There are a few ways to measure for the correct width.
Measure an old bit you know already fits. Use a tape measure to measure from the inside of each cheek piece or ring.
Use a piece of string about 12” (30cm) long. Tie a knot about 2 inches (5cm) in. Put it through your horse’s mouth, with the knot against the lips on one side. Mark the opposite side with a marker or little piece of tape.
Mark a piece of doweling the same as for the string.
Put a sewing tape measure through your horse’s mouth—the flexible cloth or plastic kind.
Use callipers. Adjust one arm of the caliper on either side of the horse’s mouth and then lay them down and measure the distance between the two arms of the caliper. Buying a pair of callipers for a one time measuring job is costly, so do this only if you have some available. (Buy it now)
Download, print and cut out a bit measurer. Bit Measurer Template from Libertyville Saddle Shop.
Write down the measurement and take a tape measure with you when you go out to buy a bit. At the store, ask if the bit is returnable if it doesn’t fit. When you get the bit home make sure that it sits comfortably in the horse’s mouth before you get rid of the receipt and packaging.