A jumper that's too tight rubs under the legs. Too loose and it slides round by the end of the first walk. Two minutes with a soft tape measure saves a return.
What you'll need
A soft tape measure (a piece of string and a ruler works too), a treat, and a dog standing square on all four feet.
The three measurements
Back length. From the base of the neck, where the collar sits, along the spine to the base of the tail.
Chest. Around the widest part of the ribcage, just behind the front legs. Keep the tape snug but not tight. You should be able to slip two fingers underneath.
Neck. Around the neck where the collar sits.
Read the size guide
Every jumper in our For the Dog edit has a size guide on its product page. Sizes vary from maker to maker, so a medium in one brand isn't always a medium in another. Check the guide for the jumper you're buying rather than going by the size your dog wore last year.
Some guides are in centimetres and some are in inches, so check the heading. One inch is 2.54 cm.
Where a guide lists breeds, treat them as a starting point. Your dog's own measurements are more reliable.
Which measurement matters most
Go by the chest. A jumper that fits the chest but is a little long in the back is fine. One that fits the back but pulls round the chest won't be comfortable.
Then check the neck. If your dog has a thick neck for its size, the neck measurement may decide it.
If your dog is between sizes, go up.
Unusual shapes
Whippets and greyhounds have deep chests and narrow waists, so a standard jumper can gape at the tummy. Choose by chest and expect a looser fit round the waist.
Dachshunds are the other way round, with long backs and short legs. Choose by chest and neck, and don't worry if the jumper stops a little short of the tail.