Take the two measurements that matter
Grab a soft fabric tape measure (a piece of string and a ruler works too). The chest girth is the key number for most harnesses, wrap the tape around your dog's body just behind the front legs, at the widest part of the ribcage, and note the measurement snug but not tight. Then measure around the base of the neck, where a collar would normally sit. Weight-based sizing on packaging is only a rough guide, because two dogs of the same weight can have very different body shapes, a deep-chested Whippet and a stocky Staffy at the same weight need different harness sizes.
Match to the chart, not the label
Once you have both numbers, check them against the specific harness's own sizing chart rather than assuming "medium" means the same thing across brands. Sizing varies a lot between manufacturers, and no-pull harnesses in particular tend to run narrower in the chest strap than standard harnesses. If your dog's measurement falls between two sizes, size up for a growing puppy or a dog carrying extra weight, and size down for a lean, narrow-chested breed.
The fit checks to run once it's on
With the harness on, use the two-finger rule: you should be able to slide two fingers flat underneath every strap without much resistance. Any tighter and it can rub or restrict breathing; any looser and your dog can twist out of it. Check behind the front legs (the girth strap) for chafing, since this is the highest-friction point on a no-pull design, and look at the chest plate to make sure it sits centred and doesn't ride up into the throat when the leash is pulled taut.
The backing-out test
Before you trust a harness on a real walk, test it in a safe, enclosed space. Clip the leash on, apply gentle backward pressure the way a startled dog would if bolting away from something, and watch whether the harness stays in place or the dog can slip a leg or their head out. This is especially worth doing with sighthounds, dogs with narrow heads, and nervous dogs who are more likely to panic-pull. A harness that passes this test at home is far more trustworthy at the vet, the groomer, or near a busy road.
Recheck as your dog changes
Puppies, dogs recovering from illness, and dogs on a weight-management plan can outgrow or shrink out of a harness within weeks. Re-measure every month or so during growth or weight change, and re-run the two-finger and backing-out checks after any adjustment. If you're not confident the fit is right, most pet stores will do a quick in-person fitting, and a vet or trainer can flag any rubbing or restriction you might miss.
