A rider can determine the exact width of the saddle tree of a fully assembled English saddle only by relying on manufacturer documentation or markings on the saddle. Additionally, different English saddle manufacturers use different designations for saddle width measurements; the width of the same size saddle tree can have a different measurement designation between two different manufacturers. A rider can, however, determine the width of saddle tree needed on a particular horse and determine if a particular saddle comes close to fitting this requirement.
Determine the width of the saddle tree you need for an English saddle by making a template of your horse's withers -- the ridge between the shoulder bones. Do this by placing a bent wire or a flexicurve tool over the horse's back at the point just rear of the withers where the front of the saddle will rest. Ensure that the measuring tool rests firmly against the horse.
Mark off the width of your horse's back on this measuring instrument by marking where the horse's back begins to slope off on each side. Write this measurement down to provide to your saddler.
Determine, in consultation with your saddle shop, the type of English saddle you require, such as dressage, all-purpose or eventing. Include, if possible, the manufacturer of the saddle type you intend to purchase. The more common sizing designations for an English saddle tree use "N" for narrow, "M" for medium, "W" for wide, and "XW" for extra wide. However, English saddle tree manufacturers use the numbers 2, 3, 4 and 5 to correspond to the width designations. Additionally, other manufacturers use centimetre measurements, applying centimetres in different ways.
Recognise that in measuring a fully assembled saddle, you can only determine the width by way of documentation included with the saddle or a marking on the saddle designating the width. Measure from the saddle nail on one side of the saddle to the saddle nail on the other side of the saddle, if for example you are dealing with a used saddle, to ensure the saddle tree has sufficient width to accommodate your horse across the back. The saddle nails are rivet-type metal pieces at the front of the saddle at the front of its skirt. Bear in mind that no standard sizes exist for saddle trees and that each saddle producer sets the standards for the saddles produced by its manufacturing facility. Additionally, the introduction of adjustable width features in saddles, as of 2010, further obscures identification of the width of the saddle tree on fully assembled English saddles.
The width of the tree is only one of the size specifications necessary to determine if a saddle is a proper fit for rider and horse. Minimally, you need to have length-of-saddle sizing from front to back. This is the measurement normally used to identify a saddle's size, such as 17 inches.