How this instrument works
A flat garbage bag opens out to roughly double its folded width, so a bag's stated 'width' only needs to cover about half the can's perimeter, not the whole thing — that's why this calculator's width formula for a rectangular can is simply length plus width (roughly half of length+width+length+width), rather than the full perimeter. For a circular can, the equivalent figure is half the circumference, calculated as diameter times pi, divided by two.
Bag height needs more than just the can's height: it needs enough extra length to gather and tie off at the top (the overhang allowance you enter), plus a bit more to account for the bag's flattened bottom corners taking up some of its own length once it's actually sitting inside a can and molded to the shape. For rectangular cans that extra allowance is one-quarter of the bag's width; for circular cans it's half the can's diameter — different fractions because the geometry of a flattened corner differs between a square-ish gusset and a round one.
This is a practical sizing heuristic that lines up with how commercial trash bag sizes are typically labeled, not a strict engineering specification — when your calculated size falls between two commonly stocked bag sizes, sizing up is the safer choice, since a slightly oversized bag can still be gathered and tied, while an undersized one may not stretch to cover the can at all.
- Choose rectangular/square or circular can shape.
- Enter your can's length and width (for rectangular/square cans) or its diameter (for circular cans).
- Enter the can's height.
- Enter how much overhang or tie-off allowance you want at the top.
- Read the recommended bag width and height.
Worked example — a 15×12×20 in kitchen trash can
A rectangular can 15 in long, 12 in wide, 20 in tall, with a 5 in overhang: bag width = 15 + 12 = 27 in. Bag height = 20 (can height) + 5 (overhang) + 27 ÷ 4 (6.75 in for the bottom-corner allowance) = 31.75 in.
A round 14 in-diameter can of the same 20 in height and 5 in overhang instead uses the circular formulas: bag width = 14 × π ÷ 2 ≈ 21.99 in, and bag height = 20 + 5 + 14 ÷ 2 = 32.00 in — noticeably different numbers from the rectangular case even at a similar overall can size, which is exactly why shape matters as an input here.
Questions
Why does the height formula divide by 4 for rectangular cans but by 2 for circular ones?
Both fractions are approximating how much of the bag's length gets consumed by its bottom corners folding to match the can's shape once it's actually in place, and that fold behaves differently for a roughly square-cornered gusset versus a round bottom — the width-over-4 and diameter-over-2 figures are the commonly used approximations for each shape respectively, not derived from the same single constant.
How much overhang should I leave for tying off the bag?
A few inches is typical for a simple gather-and-tie closure — enough to bunch the excess material together without so much overhang that it becomes unwieldy. If you use drawstring bags or twist ties rather than a hand-tied knot, you can generally use a smaller overhang allowance than a bag meant to be tied by hand.
What if my calculated size falls between two standard bag sizes?
Round up to the next commonly available size rather than down — an oversized bag can still be gathered and tied off with a bit of extra material bunched at the top, while an undersized bag may not stretch far enough to properly cover the can's rim at all.
Does this work for both small kitchen bins and larger outdoor cans?
Yes — the same proportional formulas apply regardless of scale, so you can use this calculator for anything from a small bathroom bin to a large outdoor municipal-size trash can, as long as you enter that specific can's actual dimensions.
Why is a garbage bag's stated "width" roughly half the can's perimeter instead of the whole thing?
A flat, folded garbage bag opens up into a roughly cylindrical or rectangular shape once in use, and its flat folded width corresponds to about half the can's perimeter (or half its circumference, for a round can) — the bag effectively 'unfolds' to wrap the rest of the way around, which is why the width formula doesn't need to account for the full perimeter directly.