Horse Bit Size Calculator

Fill in the fields above and press Work out the bit size.

The same mouth takes three different widths

This is the part that catches people out. You measure the mouth once, but the bit you should buy depends on how it is built, because a jointed mouthpiece curves inside the mouth and takes up space that a solid bar does not.

What you are fitting	Allowance over the mouth width	Why
Solid / mullen mouthpiece	+0 mm	It sits straight, so the bit is the mouth size
Semi-broken or flexible	+5 mm	The joint has some movement and needs room to settle
Fully broken — single or double jointed	+10 mm	It curves around the tongue and relaxes down in the mouth
Fixed cheek — eggbutt, D-ring, full cheek, Pelham	+0 mm	A fixed cheek is meant to sit snug against the lips
Loose ring	+5 mm	2.5 mm each side so the ring can turn without pinching
Bit rubbers or gel guards	+3–5 mm	They sit between the lip and the ring and take up real width

Add them up. A 135 mm mouth takes a 135 mm solid bar with a fixed cheek, and a 150 mm single-jointed loose ring with rubbers — three sizes apart, same horse, same tape.

How to measure the mouth, not the old bit

Measure from the outside of one lip to the outside of the other, with the bit out of the mouth. Most people instead measure the bit they already own, which is the wrong number twice over: it includes the cheekpieces on some designs, and it reproduces whatever guess was made when that bit was bought.

Two workable methods:

A bit measure — a purpose-made gauge that slides into the mouth and reads the width directly. Fastest and least error-prone.
A soft tape or a marked stick — run it across the mouth at the point where the bit sits, mark where the lips fall on both sides, then measure between the marks. Easier with a second person holding the horse’s head still.

Do not add anything “for room” at this stage. The allowances above are the room, and adding more on top lands you a size too wide. A bit that is too wide slides from side to side and is no more comfortable than one that pinches — it just fails differently.

Why the inch sizes do not agree

European bits are made in 5 mm steps and labelled in millimetres. American and British bits are labelled in inches, usually in quarter-inch steps. Millimetres do not divide neatly into quarter inches, so each maker published its own conversion — and they do not match.

Millimetres	Sprenger table	Bombers table
110 mm	4 1/4"	4 1/2"
115 mm	4 1/2"	4 3/4"
125 mm	5"	5 1/4"
135 mm	5 1/4"	5 3/4"
150 mm	6"	6 1/2"

From 110 mm upward the two columns drift apart, and by 150 mm they are half an inch out. So an inch figure is not a safe instruction to carry between brands: buy the millimetre size, and treat the inch label as a description of the maker’s own range.

Thickness is a separate decision

Width is not the whole fit. The common mouthpiece thicknesses are 14, 16 and 18 mm, and the space between the upper and lower jawbones varies a great deal between horses. A thick mouthpiece in a small mouth presses on the palate, which shows up as head tossing, an open mouth or snatching at the rein — symptoms that are often blamed on the rider’s hands.

The practical test is to put two fingers together in the gap where the bit sits. Pressure on both fingers means the mouth wants the thinner end of the range, 14–16 mm. Little or no pressure means there is room for 16–18 mm.

Both decisions belong with your dentist or bit fitter rather than an online calculator. What this page does is get the width right, which is the figure most people get wrong by a whole size.

Frequently asked questions

What size bit does my horse need?

Measure from the outside of one lip to the outside of the other, then add the allowance for the bit you are fitting: nothing for a solid mullion with a fixed cheek, 5 mm for a semi-broken mouthpiece or a loose ring, 10 mm for a fully jointed mouthpiece, and 3–5 mm for rubbers. Most adult horses land between 125 and 145 mm.

Is 5 inch the same as 125 mm?

In Sprenger’s table, yes — but Bombers calls 125 mm 5 1/4". The two makers’ inch tables disagree from 110 mm upward, which is why the millimetre size is the one worth remembering.

Should I size up so the bit is not too tight?

No. The allowances already account for the room the design needs. Going a size wider than the calculation gives you a bit that slides side to side, which causes its own rubbing and makes the aids less clear.

Can I use my old bit to find the size?

Only as a cross-check. An old bit often includes the cheekpieces in the measurement, and if it was bought by guesswork you would be copying the original mistake. Measure the horse.

Related tools and guides

Sources and method

Sprenger size guide — the millimetre-to-inch table, the 5 mm loose-ring clearance rule, the note that a fixed cheek sits snug against the lips, and the two-finger thickness test.
Bombers sizing guide — the mouthpiece allowances (+5 mm semi-broken, +10 mm broken, +3–5 mm for rubbers), the cheekpiece allowance, and their own millimetre-to-inch table.
Buttery Horse Co. / Bombers fitting guide — the lip-to-lip measurement method, and the warning that mouthpiece type and rubbers change the width required.

Page last reviewed 2026-10-16. Results are estimates from published formulas and are not veterinary advice.