The capital letter that means a thousand
There are two calories in circulation, and the only thing distinguishing them in American usage is a capital letter.
The small calorie (cal) is the energy needed to raise one gram of water by one degree Celsius — exactly 4.184 joules by definition. It is the one physics and chemistry use.
The Calorie with a capital C, as printed on a US nutrition label, is a kilocalorie: exactly 1,000 small calories, or 4,184 joules. So a 250 Calorie snack contains 250,000 calories. Both of those sentences are correct, which is precisely the problem.
A capital letter is a fragile way to carry a factor of a thousand. It does not survive handwriting, it does not survive a search box, it does not survive being read aloud, and it does not survive a sentence that begins with the word. Most of the world sidesteps it by writing kcal, which is unambiguous; EU labelling rules go further and require kilojoules as well, which is why European packaging carries two numbers and American packaging carries one.
This converter lists them as separate units with their own names, and never treats “calorie” as a single thing.
Reading a nutrition label
A European label gives energy twice — for example 1,046 kJ / 250 kcal — and the two are the same quantity. The ratio between them is fixed at 4.184, so if you only have one, multiplying kcal by 4.184 gives kJ and dividing kJ by 4.184 gives kcal. A rough mental version: kJ is a little over four times kcal.
A US label gives one number, headed Calories, and that number is kilocalories. A US label saying 250 and a European label saying 250 kcal are describing the same food; a European label saying 1,046 is describing the same food again in kJ.
Two label details cause more confusion than the units. First, values are per serving, and the serving size is set by the manufacturer — a packet that looks single may declare two servings, so the on-pack number is half the packet. Second, the per-100 g column, which EU labels must carry, is the one to use when comparing two products, because it is the only figure that is independent of how each brand chose to define a serving.
Energy on a utility bill
Electricity is billed in kilowatt-hours — the energy used by a 1,000-watt appliance running for one hour. It is a unit of energy, not of power, despite containing the word watt: a kWh is exactly 3.6 megajoules.
So a 2,000 W kettle running for six minutes uses 0.2 kWh, and a 9 W bulb left on all day uses about 0.22 kWh — the kettle and the bulb are comparable over those periods, which is the sort of comparison the unit exists to make.
Gas is billed differently and the units are worse. UK bills convert a metered gas volume into kWh using a calorific value that varies with the gas supplied, and print the calculation on the bill. US bills use therms — exactly 100,000 BTU, about 29.3 kWh — or CCF, meaning a hundred cubic feet, which is a volume and needs the same calorific-value step before it becomes energy.
That is why a gas bill has a conversion factor printed on it and an electricity bill does not: electricity is metered in energy directly, and gas is metered in volume and converted.
Energy is not power
Energy is a quantity; power is a rate. A watt is one joule per second, so power tells you how fast energy is being used and energy tells you how much was used altogether. Multiplying a power by a time gives an energy — which is exactly what a kilowatt-hour is, and why the unit has a time in its name.
The distinction matters when comparing appliances. A 3,000 W shower uses power at three times the rate of a 1,000 W heater, but ten minutes of shower is 0.5 kWh against the heater’s 1 kWh over an hour — so the heater costs twice as much to run despite the smaller number on its label. The rating is a rate; the bill is an amount.
This page converts energy. Power units — watts, kilowatts and the three different horsepower — are a separate dimension, and asking this tool to convert between them throws an error rather than returning a plausible number, because there is no factor that turns a quantity into a rate.
Doing it in a spreadsheet
The factors worth writing out: =A1*4.184 for kcal to kJ, =A1/4.184 back, =A1*3.6 for kWh to MJ, =A1*29.3071 for therms to kWh, and =A1*1.05506 for BTU to kJ. All are exact to the digits shown.
If a sheet mixes food energy and utility energy — a cycling or training log against a household budget, say — put the unit in the column header and convert once at the boundary rather than per row. The two calories are close enough in spelling that a column headed just “calories” will eventually be filled with both, and nothing in the sheet will show which rows are which.
For an appliance cost, the chain is: watts ÷ 1,000 × hours = kWh, then × your unit rate. Keeping the three steps in three cells makes the tariff visible and reviewable, which matters more than the arithmetic when the tariff changes.
Sources and methodology
Nothing here is fetched and there is no data feed: every figure is computed from what you type. The factors are exact by definition, and the sources settle the two things that actually cause errors — the US regulation that puts “Calories” on a label when it means kilocalories, and the EU rule that requires both kJ and kcal, which is why European packaging sidesteps the problem entirely.