a specific enough number that most people look it up rather than memorize it. This calculator handles that conversion, along with converting either one into joules, therms, or any of the other 9 supported units.
Using the swap and quick reference features
The swap button is a small but useful shortcut: instead of manually re-selecting both dropdowns to reverse a conversion, one click flips your From and To units and immediately recalculates. The Quick Reference panel serves a different purpose, showing your current input value already converted into multiple other units simultaneously, so if you entered 5 kWh, you can see its joule, BTU, and kilocalorie equivalents all at once without running separate conversions for each.
This energy conversion calculator is a straightforward unit-math tool. It applies standard, internationally recognized conversion factors between energy units, but it doesn’t account for real-world losses, efficiency ratings, or context-specific adjustments that might apply in an engineering, billing, or scientific application, so treat the converted number as the mathematical equivalent, not a substitute for a domain-specific efficiency calculation where one is needed.
Frequently asked questions
Why is there both a “calorie” and a “kilocalorie” option?
The calorie (lowercase) is a small unit from chemistry, the energy to raise 1 gram of water by 1°C. The kilocalorie equals 1,000 calories and is what nutrition labels actually mean by “Calories” (capital C). This calculator keeps them as separate selectable units specifically to avoid the common confusion between the two.
How many BTU are in 1 kilowatt-hour?
1 kilowatt-hour equals approximately 3,412.14 BTU. This calculator applies that conversion factor directly, so entering 1 in the From field with kWh selected and BTU as the To unit will show this figure automatically.
What is a therm and why would I need it?
A therm is a larger energy unit, roughly equal to 100,000 BTU, commonly used for billing natural gas usage. If you’re comparing a gas bill measured in therms against an electricity bill measured in kWh, this calculator lets you convert both into the same unit for a direct comparison.
Does the swap button change my converted result?
The swap button reverses which unit is treated as the source and which as the target, then recalculates using your current input value against the newly assigned From unit. It’s a shortcut for quickly checking the same value in the opposite conversion direction without re-selecting both dropdowns manually.
