Measurements are from buckle to buckle, you will need to allow an extra 1" compared to your normal girth size because of the fleece liner.
If the size you need isn't instock please see here - as part of the core range this can be ordered with delivery in 3 to 4 weeks.
The anatomical girth
For the horse with a proper girth groove far enough from the elbow of the horse. This girth is cut back somewhat in the front and rear in the elbow area.