How to Measure Your Horse's Mouth for a Bit at Home

Getting the width of your horse's bit right is one of the most important steps toward ensuring their comfort and wellbeing under saddle. Whilst a professional bit fitting consultation will always give you the most accurate and comprehensive assessment — taking into account not just width but palate height, tongue size, bar conformation, and much more — knowing how to take a basic mouth measurement at home is a useful skill that can help you make more informed decisions about your horse's bitting arrangement.

This guide walks you through how to measure your horse's mouth at home using a simple and accessible method. Please remember that this measurement gives you a starting point — it is not a substitute for professional bit fitting advice.

What You Will Need

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

Before You Begin

Make sure your horse is calm, relaxed, and comfortable — never attempt to measure a stressed or anxious horse
Ensure your hands are clean and free from strong smells such as feed or chemicals that might cause your horse to react
If your horse is particularly head-shy or sensitive around the mouth, spend a few minutes quietly handling their head and muzzle before you begin
Remove the bridle and bit if your horse is wearing one — you need clear, unobstructed access to the mouth

Step One — Prepare Your Measuring Tool

Take your piece of string, twine, or card and fold a small stopper knot into one end — or make a small mark approximately two centimetres from one end
This stopper or mark will prevent the string from sliding too far through the mouth and will give you a clear reference point when you take your measurement
Alternatively, a purpose-made bit measuring tool is available from most good equestrian retailers and saddleries and will give you a more consistent result if you plan to measure regularly

Step Two — 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 — this is the gap between the front teeth (incisors) and the back teeth (cheek teeth) where the bit naturally sits
Slide the string gently across the mouth so that it sits in the same position that a bit would occupy — resting on the bars and tongue, not pushed back toward the cheek teeth or forward against the incisors
Allow the stopper knot or your starting mark to rest snugly against the corner of the mouth on the near side
Draw the string gently across to the off side until it sits comfortably and naturally in the mouth without pulling or puckering the lips

Step Three — Mark and Measure

Ask your helper to hold the string in place whilst you mark it clearly with your pen at the exact point where it meets the corner of the mouth on the off side
Gently and carefully withdraw the string from your horse's mouth, keeping your finger on the mark so that it does not shift
Lay the string flat against your ruler or tape measure
Measure the distance between your starting point and your mark — this measurement, in millimetres, represents the internal width of your horse's mouth

Step Four — Calculate Your Bit Width

To this measurement, add ten millimetres — five millimetres for each side
This additional allowance accounts for the clearance that a correctly fitted bit should have on each side of the mouth, ensuring that the rings or cheekpieces do not pinch or rub the lips and corners of the mouth
The resulting figure is your horse's approximate bit width

For example:

If your internal mouth measurement is 110mm, your approximate bit width would be 120mm
If your internal mouth measurement is 115mm, your approximate bit width would be 125mm
If your internal mouth measurement is 120mm, your approximate bit width would be 130mm

Important Considerations

This measurement gives you a guide width for a standard snaffle-type bit — different bit designs may require slight adjustments to this figure, and a professional fitter will always advise on the appropriate width for the specific design being considered
Loose ring snaffles typically require a small amount of additional width — approximately five millimetres more than a fixed cheek design — to allow the rings to move freely without pinching the lips
The measurement you have taken reflects the width of the mouth at rest — some horses may need a small additional allowance if they have particularly mobile or sensitive lips
Mouth width is only one of many factors that determine whether a bit is appropriate and comfortable for your horse — palate height, tongue size, bar conformation, and dental health are all equally important considerations that can only be fully assessed by a professional fitter

When to Seek Professional Advice

Whilst a home measurement is a useful starting point, there are circumstances in which professional advice should always be sought before making any changes to your horse's bitting arrangement:

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

A professional bit fitting consultation at The Fitted Horse goes far beyond a simple width measurement — we assess every aspect of your horse's mouth conformation, dental history, way of going, and individual sensitivities to ensure that the bitting arrangement we recommend is the very best possible fit for your individual horse. Find out more and book your consultation at www.thefittedhorse.com/bit-fitting.