When evaluating a bit, most people think of it in terms of the length of the mouthpiece. However, there are other important parameters to consider that impact the comfort of your horse, the way it functions and its legality for certain disciplines.
The parameters you need to measure include the length, the thickness (diameter) of the mouthpiece, and the size (diameter) of the rings. Some bits feature ports to provide tongue relief and in those designs you can measure the port height and width.
Length: The length of the mouthpiece is measured from the center of each of the ring holes. To get an accurate measurement you should use a ruler rather than a tape (measuring tapes can stretch over time) and pull on the bit rings to make sure the mouthpiece is fully extended.
Thickness: The diameter is measured at the widest part near the bit ring. Many people believe that thicker mouthpieces are milder as it gives more weight bearing surface across the bars. However, some horses, especially those with small mouths or thick tongues may find smaller diameter bits more comfortable. To be dressage legal, the diameter of a snaffle or bridoon mouth piece must have a minimum diameter of 10 mm at the rings or cheeks of the mouthpiece (ponies are excepted)
Diameter: The diameter of the rings is the measurement directly across the ring.
Port height: To provide tongue relief, some bits feature a port. For dressage competitions, the maximum height of the deviation is 30 mm from the lower part of the tongue side to the highest part of the port. The widest part of the deviation/port must be where the mouthpiece contacts the tongue and have a minimum width of 30 mm.
Watch this helpful video below for more details.