A bit should fit your horse’s mouth so that the mouthpiece extends approximately 1/4 inch (0.6 cm) beyond the horse’s lips on either side, but not so tight that the horse might be pinched by any movable cheek pieces. You also don’t want a great length of bit hanging out of the horse’s mouth. The different parts of the mouthpieces on bits are made to sit on specific areas of the mouth. So for the bit to be effective, the bit needs to contact those areas properly. You also don’t want the bit pulling through the horse’s mouth when you pull on a rein.

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.