Metric volume

 (Photo © Reid Bramblett)

Gallons and liters and how to order wine

Gal Liter
1gal 3.79L
2gal 7.57L
3gal 11.53L
4gal 15.14L
5gal 18.93L
6gal 22.71L
7gal 26.50L
8gal 30.28L
9gal 34.16L
10gal 37.94L
50gal 189.7L
100gal 379.4L
Liter Gal
1L 0.26gal
2L 0.53gal
3L 0.79gal
4L 1.06gal
5L 1.32gal
6L 1.58gal
7L 1.85gal
8L 2.11gal
9L 2.38gal
10L 2.64gal
50L 13.2gal
100L 26.4gal

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.