Figuring out the right size liner for your outdoor trash can is simpler than you might think. The most reliable method is to grab a tape measure. First, measure the height (or depth) of your can from the inside bottom to just below the rim. Next, measure the circumference around the inside top of the can. Don't have a flexible tape? Use a string and then measure the string.
Once you have these numbers, you can find a compatible liner. Liner sizes are typically listed by gallon capacity (like 55 gallons) and dimensions. Your measured height should be a few inches less than the bag's height to allow for tying. The bag's width (when laid flat) should be close to half of your can's circumference. For example, if your can's top circumference is 60 inches, look for a bag with a flat width of about 30 inches.
A great tip is to check the product specifications online or on the box, which often list the recommended can dimensions. If your current liner sinks to the bottom or tears easily, it's too small. If there's excessive overhang that flaps in the wind, it's likely too big. Taking two minutes to measure ensures you get a liner that fits snugly, contains messes, and makes taking out the trash hassle-free.