But you're not saying "four litres of milk" that often. You're saying "a carton of milk" and typically the people understand that that is a litre or 1.5l or wha

But you're not saying "four litres of milk" that often. You're saying "a carton of milk" and typically the people understand that that is a litre or 1.5l or whatever it is you usually buy. As for beer, again, where I drink it's typically either unspecified or a "small", being 25cl and 15cl respectively (this being a nerdy craft beer bar where smaller sizes are preferred.) If you do want to say the size, you can just say 25 or 15, no one is going to think you want 15 litres because that'd be weird.