Getting your horse’s bit width right is one of the simplest things you can do to improve their comfort under saddle — and it’s something you can start at home with a piece of string and a ruler. This guide walks you through the process step by step.

A home measurement gives you a useful starting point and helps you have a more informed conversation when buying bits or booking a fitting. It isn’t a substitute for professional bit fitting, which assesses far more than width alone — but it’s a practical skill worth having.

What you’ll need

• A clean piece of string, baling twine, or a thin strip of card — approximately 30cm long
• A permanent marker or pen
• A ruler or tape measure
• A helper if possible — having someone to hold your horse makes the process considerably easier
• A quiet, familiar environment where your horse is relaxed

A purpose-made bit measuring tool is available from most equestrian retailers and saddleries if you’d prefer a more consistent result, particularly if you plan to measure regularly.

Before you begin

Take a moment to make sure conditions are right before you start — an unhurried horse gives you a much more accurate measurement.

• Your horse should be calm and relaxed. Never attempt to measure a stressed or anxious horse.
• Make sure your hands are clean and free from strong smells such as feed or chemicals.
• If your horse is head-shy or sensitive around the mouth, spend a few minutes quietly handling their head and muzzle first.
• Remove the bridle and bit if worn — you need clear, unobstructed access to the mouth.

Step 1 — Prepare your measuring tool

Fold a small stopper knot into one end of your string, or make a clear mark approximately 2cm from one end. This gives you a fixed reference point and prevents the string sliding too far through the mouth when you take your measurement.

Step 2 — Position the string in the mouth

Stand to the side of your horse’s head, level with their muzzle.

Gently insert the string into the corner of your horse’s mouth at the interdental space — the gap between the front teeth (incisors) and the back teeth (cheek teeth) where the bit naturally sits. Slide it across so that it rests on the bars and tongue in the same position a bit would occupy, not pushed back toward the cheek teeth or forward against the incisors.

Allow the stopper knot or starting mark to rest snugly against the corner of the mouth on the near side, then draw the string gently across to the off side until it sits comfortably and naturally without pulling or puckering the lips.

Step 3 — Mark and measure

Ask your helper to hold the string in place while you mark it clearly with your pen at the exact point where it meets the corner of the mouth on the off side.

Gently withdraw the string, keeping your finger on the mark so it doesn’t shift. Lay it flat against your ruler or tape measure and measure the distance between your starting point and your mark. This figure, in millimetres, is the internal width of your horse’s mouth.

Step 4 — Calculate your bit width

Add 10mm to your internal mouth measurement — 5mm for each side. This allowance ensures the rings or cheekpieces won’t pinch or rub the lips and corners of the mouth. The resulting figure is your horse’s approximate bit width.

So if your internal measurement is 110mm, your approximate bit width is 120mm. If it’s 115mm, aim for 125mm. If it’s 120mm, you’re looking at 130mm.

A few things to bear in mind

Loose ring snaffles need a little extra. Because the rings move freely, loose ring designs typically need approximately 5mm more than a fixed cheek design to prevent the rings pinching the lips. If you’re measuring for a loose ring, add 15mm rather than 10mm to your internal measurement.

This measurement is for standard snaffle-type bits. Different bit designs — particularly more specialist designs — may require slight adjustments, and a professional fitter will advise on the appropriate width for the specific bit being considered.

Mouth width is only one factor. This measurement tells you the width of the mouth at rest. It says nothing about palate height, tongue size, bar sensitivity, or dental conformation — all of which affect how a bit sits and feels. These can only be properly assessed by a professional fitter.

When to get professional advice

A home measurement is a useful tool, but some situations call for proper professional assessment before making any changes:

• Your horse is showing signs of bit discomfort — head tossing, resistance to contact, mouth opening, tongue evasions, or difficulty accepting the bridle
• Your horse has a history of dental issues or has recently had dental treatment
• You are changing disciplines or moving up a level in competition
• You are introducing a young horse to the bit for the first time
• Your measurement falls between standard bit sizes and you’re unsure which to choose
• You are considering a double bridle for the first time

A professional bit fitting consultation goes far beyond width. At The Fitted Horse, we assess every aspect of your horse’s mouth conformation, dental history, way of going, and individual sensitivities — so that whatever we recommend is the right fit for that specific horse, not just a number on a ruler.

Key takeaways

✔ Internal mouth measurement + 10mm = approximate bit width for most fixed cheek designs

✔ Add 15mm (not 10mm) for loose ring snaffles to allow the rings to move freely

✔ Always measure in a calm environment with a relaxed horse — a tense horse may hold their mouth differently

✔ Width is a starting point, not the whole picture — palate height, tongue size, and bar conformation all affect bit comfort

✔ If your horse is showing any signs of discomfort, book a professional fitting rather than adjusting equipment by trial and error

Want a full professional assessment?

If you’d like to go further than a home measurement — or if your horse is showing any signs that their current bitting isn’t working for them — a bit fitting consultation at The Fitted Horse covers everything a home measurement can’t.

View services, costs and booking → 

Further reading

• How to recognise your bit might not fit
• Size matters: bit width and thickness explained
• Understanding bit pressure
• Why a correctly fitted bit matters