Harness fit is decided by chest girth, not by weight and not by breed. Put a soft tape around the widest part of your dog's ribcage, just behind the front legs, with the dog standing. Then measure the neck where a collar would sit. Enter both below.
Widest point of the ribcage, behind the front legs.
Where the collar sits. Optional, but it catches fit problems.
The size chart this uses
| Size | Neck | Chest / girth | Back length |
|---|---|---|---|
| S | 23–31 cm (9–12") | 38–55 cm (15–21.5") | 33 cm (13") |
| M | 30–38 cm (12–15") | 42–70 cm (16.5–27.5") | 40 cm (15.75") |
| L | 35–45 cm (13.75–17.75") | 47–93 cm (18.5–36.5") | 48 cm (19") |
| XL | not published — see note | 52–103 cm (20.5–40.5") | 53 cm (21") |
What this tool can't tell you
It reads a size chart. It cannot tell you whether a harness suits your dog's shape, and that is a real limitation rather than a disclaimer.
Two dogs with identical chest measurements can need different harnesses if one is deep-chested and the other barrel-chested — the girth number is the same, the geometry is not. If your dog has a large gap between neck and chest measurements, read harness sizing for deep-chested dogs before ordering anything.
A chart also can't confirm the harness is on correctly once it arrives. Check it against how tight a dog harness should be — two fingers under every strap, tested one strap at a time — and re-check after a few wears, because webbing settles.
The measuring method itself is covered in full in how to measure your dog for a harness, including the mistakes that throw the number off: measuring over thick fur, pulling the tape tight, and measuring the belly instead of the ribcage.