Bin liners are sold by a gallon rating that has nothing rigorous behind it, in flat dimensions that most packaging never explains, and in a shape that only makes sense once you realise a bag is a flattened tube. This garbage bag size calculator takes the measurements of your actual bin and returns the flat width and length a bag needs in order to line it, fold over the rim and still reach the corners.
Arb Digital publishes this alongside the other everyday measurement tools our team builds, because the problem is genuinely annoying and genuinely arithmetic. Once you understand that the flat width of a bag is half its circumference, the whole thing becomes a two-line calculation you never have to guess at again.
What This Garbage Bag Size Calculator Does
Enter the shape and internal dimensions of a bin and it returns three things: the flat width the bag must have, the flat length it must have, and the bin's true capacity in litres and US gallons. It handles rectangular bins, straight-sided round bins and tapered round bins, and it lets you set the overhang, the width ease and the bottom allowance yourself, because those are preferences rather than facts.
It publishes no product list and no brand sizes. Bag dimensions and ratings are set by whoever makes them and vary considerably, so the tool gives you the numbers to compare against a manufacturer's own stated flat size.
How to Use It
- Pick the bin shape and measure inside the bin, at the rim, rather than around the outside of the moulding.
- Enter the footprint and height. A tapered bin needs both the top and base diameters; a straight round bin needs only the top.
- Set the overhang — how far you want the bag to fold over the rim. Three to five inches is a common preference.
- Set the width ease. Ten per cent stops the bag being stretched drum-tight; zero gives the mathematical minimum.
- Leave the bottom allowance at zero to let the tool derive it, or enter your own if you are working to a particular bag style.
The Formula: A Bag Is a Flattened Tube
A bin liner is a tube of film sealed at one end and pressed flat. Flatten a tube and its width becomes half its circumference, because both walls lie on top of each other. So the flat width you need is the rim perimeter divided by two, plus whatever ease you want.
Rectangular bin: perimeter = 2 × (length + width), so flat width = (length + width) × (1 + ease).
Round bin: circumference = π × diameter, so flat width = π × diameter ÷ 2 × (1 + ease).
Tapered bin: use the larger of the two diameters, because the bag has to pass the widest point.
Length is simpler: bag length = bin height + overhang + bottom allowance. The bottom allowance exists because a flat bag has no gusset and no shaped base. To settle into a rectangular bin's corners it has to consume some of its own length, and the amount is roughly half the shorter footprint dimension. For round bins a quarter of the diameter is the equivalent, and this tool uses those defaults when you leave the field at zero.
Worked through with the defaults — a 15 by 12 inch bin, 26 inches tall, 4 inches of overhang and 10 per cent ease: perimeter is 2 × 27 = 54 inches, half of that is 27, and with ease the flat width is 29.7 inches. The bottom allowance is 12 ÷ 2 = 6 inches, so the bag length is 26 + 4 + 6 = 36 inches. A 29.7 by 36 inch flat bag lines that bin properly. Capacity is 15 × 12 × 26 = 4,680 cubic inches, which is 76.7 litres or 20.3 US gallons.
Why the Gallon Rating on the Box Is Not Your Bin's Capacity
This is the single biggest source of confusion in the category. A bag labelled 30 gallon is not certified to hold 30 gallons of anything, and there is no standard that says what a 30 gallon bag must measure. It is a marketing size class, and two manufacturers' 30 gallon bags can differ by several inches in both dimensions.
Worse, the rating describes an idealised bag volume, not the bin it is meant to line. A bag has to be larger than the bin's capacity because a lot of its volume is consumed by the overhang, the corners it cannot reach, and the slack that lets it be tied. Matching your bin's gallon capacity to a bag's gallon rating therefore gives you a bag that is too small more often than not.
The reliable approach is dimensional. Work out the flat width and length you need, then look for those two numbers on the packaging or the manufacturer's specification. They are almost always printed somewhere, usually in small type, and they are the only figures that mean anything.
Ease, Overhang and Why Bags Tear
A bag stretched tight across a bin rim is a bag under tension before anything has been put in it. Every sharp item then has a film that cannot deform to absorb it, which is why an exactly-sized bag splits at the shoulder while a slightly loose one survives. Ten per cent width ease is a modest allowance that costs nothing and helps a lot.
