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If you do not have an existing rug to measure, you can measure your horse. The measurement should be taken from the centre of the animal's chest (where the front of the rug would meet), around the shoulder and along the side of the horse in a continuous straight line to the point level with the top of the tail.
Swa is in 5ft 3" (he is a fine built section A, 11h2") & Hami is is a 4ft 9" (well built dartmoor, 11h2) same height and yet the chunkier of the 2 is in the smaller rugs due to having a shorter back.
Id always recomend measuring before you buy, oh and trying it on for the first time with a thin summer sheet (or old bed sheet) under it then if its wrong you can normally return it (has no hairs on the then)
yes i'd measure, my 11.2 sec a is in a 4'6" but bear in mind rugs tend to differ in size sometimes as well ( eg around the neck) so best to take a tape measure with you to buy. i've found that cuddly ponies and rhino are the only type that fit mine so far.