Tack Fitting Calculator

Gullet width measures the space between the saddle tree's points at the withers, so it purely reflects how well the tree matches the horse's shoulder and wither shape — narrow under 6", medium 6-7", wide 7-8", extra-wide 8"+. Seat size, by contrast, is about fitting the rider rather than the horse, so this calculator derives it purely from rider weight in half-inch steps at the 120, 150, and 180 lb thresholds. The two dimensions are independent: a horse needing a wide tree could be paired with anything from a 15" to an 18" seat depending on who's riding it.

It applies the common 20%-of-bodyweight guideline to a horse weight that's itself estimated from wither height alone (roughly hands × 100 − 400 lb), so two horses of the same height but very different bone density or body condition get identical numbers even though their true carrying capacity differs. Treat it as a rough screening check, not a precise limit — a scale-measured horse weight would give a materially more accurate result.

Both are approximated as simple functions of height because head and mouth size roughly scale with overall horse size: bridle size steps from Pony to Full to Oversize at the 14.2 and 16.2 hand marks, and bit width scales linearly (height × 0.33 + 0.3, rounded to a tenth of an inch). This is a shortcut — an individual horse's head or mouth can be proportionally larger or smaller than its height suggests — so it's meant to get you in the right size range before trying an actual bit or bridle on the horse.

The calculator treats the horse's measured back length (wither to last rib) as the panel's maximum allowable length, since a saddle panel extending past roughly the 18th rib puts pressure on the lumbar area behind where the horse's back can safely bear weight. It's a ceiling, not a target — a well-fitted panel is usually somewhat shorter than this maximum, and the exact placement still depends on the horse's individual back shape and the saddle's own panel design.