Overhang does two jobs. It holds the bag in place so it does not fall into the bin when something heavy lands in it, and it gives you something to grip when tying. Too little and the bag drops; too much and you are paying for film that gets thrown away. Three to five inches suits most household bins, and taller kitchen bins with a lid mechanism sometimes need more so the bag clears the mechanism.
Film thickness is the other half of tear resistance and is not something this calculator addresses. It is quoted in mils or microns and differs enormously between a light office liner and a contractor bag. A correctly sized bag in film that is too thin will still fail.
Bin Capacity, and What Actually Goes In It
The capacity figures here are the geometric volume of the bin, which is more than it will ever hold in practice: waste does not pack perfectly, and nothing gets filled to the rim. Treat the number as an upper bound for comparing bins rather than as a quantity of rubbish.
For context on how much household waste that represents, the US Environmental Protection Agency's National Overview: Facts and Figures on Materials, Wastes and Recycling puts municipal solid waste generation in the United States at 4.9 pounds per person per day in 2018, up from 2.68 pounds in 1960. That is a mass figure rather than a volume, and the density of household waste varies wildly, but it explains why bin sizing is a recurring domestic question rather than a one-off.
The volume conversions used here rest on exact definitions: one US gallon is 231 cubic inches, which is exactly 3.785411784 litres, and one cubic inch is exactly 16.387064 cubic centimetres. NIST's Unit Conversion page points to the conversion factor tables that carry these, with the standing instruction to use exact factors whenever they exist.
Measuring Awkward Bins
Most kitchen bins are neither a clean rectangle nor a clean cylinder. Some practical rules: for a bin with rounded corners, measure length and width as if it were rectangular — the rounding slightly reduces the perimeter, so you end up with a touch more ease rather than too little. For a strongly tapered bin, always use the widest diameter, since a bag that will not pass the rim is useless regardless of how well it fits the base.
For an oval or D-shaped bin, measure the actual perimeter with a tape run round the inside of the rim and divide by two. That is the general form of the rule, and every shape-specific formula above is just a shortcut to the same number. If you need the volume of an awkward shape, the cylinder volume calculator handles round bins directly and the grain bin capacity calculator deals with tapered cylindrical vessels on a larger scale.
To move the resulting figures between unit systems, the volume converter and the litres to gallons converter do the direct arithmetic, and the rectangle area calculator covers footprint area if you are working out whether a bin fits a cupboard.
Arb Digital builds free calculators that solve a real everyday problem properly. Browse the library, or get in touch about a set built for your audience.
Browse Free Tools Get in TouchCommon Mistakes to Avoid
- Matching bin gallons to bag gallons — bag ratings are unstandardised marketing sizes and are not your bin's capacity.
- Using the bin's width as the bag's width — flat bag width is half the perimeter, which is much more than one side.
- Forgetting the bottom allowance — a flat bag consumes length reaching into corners, so height plus overhang alone is short.
- Measuring the outside of the bin — mouldings, ribs and handles add width the bag never has to cover.
- Sizing a tapered bin from its base — the bag has to clear the widest point, which is the rim.
Related Free Tools From Arb Digital
Work out volumes with the cylinder volume calculator and the grain bin capacity calculator, convert them with the volume converter or the litres to gallons converter, and check footprints with the rectangle area calculator. The free online tools hub lists the rest.
Frequently Asked Questions
Measure the inside of the bin at the rim. The flat bag width is half the rim perimeter plus a little ease, and the bag length is the bin height plus the overhang you want plus a bottom allowance so the bag can reach the corners.
Because a bag is a tube of film pressed flat, so both walls lie on top of each other. A tube with a 54 inch circumference measures 27 inches across when flattened.
Usually not. Bag gallon ratings are unstandardised size classes describing the bag rather than the bin, and much of a bag's volume is consumed by overhang and slack. Compare flat dimensions instead of gallon labels.
A flat bag has no gusset or shaped base, so it has to use some of its own length to settle into the corners of a bin. This tool defaults to half the smaller footprint dimension, or a quarter of the diameter for a round bin.
That is a preference rather than a rule. Three to five inches is common for household bins — enough to hold the bag in place and to grip when tying, without wasting film. Bins with a lid mechanism sometimes need more.
Inside, at the rim. External mouldings, ribs and handles add width that the bag never has to cover, and sizing from them produces a bag that is loose and prone to slipping.
No. Film thickness, quoted in mils or microns, governs tear resistance and is entirely separate from fit. A correctly sized bag in film that is too thin will still split.
