Half the way round, not all the way across
A flat liner has two faces. Open it into a tube and the tube circumference is twice the flat width, so the flat width a can needs is half the distance around the can. For the 22 inch round can in the defaults that is pi times 22, halved: 34.56 inches. Not 22, and not 44.
Rectangular cans are tidier. Half the perimeter of a rectangle is one width plus one depth, so a 15 by 11 inch can wants a 26 inch flat bag. That rule is easy enough to do in the aisle and it is still routinely skipped in favour of a gallon number on the box, which describes neither dimension of the bag and only loosely describes the can.
The length is three things added up
Thirty inches of can height, four inches of overhang past the rim so the bag does not disappear inside when it fills, and something for the bottom. The bottom is the awkward one: a flat-seamed bag has a straight seam across its base and the can has a round floor, so the bag gathers instead of folding into the corner, and that gathering eats length. Roughly half the diameter is a reasonable starting point — 11 inches here — which gives 45 inches of length.
So the 22 by 30 inch can wants a 34.56 by 45 inch liner, and you buy the next stock size up in both directions.
What the wrong size costs
The default comparison is a 43 by 48 inch liner, which is a normal thing to find in a closet next to a can this size. It fits, comfortably: 8.4 inches of spare width and 3 inches of spare length. It is also 2,064 square inches of film against the 1,555 the can needs — 33 percent more, and the actual overhang past the rim comes out at 7 inches rather than the 4 you wanted, which is a bag flapping around the outside of the can.
That 33 percent is a rough proxy and it should be read as one. It holds only if the two liners are the same film at the same thickness, and thickness is exactly where liners differ most. A thinner bag that tears on a corner costs far more than the film it saved. The comparison is worth making because a third is a large number, not because a third is a saving you can bank.
The route arithmetic
Forty-six cans emptied five times a week is 230 liners a week and 11,960 a year — 59.8 cases of 200, so 60 to buy, and $2,040 at $34 a case. That is $44.35 a can per year, or 17 cents a liner. Numbers like these are why a two-cent difference per liner is worth a phone call and a four-inch difference in bag width is not, and it is useful to know which of the two you are actually arguing about.
Measure one, then stop estimating
Every figure above the route section is geometry except the bottom allowance, which is a guess dressed as a formula. The way to replace it takes two minutes: put one liner in one can, pull it down properly the way your own people do, and measure what hangs over the rim. Subtract that from the liner length, subtract the can height, and what is left is your bottom allowance for that combination of film and can. It will not match half the diameter, and after that it does not have to.
Questions people ask
What size trash bag fits a 22 inch round can?
Flat width is half the circumference: pi times 22, halved, is 34.56 inches. Length is the can height plus the overhang you want plus what the bag loses gathering across the bottom — 30 plus 4 plus about 11 is 45 inches. So a 35 by 45 inch liner, buying up to the nearest stock size in both directions.
Why is the liner width not the same as the can diameter?
Because a flat bag has two faces. Opened into a tube its circumference is twice the flat width, so the flat width has to be half the way round the can, which is pi times the diameter divided by two — about 1.57 times the diameter. A bag the width of the can cannot open into it at all.
How do I size a liner for a square or rectangular can?
Half the perimeter, which for a rectangle is simply the width plus the depth. A 15 by 11 inch can wants a 26 inch flat bag. The length is the same three-part sum as for a round can: height, plus overhang, plus a bottom allowance of roughly half the shorter footprint side.
Does the gallon rating on the box tell me what will fit?
Not reliably. A gallon rating is about the volume a bag can hold when it is filled, not about the two flat dimensions that decide whether it goes into a particular can and hangs over its rim. Two liners with the same gallon number can have quite different width and length combinations, which is why closets end up with three sizes that all half-fit.
Is a thinner liner cheaper?
This page cannot answer that. It compares film area, which is only a proxy for cost and only holds if two liners are the same material at the same thickness — and thickness is where liners differ most. Whether a given liner is strong enough for what goes in it is on the case and in your own experience of it tearing or not, and nothing here is a claim about strength.