US gallons to milliliters (gal to ml)

One US gallon is exactly 3,785.411784 milliliters. 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: 1 gal x 3,785.41178 = 3,785.41 ml.

Going the other way, one milliliter is 0.0002641721 US gallons.

For mental arithmetic: Multiply by 3,800 and take off 0.4%. Or 3.8 liters, then three zeros.

The direction for dilution. Garden, cleaning and pool products say 'mix X per gallon', and the measuring cup in the cupboard is marked in milliliters. A US gallon is 3,785 ml, so a rate of 2 fluid ounces per gallon is 59 ml in 3,785, which is about 1 part in 64. The ratio form is often printed on the label too, and it is the safer number because it does not depend on which gallon. An imperial gallon is 4,546 ml, and a British label's 'per gallon' rate applied to a US gallon of water makes the mix about 20% stronger than intended.

Precision is worth matching to the input. A measurement given as 0.1 gal carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.