Ml to gallons (ml to gal)

One milliliter is exactly 0.0002641721 US gallons. That is a definition rather than a measurement: the imperial and US customary units were fixed against metric ones by the 1959 international yard and pound agreement, so this factor does not drift and will not be revised.

Worked through: 500 ml x 0.0002641721 = 0.132086 gal.

Going the other way, one US gallon is 3,785.411784 milliliters.

For mental arithmetic: Divide by 3,785. Or multiply by 0.00026.

Small containers read against a big unit. A 500 ml water bottle is 0.132 US gallons, so a gallon is about 7.6 of them, and a 2-liter bottle is just over half a gallon. This direction is also where the two gallons quietly diverge: 1,000 ml is 0.264 US gallons and 0.220 imperial ones, and fuel economy and tank sizes quoted by British and American sources differ by that 20% without either saying which gallon they mean.

Precision is worth matching to the input. A measurement given as 100 ml carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.