Why a bad fit matters more than the size label
Dog coat sizing isn't standardised across brands, so a "medium" from one manufacturer can be a "large" from another. A coat that's too loose lets cold air in around the neck and belly and can slip sideways or catch on things during a walk. A coat that's too tight rubs the skin, restricts shoulder movement, and can chafe under the front legs, so taking your own measurements matters more than trusting a size chart alone.
The three measurements that matter
Most coats are sized off three numbers: neck circumference, chest girth, and back length. Neck circumference is measured around the base of the neck where a collar sits, with two fingers of slack so it isn't tight. Chest girth is the widest part of the ribcage, just behind the front legs, measure with the dog standing normally, not sucking in or puffing out. Back length runs from the base of the neck (where a collar sits) to the base of the tail, following the curve of the spine rather than a straight line.
Getting an accurate measurement
Use a soft fabric tape measure, not a rigid one, and measure with your dog standing in a natural, relaxed position rather than sitting or leaning. If your dog fidgets, a second person to hold treats or steady the dog makes the numbers far more reliable. Write down all three measurements in centimetres or inches, then check them against the specific brand's size chart rather than assuming your dog's weight alone tells you the right size, two dogs of the same weight can have very different chest girth and back length.
Checking the fit once the coat is on
A well-fitted coat should allow full leg movement at the shoulders, sit close enough that it won't shift or twist during play, and not gap open at the chest or neck where cold air or rain can get in. Run two fingers under any strap or closure, if they slide in easily, the fit is close to right; if you can't fit a finger, it's too tight. Watch your dog's first few minutes in a new coat: excessive scratching at it, freezing in place, or trying to walk backwards out of it usually means the fit or the coat itself isn't right for that dog.
When to size up or down
Deep-chested breeds and dogs with a barrel-shaped ribcage often need a coat sized on chest girth rather than back length, and many brands offer adjustable straps for exactly this reason. Puppies and dogs mid-weight-change need re-measuring every few months rather than assuming last season's coat still fits. If a dog sits right between two sizes, sizing up and using the adjustable straps to take up slack is usually safer than sizing down and restricting movement.