Energy Converter

Convert energy between fourteen units — joules, calories, kWh, BTU and more — one value or a pasted column.

Runs on your deviceConvertersWorkingNo account needed

What this tool does

Energy Converter measures everything against the joule. Fourteen units sit in its table: joule, kilojoule and megajoule; the thermochemical calorie and the kilocalorie; watt-hour, kilowatt-hour and megawatt-hour; the British thermal unit and the therm; and the foot-pound, electronvolt, erg and ton of TNT. Each holds a fixed factor to the joule, so a conversion is one multiplication into joules and one division back out, and any pair in the list works in either direction.

Values go in one per line, and whatever is written on the line outranks the two dropdowns above it. A bare 1 takes the From and To menus, which start on kilowatt-hour and kilojoule; 100000 BTU replaces the From menu for that line alone; 2000 kcal to kJ replaces both. Spelling is matched loosely, so joules, joule and J all reach the same unit, and kWh answers to the bare word unit, which is what an electricity bill calls it in India and several other places. Every converted line becomes a row of an Input / Result table, and a single hint line beneath the output counts the values and prints the factor for the last pair converted — 1 kWh = 3600 kJ — rather than one factor per row. Copy output hands that table over as CSV, factor line excluded.

The calorie is where this goes wrong most often, so it is worth stating exactly. Here cal is the thermochemical calorie of 4.184 J and kcal is the kilocalorie of 4184 J, the unit a food label prints with a capital C. Capitalisation is not read as a difference: Cal, cal, calories and Calories all resolve to the small calorie, so a 2000-Calorie daily intake typed as 2000 cal returns 8.368 kJ instead of the 8368 kJ you meant. The plural is worse, because the unit table registers cals as a spelling of the kilocalorie: 2000 cals comes back as 8368 kJ while 2000 cal comes back as 8.368 kJ, one letter apart and a thousandfold different. Type kcal whenever the figure came off a nutrition panel and neither trap can reach you.

Three limits decide when this is the wrong page. It knows energy units only, so 1 kWh to W is refused with an unknown-unit error whose suggestion list stops after eight symbols, J through MWh, leaving the other six behind an ellipsis: watts, kilowatts and horsepower are power, and turning a power rating into energy means multiplying by a number of hours, which nothing here does for you. It converts rather than prices — there is no tariff field, so a kilowatt-hour comes out as kilojoules and never as money. And the BTU used is the International Table value of 1055.05585262 J, which makes the therm exactly one hundred thousand of those; the therm on a United States gas bill is built on a different BTU and is roughly 25 kJ smaller, about 0.02 per cent. The unit table ships with the page and the sums happen in this tab, so the figures you type stay on your machine, and once the page has been fetched the network can be switched off without stopping the conversions.

Two details of the number formatting are worth knowing before you trust a printed digit. Automatic precision is ten significant figures in the plain decimal range, but answers written as exponents carry eight, which is why a kilowatt-hour in electronvolts reads 2.2469433e25 and not a longer figure. Picking a fixed number of decimals does not force such a value into ordinary digits either: from 10^21 upwards the fixed setting is ignored and the raw exponential is printed in full, so the same conversion at 2 decimals shows 2.2469432668058746e+25.

How to use it

  1. Type the value, or a column of them. One figure to a line. Load sample fills the box with a bare 1, a 2000 kcal to kJ line and 100000 BTU to kWh, which covers the three input styles.
  2. Set From and To, or write the units in the line. The menus default to kilowatt-hour and kilojoule. A unit typed after the number overrides From, and anything after "to" overrides To.
  3. Decide how many digits to keep. Automatic gives ten significant figures with trailing zeros trimmed, dropping to eight on any answer that comes out as an exponent. The fixed options round to 0, 1, 2, 3, 4, 6 or 8 decimal places instead.
  4. Read the table, or ask for every unit. Converted lines land in an Input / Result table with the count and the last factor on the line below. Set To to All units (table) for one figure in all fourteen at once — that view prints no factor — then Copy output for CSV.

Processing & privacy

Runs on your device. Your file is processed directly in this browser.

Common use cases

Comparing a gas bill with an electricity bill

Gas is billed in therms in some places and kWh in others. One therm is 29.30710702 kWh, which is the figure that puts the two invoices on the same scale.

Reading a food label written the other way round

European panels print kJ beside kcal while American ones give only Calories. 300 kcal to kJ returns 1255.2 kJ, the number the European label would carry.

Checking a battery against an airline limit

Cells and power banks are labelled in watt-hours. Converting 100 Wh to kJ, or a laptop pack into kWh, puts two spec sheets into one unit before you compare them.

Turning a physics figure into everyday units

One kilojoule is 6.2415091e21 electronvolts and a kilowatt-hour is 2.2469433e25 — a jump far quicker converted than counted out.

Normalising a pasted column of readings

Meter readings or lab results copied from a spreadsheet drop in as lines, each converts on its own terms, and the table copies back as CSV.

Questions

Does anything I type leave this tab?
No. The unit factors ship with the page and the arithmetic is done by your browser, so no value is transmitted or logged. After the page has loaded it carries on converting with the network disconnected, although nothing is cached for offline use, so the page itself has to be fetched while you still have a connection.
Why did 2000 calories come out as 8.368 kJ?
Because cal here is the thermochemical calorie rather than the food Calorie, and the capital C on a label is not treated as a separate unit. Type 2000 kcal to get 8368 kJ.
Is cals the same as cal?
No, and that is a flaw in the unit table rather than a convention: cals is listed among the kilocalorie's spellings, so it converts as kcal while cal, Cal and calories convert as the small calorie. Avoid the plural and type kcal or cal outright.
Can it convert watts, kilowatts or horsepower?
No. Those are power — energy divided by time — and they come back as unknown units. A 2 kW heater running for three hours uses 6 kWh, and that multiplication has to happen before you reach this page.
Can it tell me what a kilowatt-hour costs?
No. There is no tariff or currency setting, so it converts kWh into other energy units and stops. Multiply the result by your own unit rate to reach a cost.
Which BTU and which therm does it use?
The International Table BTU of 1055.05585262 J, and a therm of exactly 100,000 of them. The US therm rests on a slightly different BTU and is around 0.02 per cent smaller, which shows up only when reconciling a large account.
Which energy units are missing?
Tonne of oil equivalent, quad, hartree and kilotonne of TNT are not in the table and are rejected as unknown units. The list is the fourteen named on this page, and nothing outside it can be typed.
Why is my answer written as 1.6021766e-19?
Automatic precision turns exponential below one millionth and from 10^15 upwards, which an electronvolt expressed in joules cannot avoid, and those exponential answers hold eight significant figures instead of ten. Fixed decimals will not rescue it: at 8 decimals that value rounds to 0.00000000, and above 10^21 the decimal setting is disregarded and the untrimmed exponential is printed.

Related tools