What size bit does my horse need?

In the US and UK it is quoted in inches, and many European makers use millimeters. Two other numbers decide how a bit sits: the thickness of the mouthpiece and the size of the rings.

Getting it wrong costs something either way. Horse & Hound's piece on measuring says a bit that is too narrow squeezes the lips and cheeks, and one that is too wide can slide from side to side and bruise the inside of the mouth. Riders ask about size in several forms: what sizes a bit comes in, what the ring measurement is, and what fits a tall horse. This page takes those in order.

Here are the three measurements, with the numbers the sources give.

Horse & Hound describes a bit sizer as an inexpensive plastic gauge that slides into the mouth and shows the size. Some have a nylon strap that goes over the head, which gives a reading that reflects how high the bit sits. Some makers also supply gauges for their own bits, and some will advise on size from a photo of the bit in the mouth.

No gauge? Mark the string at each side of his mouth, then measure between the marks along a ruler. Bring a helper to hold him, keep your fingers clear of his teeth and stop if he objects. A horse who will not let anyone near his mouth is a question for your vet or equine dentist, not something to work around.

Treat the number as a starting point. Horse & Hound's expert says bits with more curve or a different thickness fit differently, so 'my horse is a 5 1/2' is only a rough guide.

For a loose ring, measure from the inside of the hole the ring passes through. Horse & Hound suggests a rigid ruler rather than a floppy tape, and a digital caliper for thickness. Without one, lay the bit on a ruler on a hard surface and read the diameter from above, taking the widest point next to the cheek if the mouthpiece tapers.

Makers' small print differs, so read the chart for the style you want. On our Happy Mouth bits page, kimberwick styles measure 1/4 inch less than the listed size because of the cheek construction. On our Korsteel page, a jointed bit is said to sit differently from a straight one, so a measurement between sizes calls for the larger width.

Converting is simple: multiply inches by 25.4 for millimeters. Rounded, that gives the table below. Nathe lists widths from 110 to 145 mm in 5 mm steps, so a 135 mm bit is just over 5 1/4 inches.

Horse & Hound's expert suggests standing in front of him with a little rein contact and looking at the clearance. As a very general guide, she wants about an index finger's width.

For loose rings, the same source says the horse's lips should clear the ring holes by about 1/8 inch on each side, to prevent pinching. If you can see half an inch or more, the bit is too big. When you turn a mouth measurement into a bit size, the guidance on our Korsteel page is to add about a quarter inch on each side so the bit rests without pinching or sliding. As a worked example, if the string measures 5 inches between the marks, that guidance points to a 5 1/2 inch bit (5 plus a quarter inch on each side). You would then check the clearance at his lips before you rely on it. Or say his mouth measures just over 5 inches and the chart offers 5 and 5 1/2: Horse & Hound would have you try the 5 1/2, then look at the loose ring holes. If half an inch or more of lip shows beyond them, it is too big.

The sources do not agree on which mistake is likelier. The 2006 Horse & Rider how-to says that for snaffle mouthpieces wider is often better. Horse & Hound's expert says fitting a bit too big is perhaps the most common fault she sees, and that a roomier bit is not a kinder one. Both agree that the bit has to fit this particular horse, so check it in his mouth, not just on paper.

Horse & Hound's expert calls 16 mm the most common, 12 to 14 mm very fine and 20 mm very generous, which is more usual on a mullen mouth or a rubber mouthpiece.

Nathe sorts its bits by thickness and says a thinner mouthpiece leaves more room in a small mouth, while a thicker one fills it more. On our Nathe bits page, thickness is the number in the style name: 15, 16, 18 or 20 mm. On our JP Korsteel page, the suggestion for a horse with a very thick tongue or a low palate is to look at a thinner or French link mouthpiece rather than a heavyweight bar. Those are maker suggestions, not rules, and horses vary.

Rule books can set a minimum. USEF dressage rule DR121 requires a snaffle mouthpiece to measure at least 10 mm where it meets the rings, or 12 mm in Young Horse and Children's classes, with an exception for ponies. We go through the rest of that rule in Is this bit legal for dressage?.

A different design can change which size is right, so check again whenever the bit's shape changes.

Height does not decide mouth width. 'What size bit for a 16.2 horse' is a search we see often, and none of the sources we read gives a bit size by height. Horse & Hound says the old rule of 4 1/2 inches for a pony, 5 for a horse and 5 1/2 for a draft horse does not hold, because mouth sizes and shapes vary too much. Measure him.

Design matters too. A bit with more curve, a different link or a different cheek can fit differently at the same listed width, which is why the maker notes in the sections above exist. If the rest of your tack is fitted by measurement, What size girth does my horse need? shows the same measure-then-check approach.

This is what our pages list for four makers. They are listed ranges, not recommendations for any horse.

Wikipedia's article on bit mouthpieces makes the point that a heavy-handed rider can make even a bit labeled mild uncomfortable, and a skilled, light rider can use a much stronger mouthpiece without distressing the horse.

If he fusses in a bit that measures right, have your vet or equine dentist check his mouth before you work through a box of bits. Then change one thing at a time. The next two pages in this series cover what the cheek does, in Loose ring, eggbutt or D-ring, and what the metal does, in Copper, sweet iron or rubber. Our bits are filterable by cheek, mouthpiece and material once you know what you are after.