Most harnesses are returned because of fit, and most fit problems come from choosing by breed instead of measuring. Two measurements take a minute and remove the guesswork.
What to measure
Chest girth — the one that matters
Put a soft tape around the widest part of the ribcage, just behind the front legs, and bring it all the way around. Keep the tape snug against the coat but not compressing it.
This is the measurement nearly every harness is sized by. If a listing gives one range, this is almost always it.
Neck base
Around the base of the neck, where a collar naturally sits — low, at the shoulders, not up under the jaw.
The two-finger rule
Once fitted, you should be able to slide two fingers flat under any strap. Tighter than that restricts movement and rubs. Looser and a dog can back out of it, which is the failure that matters most near traffic.
If your dog is between two sizes
Take the larger one, provided the adjustment range still covers your measurement comfortably rather than at the very end of its travel. A harness adjusted near the middle of its range fits better than one at its limit, and leaves room if your dog is still filling out.
If the measurement sits at the very top of one size and the very bottom of the next, ask us before ordering. We would rather check than have you post something back — and for most of what we sell, you would not be posting it back anyway.
Measure the dog you have now
Coat length changes girth. A double-coated dog measured in full winter coat will read larger than the same dog after shedding season. Measure against the coat as it is, and if your dog is mid-moult, take the measurement again once it settles.
Puppies outgrow harnesses quickly. A wide adjustment range is worth more than a precise fit at eight months old.
Collar or harness?
A collar is for identification and for attaching a lead to a dog who walks without pulling. A harness spreads the load across the chest and shoulders.
If your dog pulls, use a harness. A collar concentrates that force on the throat, and that is not something to design a walk around. This is also why we suggest attaching a retractable lead to a harness rather than a collar — a dog reaching the end of several metres at speed generates a real shock.
What we publish
Where a supplier gives a per-size measurement table, we print it on the product page — one of our collar and lead sets publishes the full range from a 23 cm to a 62 cm neck. Where a supplier does not publish sizing, we say that rather than estimating it, because a guessed measurement on a worn item is how a dog ends up in something that does not fit.
See walking.