Have you ever wondered if you have the right size bit in your horse’s mouth, or stared at the size options for the bit you’re about to order hoping you are picking the right one? Over my years as barn manager for top riders in several disciplines as well as helping thousands of customers as manager of SmartPak’s Retail Store in Natick, MA, I have learned and developed some easy processes to measure your horse for bit size and assess bit fit in your horse’s mouth.

Using the right size bit can make a big difference in how effective it is as well as how comfortable your horse will be with a bridle on. A too small bit can pinch and rub, and a too big bit can slide around in the mouth and may not be as effective. Bits have several measurements – the width of the bit is the main measure, displayed in inches or mm. Common horse sizes include 5”, 5 ¼” and 5 ½” (130mm, 135mm, 140mm.) Pony bits can run as small as 3” and draft horses run as large as 7”! There is also mouthpiece thickness, typically 14mm to 18mm, and ring diameter or shank length.

Image courtesy of Neue Schule

Fun Bit Fact: Metal bits are thought to have been developed and used in riding sometime between 1300 and 1200 BC. Other materials, such as bone and wood, may have been used prior to that time, but non-metal materials would not have durable.

Step One: Measuring
You can use one of several options to measure your horse’s mouth. If you have a bit that you know fits well, you can measure the bit. Bits are measured from inside corner to inside corner, across the width that actually sits in the horse’s mouth. Other options include using a measuring tool, such as a dowel, string, or soft measuring tape and marking both ends where the lips end. Make sure whatever you use to measure sits approximately where the bit will sit. You can measure between the marks once you remove the dowel or string from the horse’s mouth. Try to use a measuring tape that shows inches as well as metric cm/mm, as bit measurements can be shown in both.

Step Two: Assessing
When you have the bit attached to the bridle or headstall and in the horse’s mouth, you can check several aspects for fit. Before you can ensure the bit itself fits correctly, make sure the headstall or bridle is adjusted property so the bit is sitting at the right height in the horse’s mouth. How should it look? For snaffle bits, you should see 1-2 wrinkles at the corner of the mouth and there should be tension on the cheek pieces without strong pressure. For curb bits, you won’t see any wrinkles, but the bit should be resting against the corners of the horse’s mouth and should not be low enough to touch or bang the teeth.

Once the bit sits properly in the horse’s mouth, you can assess the width of the bit. Does it feel tight on the sides of the mouth, with no room to move the bit side to side at all? If so, it may be too narrow. Ideally you want 1/8” to ¼” of space on either side of the horse’s mouth, but this depends on how fleshy your horse’s lips are. Some horses have a lot of lip spread, so there will still be a lot of play side to side even if the lips are touching the bit cheekpieces or rings. Other horses have very firm lips, so you can clearly see the space. If you can easily slide the bit back and forth with ½” or more showing on either side of the horse’s mouth and the rings or cheekpieces, your bit may be too wide.

Another measurement to assess is the thickness of the bit. This measurement is mostly seen in loose ring snaffles used in dressage, but can be a factor in any bit. Just like people, some horses have smaller mouths than others, which can make a thicker bit uncomfortable. Generally speaking, larger diameter/thicker bits tend to be milder in their action than narrow diameter bits, but there is also a comfort factor for the horse. If your horse seems to have a small mouth, a narrower diameter bit may fit better. The most common thickness is 16mm (approx. 5/8th of an inch), although there are large ranges. Most commonly seen are 14-18mm, with some bradoon bits designed for double bridles as narrow as 12mm.

Ring diameter or shank length are both aspects to consider with bits. Decisions here are generally more related to bit action and function, but there are a few fit factors to think about. A horse with a narrow muzzle and a wide cheek may need a curved or flared purchase/cheek ring (portion above the mouthpiece that attaches to the headstall or bridle cheek pieces) to keep it from jabbing into the horse’s cheek. Some horses have sensitive lips, so a cheekpiece that isn’t smooth and fixed, like a D ring, full-cheek or eggbutt snaffle or fixed shank bits, could rub and create bit sores in the corners of the mouth. This is seen more commonly in bits with loose cheek attachments, such as loose-ring snaffles and non-fixed shank bits.

There are a wide range of mouthpieces, and each will sit differently in the horse’s mouth. Deciding what mouthpiece to use is a training and riding preference, but you should take mouth confirmation into account here. Horses with fat tongues and low palates could find certain bits uncomfortable, such as thick diameter bits or single jointed snaffles. For those horses who need tongue clearance, a ported bit or double-jointed snaffle could provide more comfort.

Fitting a double bridle, which includes both a snaffle bit, also known as a Bradoon, and a curb bit, also known as a Weymouth, can be tricky as you need to make sure both bits fit comfortably in the horse’s mouth. The above guidelines apply, and it is important to ensure both bits are seated within the mouth in such a way that they are not too low and also do not interfere with each other.

Even the most well-fitting bit may not give you the results you are looking for, so consulting with your riding instructor, trainer or clinician can help you determine the right type of bit to use. Your horse gets the final say on any new bit, and sometimes they have preferences that don’t follow the rules. I had a sensitive mare who preferred her bits to be wider than normal, and a pony who tolerated bits only if they were quite snug and didn’t move around at all (although she much preferred a hackamore!) Best of luck with your bit fitting, and contact SmartPak for questions about any of the bits or products that we sell. Have a great ride!