Calories to kilojoules
One calorie is 0.004184 kilojoules, and this shows the working rather than just the answer. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
The factor
One calorie is 0.004184 kilojoules. That is the whole of the conversion: multiply by it, and nothing else happens.
What this page will not claim
It will not call this conversion exact. The factor is right to every figure shown, but at least one of these two units is not defined exactly in terms of the other, so there is a limit to how far the decimals mean anything.
The calorie is the small calorie, 4.184 J. NOT the one on a food label.
The number in your head
Most people use 0.0042 for this, which is the factor to two significant figures. It is high by 0.3824% — 3.824 kilojoules too many in every thousand, and 38.24 in every ten thousand.
Going the other way
The reverse is not this sum backwards in your head. It is a different factor, 239.005736137667, and dividing by one is not the same as multiplying by a rounded version of the other.
Rounded, the way back is 239, which is low by 0.0024%. So the two roundings do not cancel: go out and back with both and you do not land where you started.
How many of those decimals mean anything
The box above will give you ten decimal places because it can. Whether they mean anything is decided by your measurement, not by the arithmetic.
If you measured to the nearest calorie, your figure is already uncertain by half of one — so the answer is uncertain by about 0.002092 kilojoules, and every digit past that is decoration. Quoting 0.0051654 when you measured with a tape is claiming a precision the tape never had.
Where these two sit
Calories and kilojoules both measure energy, which is why they can be converted at all. A conversion between families — a length into a weight — is not a conversion, it is a question with a missing quantity in it.
The unit converter pairs calories with joules rather than with kilojoules, because that is the conversion people ask for most. This page exists because a good many ask for this one instead.