Us gallons to millilitres
One US gallon is 3,785.411784 millilitres, 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 US gallon is 3,785.411784 millilitres. That is the whole of the conversion: multiply by it, and nothing else happens.
And it is exact
Not rounded, not measured, not a convention that might be revised. The factor above is the definition, so an answer from it is as exact as the number you typed.
The US gallon is 231 cubic inches, so exactly 3.785411784 litres. About 20% smaller than the imperial gallon.
The number in your head
Most people use 3,800 for this, which is the factor to two significant figures. It is high by 0.3854% — 3.854 millilitres too many in every thousand, and 38.54 in every ten thousand.
Going the other way
The reverse is not this sum backwards in your head. It is a different factor, 0.000264172052, and dividing by one is not the same as multiplying by a rounded version of the other.
Rounded, the way back is 0.000264, which is low by 0.0651%. 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 US gallon, your figure is already uncertain by half of one — so the answer is uncertain by about 1,892.705892 millilitres, and every digit past that is decoration. Quoting 4,673.31797206 when you measured with a tape is claiming a precision the tape never had.
Where these two sit
Us gallons and millilitres both measure volume, 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 US gallons with litres rather than with millilitres, because that is the conversion people ask for most. This page exists because a good many ask for this one instead.
Other conversions from US gallons
litres · cubic metres · imperial gallons · imperial pints