How do I measure my horse for the right bit size?

Bit size is the mouthpiece width in inches, and the easiest way to measure it is the string method: run a piece of smooth string through your horse's mouth where the bit sits, mark where it exits at each corner of the lips, and measure between the marks. Add about 1/4" to that measurement for a loose ring bit (so the rings don't pinch), or use the measurement as-is for fixed cheeks like an eggbutt, D-ring, or full cheek, which should sit snug against the lips without pressing.

Common sizes: 4 1/2" for small ponies, 5" for most horses (by far the most common), 5 1/2" for larger warmbloods and drafts crosses, 6"+ for drafts. A correctly sized bit shows about 1/8"–1/4" of mouthpiece on each side of a jointed loose ring, and creates one to two gentle wrinkles at the lip corners when the bridle is adjusted.

Too wide is as bad as too narrow: an oversized jointed bit droops in the middle and hits the bars unevenly, while an undersized one pinches the cheeks. If your horse suddenly resists a bit that used to fit fine, check teeth with your vet or equine dentist before shopping — sizing rarely changes on its own.