How this calculator works
A bag sold flat opens into a tube. Laid flat its width wraps half of the rim, so the flat width you need is half the perimeter of the bin. For a round bin that is half of pi times the diameter; for a square it is twice the side; for a rectangle it is the sum of the two different sides. Height works differently. The bag has to reach the bottom of the bin, which costs the bin height, then fold back up and down the outside, which costs the overhang you want, and its folded bottom sits half the narrower bin dimension below the rim, so that amount is added as well. Buy the next standard size up if the figures land between two sizes.
Formula
flat bag width = rim perimeter / 2. Rim perimeter is pi x diameter for a round bin, 4 x side for a square, and 2 x (length + width) for a rectangle. Bag height = bin height + overhang + (half the diameter, half the side, or half the shorter side).
Worked example
A round bin 30 cm across and 45 cm tall with a 10 cm overhang has a rim of pi x 30 = 94.2 cm, so the bag must be 47.1 cm wide. Its height needs 45 + 10 + 15 = 70 cm. A rectangular bin 50 by 35 cm needs 85 cm of flat width and 45 + 10 + 17.5 = 72.5 cm of height.
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Frequently asked questions
Why is the bag width only half the rim?
Because the bag is folded flat. One side of the flat bag becomes the front of the rim and the other side becomes the back, so the flat width covers half the perimeter once opened.
Do bag sizes on the box match this figure?
Usually. Bags are sold by flat width and height, so a 47 cm by 70 cm bag lines a 30 cm round bin with a generous overhang. Sizes are sold in steps, so round up.
How much overhang should I allow?
About 5 cm to 10 cm gives enough to hold the bag without it slipping in. Add more if the bag will be tied closed over a full load.
Why add half the bin width to the height?
The bag is a tube with a seam at the bottom, not a flat sheet. When it drops to the bottom of the bin, the seam sits half the bin width below the rim, so the bag must be that much longer to reach the floor of the bin and still fold over the top.