If you have a new horse or are just unsure which size rug your horse needs, using a tape measure (or a piece of string you can measure later) measure your horse from the wither, horizontally along to the top of the tail. Rugs are sized in feet and inches, every 3”. If your horse is between sizes you are usually better to go up a size than down, unless the horse is narrow chested or petite. Ideally, an existing rug in the correct size can be measured all the way along the bottom edge to give the size required.
