How dog sweater sizing works
A sweater is not sized on one number, because it has to do two jobs at once: pull over the head and around the ribcage, then hang the right length down the back. Most dogs give you a different size on each measurement, and every published chart quietly picks one to trust without saying so. This page tells you which one and why.
The short answer is chest girth, because a pullover has to clear the head and the ribcage in the same motion, and a knit that is tight there chafes behind the front legs on every wear. A sweater that runs a little short is just a short sweater. A sweater that will not go over the ribcage is one you send back.
The formula
Chest girth → a size from the chest chart
If they agree: that size.
If they are one size apart: the chest size wins.
If they are two or more sizes apart: standard lettered sizing will not fit well; buy on the numbers.
Both charts run XS through XL. Back length bands are 8 to 10 in (XS), 10 to 14 in (S), 14 to 18 in (M), 18 to 22 in (L), and 22 to 28 in (XL). Chest girth bands are 12 to 16 in (XS), 16 to 22 in (S), 22 to 28 in (M), 28 to 34 in (L), and 34 to 42 in (XL), the same chest bands our dog harness size chart uses, since a ribcage is a ribcage regardless of what is going around it.
Worked example
How to measure
Back length runs from the base of the neck, roughly where a collar sits, to the base of the tail, not the tip. Keep the dog standing square. Measuring from the skull instead of the neck is the single most common error here, and it adds a few inches that sends you up a whole size for no reason.
Chest girth is the widest part of the ribcage, just behind the front legs, with the tape snug against the fur but not compressing it. Our dog harness size chart walks through the same measurement in more detail if you want a second look at it. No tape measure in the house: run a piece of string around the dog and measure the string against a ruler instead.
Hard to fit builds
A few common shapes throw the two measurements far enough apart that a single lettered size never quite works, and it is worth knowing which one is lying to you before you order.
Long and low (dachshund, corgi, basset): back length reads high while chest girth reads low. The chest number is the one to trust for whether the sweater goes on at all; look for a maker that sells a specifically long-body cut rather than sizing up the whole garment, which just adds width you do not need.
Deep chest, tiny waist (greyhound, whippet): chest girth reads high relative to back length, so a sweater sized to the chest can sit loose and slide back over the narrow waist behind it. A garment with its own waist shaping, rather than a straight tube, holds its position far better on this build.
Broad chest, short back (bulldog, pug, French bulldog): chest girth again reads high against a short back, but here the risk is the opposite of sliding: a tight neck and armhole opening on a barrel-chested dog. Favor a wider neck opening and check the armholes specifically rather than trusting the size letter to have accounted for the shape.
Where a sweater actually goes wrong
People worry about length and the sweater almost never fails there. It fails at the armholes, where a tight knit rubs with every stride, and at the belly, where a too-long body panel drags and gets soiled. Measure the dog's current coat, not a groomed or clipped version of it: a groomed double coat and an ungroomed one can differ by close to an inch of apparent girth, enough to move a size.
Compare that to a woven coat, which is a different garment with a different failure point. A coat does not stretch, closes with straps or a zip instead of pulling over the head, and the dangerous direction there is a tight shoulder that shortens the dog's stride rather than a rubbing armhole. Our dog coat size chart covers that garment and explains why coat makers pick a different primary measurement depending on how the coat closes.