Measure your can’s top opening — width plus depth for square or rectangular cans, diameter times 3.14 divided by 2 for round ones — then pick a liner as wide as that number and a few inches longer than the can.
The liner that fits is almost never the size you guessed. It hangs on one number: the width of the opening at the rim, not the can’s height or its narrower base. Get that right and the bag folds over the edge, stays put, and ties off without a fight.
Fit comes down to two measurements — the opening’s width and the can’s height. These steps use the formulas described by Hefty’s sizing guide, Interplas’s fit blog, and Plasticplace’s measurement tutorial.
Measure the Top Opening, Not the Body
The opening governs width, because that’s what the liner stretches around. A tapered or dual-rim can is wider at the top than halfway down, so measuring the body understates the bag you need and produces liners that won’t clear the rim.
Stretch a tape measure straight across the widest point of the opening, lip to lip. For a round can, that one number is your diameter. For a square or rectangular can, you need the width and the depth front to back.
Turn the Opening Into a Bag Width
A trash bag is a flat tube, so its stated width describes the lay-flat sheet, not the circle it can wrap. Skipping this conversion is why a bag looks big enough on the box and still won’t fit.
- Round cans: measure the diameter, multiply by 3.14 to get the circumference, then divide by 2. A 24-inch opening gives a circumference of about 75 inches and a minimum flat bag width near 38 inches.
- Square or rectangular cans: add the opening’s width and depth together. A can 20 inches across and 16 inches deep needs roughly 36 inches of flat bag width.
Treat these results as minimums. Manufacturer guides suggest moving up to the next standard size when your measurement falls between sizes — an oversized liner is easier to deal with than one that splits at the seam.
Add Overhang Length So the Liner Ties Off
Measure from the inside floor of the can straight up to the rim, then add extra so the bag can fold over the lip and leave slack for a knot.
Plasticplace sums up why the method matters: a “100-gallon” label describes capacity, not fit. Two cans with the same gallon rating can have completely different opening shapes, and the bag only cares about the opening. If you’d rather skip the arithmetic, our roundup of the best 100 gallon trash bags lists flat widths and lengths so you can match them against your numbers directly.
Match Your Measurements to a Bag
Compare your two numbers against the flat dimensions printed on the box.
| Can Measurement | Formula | Bag Size to Buy |
|---|---|---|
| Round, 24 in. diameter | 24 × 3.14 ÷ 2 | At least 38 in. flat width |
| Round, 30 in. diameter | 30 × 3.14 ÷ 2 | At least 47 in. flat width |
| Square, 20 in. wide × 16 in. deep | 20 + 16 | At least 36 in. flat width |
| Square, 24 in. wide × 20 in. deep | 24 + 20 | At least 44 in. flat width |
| Any can, 36 in. tall to rim | 36 + 3 to 6 | At least 39–42 in. long |
| Any can, 40 in. tall to rim | 40 + 3 to 6 | At least 43–46 in. long |
| Measurement falls between sizes | Round up | Choose the next larger size |
Both formulas come from the sizing guides published by Interplas’s trash bag sizing guide, which lays out the round and rectangular conversions side by side.
Where the Fit Usually Goes Wrong
- Measuring the body instead of the rim. A tapered can is widest at the top, so a body measurement comes up short.
- Skipping the opening measurement entirely. Height alone tells you nothing about width, which determines whether the bag fits at all.
- Leaving out the overhang. A liner cut to exact can height has nothing left to fold over the rim or tie, so it slips down.
Oversizing has a limit: a liner far too big will bunch, slip down the sides, and waste material — so the next standard size up, not the largest bag on the shelf, is the right target.
FAQs
Do I measure the inside or outside of the rim?
Measure across the widest point of the opening, lip to lip, which is the outside edge on most cans. What matters is the full distance the bag stretches around. If your can has a thick rolled edge, include it.
What if my can isn’t perfectly round or square?
Use the widest span across the opening for width and the longest front-to-back distance for depth, then run the matching formula. Round up to the next standard size to cover gaps left by curves and corners.
Can I skip the math and just buy a 100-gallon bag?
You can, but the label alone won’t tell you if it fits. Two 100-gallon cans can have very different opening shapes, so compare the printed dimensions — flat width and length — against your measurements before buying.
References & Sources
- Hefty. “Standard Trash Bag Sizes: How to Find the Best Bag for Your Bin” Supports the measure-the-opening method and rounding up between sizes.
- Interplas. “What Size Trash Bag Do I Need?” Source of the round and rectangular bag-width formulas used in the table.
- Plasticplace. “What Size Trash Bag Do I Need? 2 Easy Steps” Supports the capacity-versus-fit distinction and the overhang allowance.
