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Choosing the right bit size is one of the most important tack decisions you will ever make for your horse. A bit that is too wide slides from side to side and delivers vague, imprecise aids; one that is too narrow pinches the lips and presses on the delicate bars of the mouth, causing real pain. Either way, your horse cannot respond to you clearly, and discomfort quickly becomes resistance. This step-by-step guide shows you exactly how to measure for a bit, includes a full horse bit size chart, and explains how to know when a bit fits.
Why Getting Bit Size Right Matters
Bit size refers to the length of the mouthpiece, measured across the part that sits in the horse’s mouth. The mouthpiece rests on the bars, the toothless gap between the front incisors and the rear molars, and across the tongue. If the fit is wrong, the bit cannot sit correctly and the rein aids become muddled. Signs of a poorly sized bit include head tossing, mouth gaping, leaning, tongue evasion and rubbed or sore lip corners. Getting the size right is the foundation on which every other aspect of bitting, from the mouthpiece design to the cheekpiece, depends.
How to Measure Your Horse for a Bit
You do not need to own a bit to find your horse’s correct size. The simplest method uses a piece of string or a wooden dowel:
- Take a length of soft string (or a thin, smooth wooden dowel) about 30cm long.
- Gently place it in the horse’s mouth where the bit would sit, across the tongue and resting on the bars, just as a bit would.
- Mark the string with a pen at each corner of the lips, right where they meet.
- Remove the string and lay it flat against a ruler or tape measure. Measure the distance between your two marks.
- Add 6mm to 12mm (about a quarter to a half inch) to allow a small amount of clearance on each side, so the bit does not pinch.
That final figure is your horse’s bit size. A purpose-made plastic bit sizer does the same job and is a worthwhile buy if you fit bits regularly.
How to Measure an Existing Bit
To measure a bit you already own, lay it flat on a hard surface and pull the rings or cheekpieces apart so the mouthpiece is straight and at its full length. Measure the mouthpiece from the inside edge of one ring (or cheekpiece) to the inside edge of the other. That measurement, in inches or centimetres, is the bit’s size. A jointed bit should be straightened out fully before measuring.
Horse Bit Size Chart
Bit sizes are most commonly given in inches, in quarter-inch increments, though many European brands use centimetres. As a starting guide, the table below shows typical bit sizes by type of horse or pony. Always confirm with an actual measurement, as individual conformation varies widely.
| Type of horse or pony | Typical bit size (inches) | Approx. centimetres |
|---|---|---|
| Miniature horse / very small pony | 3.5″ – 4″ | 9 – 10 cm |
| Small pony (e.g. Shetland, Welsh A) | 4″ – 4.5″ | 10 – 11.5 cm |
| Medium pony (e.g. Welsh B, New Forest) | 4.5″ – 5″ | 11.5 – 12.5 cm |
| Large pony / small cob | 5″ | 12.5 cm |
| Cob | 5″ – 5.25″ | 12.5 – 13.5 cm |
| Full size / horse (e.g. most warmbloods, Thoroughbreds) | 5.25″ – 5.5″ | 13.5 – 14 cm |
| Large horse / draught (e.g. Shire, heavy cob) | 5.75″ – 6″+ | 14.5 – 15 cm+ |
A 5-inch bit is the most commonly sold size because it suits a large proportion of cobs and smaller horses, but never assume. A finer-muzzled Arab may take a 4.75″ while a broad-jawed warmblood needs 5.5″.
Bit Thickness and Why It Counts
Length is only half the story. The thickness, or diameter, of the mouthpiece is measured at its widest point, usually just next to the bit ring, and it has a major effect on how the bit feels. As a general rule, a thicker mouthpiece spreads pressure over a larger area and is considered milder, while a thinner mouthpiece concentrates pressure and acts more sharply.
Crucially, thickness must suit the horse’s mouth, not just the rider’s preference. Horses with a fleshy tongue or a low palate have less room and often go better in a thinner or more anatomically shaped mouthpiece, whereas a horse with more space can be comfortable in a thick, mild bit. A mouthpiece that is too bulky for the available space causes just as many problems as one that is too thin.
Sizing Differences by Bit Type
Different families of bit are measured and fitted slightly differently:
- Loose ring snaffles are measured between the inside edges of the rings. Because the mouthpiece slides through the ring, allow a fraction more width (around 6mm extra) so the lips are not pinched in the ring holes. See our guide to the types of snaffle bit for more detail.
- Eggbutt, fulmer and full-cheek snaffles have fixed cheeks, so the mouthpiece should sit snugly against the lips with little or no gap, as it cannot slide.
- Curb bits, Pelhams and Weymouths fit closer than a snaffle. A curb bit or Pelham should be snug so it sits still and the shanks act evenly. In a double bridle, the bradoon is usually a touch wider than the Weymouth that sits below it.
How to Tell if a Bit Is Too Big or Too Small
Once the bit is on, a quick check tells you whether the size is right:
- Too big: you can see more than about 6mm of mouthpiece sticking out on each side, the bit slides noticeably from side to side, or a jointed bit hangs too low and bangs the teeth. With a loose ring, the lips may get drawn into the ring holes.
- Too small: the bit pinches and wrinkles the lips, the rings or cheeks press hard into the sides of the face, and the horse may rub its mouth or fuss constantly.
- Just right: with the bit drawn up so it just wrinkles the corners of the lips once or twice, you should see roughly 6mm (a quarter inch) of mouthpiece on each side, between the lip and the ring.
Common Bit Fitting Mistakes
Even experienced owners slip up. The most frequent errors are guessing the size from the previous horse, buying the “standard” 5-inch without measuring, ignoring mouthpiece thickness entirely, and confusing height in the mouth with width. Remember that bit size (width) and bit height (how many lip wrinkles it sits at) are two separate adjustments. Width comes from the bit itself; height comes from the bridle cheekpieces. If you ride in a double bridle, also revisit how you handle the two reins in our guide to holding double reins.
Frequently Asked Questions
How do you measure a horse for a bit?
Place a length of string or a thin dowel in the horse’s mouth where the bit sits, mark it at each corner of the lips, then measure the distance between the marks against a ruler. Add 6mm to 12mm for clearance, and that figure is the bit size you need. A plastic bit sizer does the same job.
What is the most common horse bit size?
The 5-inch bit is the most widely sold size, as it fits many cobs and smaller horses. Full-size horses usually take 5.25″ to 5.5″, while ponies generally need 4″ to 5″. Always measure rather than assume, because muzzle width varies considerably between individuals even of the same height.
How do I know if my horse’s bit is too big?
If more than about a quarter of an inch of mouthpiece shows on each side, the bit slides from side to side across the mouth, or it hangs low and knocks the teeth, it is too big. With a loose ring snaffle, a bit that is too wide can also pinch the lips in the ring holes.
Are horse bits measured in inches or centimetres?
Both are used. British and American bits are usually sized in inches, in quarter-inch steps such as 5″, 5.25″ and 5.5″. Many European brands, including several popular bit makers, give the measurement in centimetres. Our chart above shows the rough equivalents so you can convert between the two.
Does bit thickness matter as much as length?
Yes. Length determines whether the bit fits across the mouth, but thickness determines how it feels. A thicker mouthpiece is generally milder because it spreads pressure, while a thinner one is sharper. The thickness must also suit the space in your horse’s mouth, as a bulky bit in a shallow mouth causes discomfort.
