Quart is short for Quarter of a Gallon. It’s oddly relevant when buying second hand floor mixers, (the big ones that stand on the floor) which are often referred to by their original sales pitch, e.g. a 40 quart mixer.
In metric a quart is UK 1.13 litres and US 0.94 litres, so completely useless for making a quick calculation of volume.
Litres convert to grams at 1:1, so a 40 quart mixer is UK 45.2 litres, or roughly 45kg of dough.
Bear in mind that’s a full bowl, which you would never actually fill, so if you want to mix 45kg of dough at a time, buy at least an 80kg mixer, or 80 quart.