A snaffle bit size guide only works if you measure three things correctly: bit width, mouthpiece thickness, and cheek clearance. Get any one of them wrong, and even a well-made bit can pinch, slide, or make your horse fight the contact. This guide breaks each measurement down into a repeatable process, so you can size a bit with confidence instead of guesswork.
1. Snaffle Bit Size Guide, Step 1: Measuring Bit Width Correctly
Bit width is the distance between the two cheekpieces — the rings or shanks — and it's the single most important number in any snaffle bit size guide. A bit that's too wide slides side to side and can knock against the front teeth; a bit that's too narrow pinches the corners of the mouth and rubs the skin raw.
The String or Dowel Method
- Take a soft cotton rope, a wooden dowel, or a piece of flexible plastic.
- Rest it across the bars — the toothless gap between the incisors and molars, right where the bit sits.
- Mark both edges of the mouth where the material meets the lips.
- Remove it and measure the distance between the two marks with a ruler.
The Bit Caliper Method
A dedicated bit caliper — sold through specialty tack retailers — slides two markers until they rest against the lips, giving a more precise reading than the string method. This is the preferred approach for professionals fitting multiple horses.
When measuring an existing bit, always measure from the inside of one ring to the inside of the other. Including the outer diameter of the rings will overstate the width and lead to an oversized purchase.
| Horse Type | Typical Bit Width | Notes |
|---|---|---|
| Small Pony | 4.0"–4.25" | Shetlands and small pony breeds |
| Large Pony | 4.5" | Welsh section C/D, small cobs |
| Cob / Small Horse | 5.0" | Cob breeds, smaller riding horses |
| Average Horse (15–16 hh) | 5.0"–5.25" | The most common size range |
| Large Warmblood / Draft | 5.5"–6.0"+ | Thoroughbreds, Friesians, draft crosses |
Rule of thumb: add roughly ¼–½ inch to your horse's measured mouth width so the bit sits comfortably without pressing directly into the corners of the lips.
2. Understanding Mouthpiece Thickness
Mouthpiece thickness — measured in millimeters at the widest point, usually near the rings — is the measurement most riders skip, yet it has the biggest effect on tongue room and how comfortably a horse can close its mouth.
| Thickness | Classification | Best For |
|---|---|---|
| 10–12mm | Ultra-thin | Refined heads, horses with large tongues |
| 12–14mm | Thin | Fleshy mouths, low palates, large tongues |
| 16mm | Standard | Average conformation — the most widely used option |
| 18–21mm | Thick | Wide mouths, high palates |
The Two-Finger Test: gently insert two fingers into the interdental space where the bit sits. If there's limited room between the upper and lower jaw, a thinner mouthpiece (12–14mm) is usually the more comfortable choice.
A thicker mouthpiece is often described as "milder" because it spreads pressure over a wider surface — but that's only true if it actually fits. In a small mouth, an 18mm mouthpiece can prevent the horse from closing its jaw comfortably, causing it to gape open. 16mm remains the safest starting point for most horses trying a new bit for the first time.
3. Cheek Clearance and Ring Fit
Cheek clearance is the space between the horse's lips and the bit's cheekpieces. Getting it right prevents pinching, rubbing, and soreness at the corners of the mouth — and the correct amount depends on the type of cheekpiece.
Loose Ring Snaffles
Because the ring slides freely through the mouthpiece, there's a real risk of skin getting caught in the hinge. Leave approximately ⅛–¼ inch (3–6mm) of space on each side, and consider rubber bit guards to reduce pinching further.
Fixed Cheek Snaffles (Eggbutt, D-Ring, Full Cheek)
With no sliding hinge, these cheekpieces should sit flush against the face — no visible gap, but no bulging or outward pressure on the lips either. Because there's no moving part to catch skin, fixed-cheek bits can be fitted slightly truer to actual mouth width.
4. Signs of a Poorly Fitted Bit
| Problem | Likely Cause | What to Do |
|---|---|---|
| Bit slides side to side on one rein | Too wide | Re-measure the mouth; try a narrower bit |
| Joint hits the roof of the mouth | Too wide | Switch to a narrower or double-jointed mouthpiece |
| Rubbed hair or pinched skin at the corners | Too narrow | Move up to a wider bit |
| Horse constantly gapes its mouth open | Mouthpiece too thick | Try 12–14mm thickness |
| Horse feels overly sharp or "behind the bit" | Mouthpiece too thin | Try a 16mm standard thickness |
| Sores at the corners near the rings | Insufficient clearance | Add ⅛–¼ inch clearance or switch to fixed-cheek style |
5. Quick Reference: Snaffle Bit Sizing Summary
| Measurement | How to Measure | Standard | Key Notes |
|---|---|---|---|
| Width | Inside ring to inside ring, or string/dowel across the bars | 5.0" average for 15–16 hh | Add ¼–½" beyond actual mouth measurement |
| Thickness | Millimeters at the widest point near the rings | 16mm standard | Thinner for small mouths, thicker for wide palates |
| Cheek Clearance | Visual gap between lip anD Ring with bridle on | ⅛–¼" loose rings; flush for fixed cheeks | Prevents pinching and rubbing |
Frequently Asked Questions
Do I need to add extra room beyond my horse's measured mouth width?
Yes. Adding ¼–½ inch beyond the measured width keeps the bit from pressing directly on the corners of the mouth while still preventing excess sliding.
Is a thicker mouthpiece always milder?
Not necessarily. A thicker mouthpiece spreads pressure over a larger area, but only if the horse has enough palate and tongue room to accommodate it comfortably. In a small mouth, a thick bit can be less comfortable than a properly fitted thin one.
How much cheek clearance should a loose ring snaffle have?
Around ⅛ to ¼ inch (3–6mm) on each side is typical, which allows the ring to move freely without pinching the skin at the hinge.
What should I do if my horse keeps gaping its mouth open?
This is often a sign the mouthpiece is too thick for the available tongue and palate room. Try a thinner mouthpiece in the 12–14mm range and reassess.
Should I consult a professional before changing bit size?
Yes, especially if discomfort persists. A bit should always be selected and fitted for the individual horse, and persistent resistance or soreness should be checked by a qualified trainer, equine dentist, or veterinarian.