Garbage Bag Size Calculator (bin volume & flat bag width × length)

Understanding the garbage bag size calculator — concept and principle

This calculator measures a bin and tells you the flat width and length of bag that will fit it, plus the bin's volume in litres and US gallons. Use it before buying bin liners for an odd-shaped bin, so the bag reaches the bottom and still folds over the rim.

A bag that wraps the bin: with the default inputs the result is 47 × 75 cm bag (for a 35.3 L bin).

How it is calculated

Volume: round bin = π × (diameter ÷ 2)² × height; rectangular bin = width × depth × height. 1 litre = 1,000 cm³; US gallons = litres ÷ 3.785.
Flat bag width: round bin W = π × diameter ÷ 2; rectangular bin W = width + depth.
Bag length = height + bottom allowance + overhang, where the bottom allowance is half the diameter (round) or half the shorter side (rectangular).

The principle

A flat bag of width W opens into a tube whose perimeter is 2W, because it has a front and a back. To fit round the rim, that perimeter must be at least the rim's perimeter, so W must be at least half of it: π × d ÷ 2 for a circle, width + depth for a rectangle. The sealed bottom of the bag has to spread across the floor of the bin, which uses up about half the diameter of length, and the overhang folds over the rim to hold the bag in place. Bags are sold by flat width × length, so these numbers can be compared directly with the packet.

Worked example

Default inputs: round bin, 30 cm diameter, 50 cm high, 10 cm overhang.

Volume = π × 15² × 50 ÷ 1,000 = 35.3 L = 9.3 US gallons.
Width = π × 30 ÷ 2 = 47 cm (18.6 in).
Length = 50 + 30 ÷ 2 + 10 = 75 cm (29.5 in).

Each point is the calculator re-run with only “Diameter (round) or width (cm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Diameter (round) or width (cm)	Bin volume (litres)
7.5	2.2
15	8.8
22.5	19.9
30	35.3
37.5	55.2
45	79.5
60	141.4

How to read the result

Choose a bag at least this wide and long. A slightly larger bag is fine; a narrower one will tear or slip in. The volume figure helps when bags are labelled in litres or gallons, but the flat size is the more reliable guide, because the same volume label can come in different shapes. For bins that taper, measure the top rim.

Common mistakes

Using the bin's diameter as the bag width; the bag must be about 1.57 times wider.
Measuring the bottom of a tapered bin instead of the rim.
Forgetting the overhang.
Choosing by litres alone.

Keep in mind: Bag thickness and strength matter as much as size for heavy or sharp waste. Check the product's load rating.

Frequently asked questions

What size bag fits a 30 cm round bin?

About 47 cm wide and 75 cm long with a 10 cm overhang.

How do I measure a bin for a bag?

Measure the rim's diameter (or width and depth) and the inside height.

Why is the bag wider than the bin?

A flat bag's width is half its opened perimeter, so it must be half the rim's circumference.

Sources: Elementary geometry of cylinders and boxes (any geometry textbook) · NIST Handbook 44, Appendix C (1 US gallon = 3.785411784 L)

Checked 9 Nov 2026. Geometry: cylinder and box volume, perimeter of the bag opening (flat width × 2) and bottom allowance; 1 US gallon = 231 in³ = 3.785411784 L (exact). The overhang is an example value.

Your numbers