Metric & imperial volumes
Gallons and litres and pints, oh my!
One liter ("litre" in British English) equals roughly a quarter of a gallon (i.e. a quart).
Actually, there are 3.8 liters in 1 gallon, so a litre is a tad less than a quart, but for estimating, that will work.
Now, Brits have started accepting that water and wine and such are sold by the litre, but when it comes to a glass of beer in the pub, those are still strictly sold by the pint (or half-pint).
A pint is 20 oz. (in metric, 568 ml—or a shade more than half a liter).
An imperial gallon is equal to 8 pints (4.5 litres).