How it works
Dog shoes, booties and boots are all sized the same way underneath the marketing name: by paw width, not by weight and not by breed. Enter the widest point of the front paw and, if the back paws look different by eye, the back paw too. Each gets its own size, because plenty of dogs genuinely need two.
The width has to come from a paw that is bearing weight. A paw splays when the dog stands on it, noticeably enough to change the size, so a paw measured while it is lifted or while the dog is lying down reads narrower than the paw that will actually be wearing the boot, and the boots come in too small.
The formula
Front and back are looked up separately, and either can land in a different band
Each size covers a published range of paw widths, and the ranges share their edges rather than leaving gaps: a paw exactly on the line between two sizes belongs to the larger one. If the front and back paws fall in different bands, both are reported, since a dog wearing one size on the front paws and a smaller size on the back paws is the common case rather than the exception.
Worked example
Back paw: 2.25 in (57 mm), which falls inside the S band of 2.0 in (51 mm) to 2.5 in (64 mm).
Result: M front, S back. The back paw is a size smaller, which is the usual pattern, since front paws carry more of a standing dog's weight.
How to measure a paw for boots
Stand the dog on a sheet of paper. Lift the opposite front paw, not this one, so the paw you are measuring takes the dog's full weight and splays the way it will inside a boot. Mark the paper at the widest point on each side of the paw, then measure the distance between the marks. Trim the nails first if a maker asks for a length measurement too, since long nails add to that number without adding to the paw underneath them.
No ruler handy? A credit card is 3.37 in (85.6 mm) wide, fixed by ISO/IEC 7810, the same standard behind the card trick used elsewhere on this site. Mark the paper as above, then lay the card against the marks and read off the width in card-widths, or use its edge as a straightedge against a tape.
Why front and back differ
A standing dog carries more of its weight on the front legs than the back ones, which is also why front paws tend to be built a little wider. The difference is not universal and not huge, but it is common enough that a lot of returns trace back to buying one size and assuming it covers all four paws.
If a maker sells boots in pairs, buy the correct pair for the front and the correct pair for the back. If they only come as a set of four in one size, fit that size to the front paws, since a boot too big up front is the one doing most of the work, and expect to cinch the back boots a notch tighter to compensate.
Why the advice here is to go down
Most sizing advice on this site says the opposite: when you are between sizes, size up, because a loose shoe is fixable and a short one is not. A boot flips that. A boot with room to spare rotates on the paw within the first few steps and is often gone within minutes, while a snug one stays where it was put.
The part that should feel firm is the strap, not the boot itself: tight enough that the boot cannot be pulled off, loose enough that a finger still slides underneath it, and never sitting on the dewclaw. A high stepping walk for the first few minutes is normal and is not the signal to watch. A boot that rotates or slips is.