For most standard urban trash cans, the liner size is usually one step up from the can’s stated capacity—especially with wheeled municipal carts. A typical 32-gallon urban trash can is best matched with a 33-gallon liner. That gives enough slack to fold over the rim without swimming in excess plastic. If you have a 45-gallon cart, choose a 45–55 gallon bag. For a 65-gallon cart, go with a 65–70 gallon liner, and for a 95-gallon cart, use a 95–100 gallon bag. Smaller indoor-style urban cans are different: a 13–16 gallon kitchen can usually takes a 13–16 gallon bag, or an 18-gallon liner if you want more overhang.
Don’t rely on gallons alone, though. Bag fit depends on the can’s top diameter and height, and rim shape matters. Measure across the top opening and down the inside height, then check the bag’s stated dimensions. The liner should cover the rim by at least an inch or two so it stays put when you pull it out. If your can sits between sizes, size up rather than down; a slightly loose bag is easier to handle than one that tears at the rim. For a standard 32-gallon urban cart, 33-gallon liners are the safe everyday choice.