Join Date: Nov 2011
Location: Beautiful Pacific Northwest
• Horses: 0
Generally, you add 2" to your Western saddle size to get your English size. That's ballpark.
For tree widths: A medium tree has a tree-point angle pretty close to 90°, medium/narrow and narrow have tighter angles, and med/wide and wide have more open angles. Some saddle-makers also have x-tra wide and XX wide sizes.
Keep in mind that in addition to the tree angle, you also have various widths of the gullet. So a medium tree in one brand may fit tighter around the withers than a medium width in another. I have a medium Passier and a medium Kieffer. Both have a 90° angle between tree points. The Passier has lots of width for the withers, while the Kieffer tens to sit much narrower and higher up.
Also, when you're shopping, keep in mind that what one saddle-maker calls a medium can be very different from what another calls a medium. There is no standard. People often will know that their horse takes a 31cm tree (med/wide) in a Stubben. That doesn't mean that they can go out and buy a 31cm tree or a med/wide in another brand and have it fit, because another brand's 31cm tree may have shorter or longer tree points, making the angle different, or their idea of a med'wide may have a narrower gullet, making it pinch, etc.