Gallons and liters and how to order wine
One liter 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.
The French often further divide the liter, ordering wine and water by the demi (half liter)—or even just un demi de vin rouge (a half liter of red wine).
Dining by yourself, you might even ask for a quarto litro, nicknamed un quartino (a quarter liter).
When it comes to smaller volumetric measurements... Europeans still use liters, just in milliliter form. So 1 cup equals 237 ml.