A harness that is too loose lets a worried dog back out of it. One that is too tight can rub the skin and stop the front legs from reaching forward. There is no single standard for sizes: one brand sizes by the dog's measurements, another by the harness's own dimensions, so "medium" can mean very different things. The fix is two steps: measure your dog once, then compare those numbers with the size chart of the exact harness you are looking at.
Use a soft tape measure (or a piece of string, then lay it against a ruler), and keep a few treats handy so measuring feels like a game. Measure while your dog is standing.
Measure the chest, compare with that harness's own chart, and always do the finger and shoulder check on the dog.
Check first whether the chart lists the dog's measurements a size suits, or the harness's own dimensions. If it lists the harness, the strap needs to open wider than your dog's chest measurement, with room for the fit check; if you can't tell, ask the maker. If your dog falls between two sizes, the American Kennel Club advises taking the larger one, and looking for an adjustable model if neither feels right. Never rely on "small, medium, large" from a past purchase, as sizes change from brand to brand.
Fit does not stay the same either. Puppies outgrow harnesses quickly, and dogs that gain or lose weight, or grow a thick winter coat, may need a re-check.
The American Kennel Club suggests two fingers under the straps. Dogs Trust says a finger should slide between strap and body, while the harness must still stay put and not let the dog slip out. In practice that means one to two fingers: loose enough to be comfortable, snug enough that it cannot rotate or pull over the head. If the maker's instructions say otherwise, follow them.
A harness spreads the pressure over the chest and shoulders instead of the neck. Some dogs prefer a step-in design, which does not need to go over the head, a good option if your dog dislikes things near the face. Whatever the style, choose one that does not restrict the natural movement of the shoulders. A front clip can make steering easier for a dog learning loose-lead walking, but it works best alongside reward-based training rather than replacing it. Avoid designs that tighten when the dog pulls: a harness should never be used to punish.
Many dogs need a little time to accept wearing one. Dogs Trust suggests going slowly: let your dog sniff it with treats around it, feed treats through the head hole, then add the clip sounds, then fasten it with more treats, then a short walk. If your dog seems worried, go back a step.
Budget: a good harness usually costs around €15–50, and a higher price does not guarantee a better fit.
If your dog limps, yelps, changes the way they walk, loses fur or has sore skin where the straps sit, stop using the harness and ask your vet. They can check whether the cause is the fit or something else. Do not try to diagnose it yourself.