A collar is the one piece of gear your dog wears all day, every day, and it is the thing that carries the tags that get your dog home. It is also the single easiest piece of gear to order wrong, because almost every buyer skips the measurement and picks from the weight column. Weight tells you nothing about a neck. Here is the thirty-second version that gets it right.
The one measurement you need
Neck circumference, measured where the collar will sit. That means roughly the middle of the neck — below the base of the skull, above the shoulders. Not tucked up under the jaw, which reads too small, and not down where the neck meets the shoulders, which reads too large.
Use a soft tailor’s tape. Wrap it around, keep it snug against the coat without squashing the fur down, and read the number in inches. No tape in the house? Use a length of string or a phone charging cable, mark where it meets, then hold it against a ruler. The manufacturer measuring guides we cite from Kurgo and Ruffwear describe the same approach for neck and girth measurements — a soft tape, snug, against the coat.
The two-finger rule
With the tape in position, slide two fingers flat between the tape and your dog’s neck. Two should fit comfortably; three should not. That is the fitted measurement, and it is the same standard used for harnesses. It gives you the margin that stops a collar restricting the throat while still being too tight to pull off over the head.
Two flat fingers — not two fingers stacked or wedged in edgeways. It sounds pedantic, but the difference between flat and stacked fingers on a small dog is most of an inch, and on a small dog that is most of a size.
Reading the size chart
Every collar is sold as an adjustable range, e.g. “Medium: 14.5 to 20 inches.” The rule that saves you a return: land in the middle of the range, not at either end. A dog measuring 14.5 inches technically fits that Medium, but it will be on the very last hole with the strap tail flapping, and there is no room left to tighten if the dog loses weight or gets a summer clip. If your measurement is at the boundary between two sizes, the smaller size at its loosest is usually the better buy for an adult, and the larger size at its tightest is the better buy for a puppy.
| Neck measurement | Typical size label | Rough build |
|---|
| 8–12 in | X-Small | Toy breeds, small puppies |
| 10–14 in | Small | Small terriers, Dachshunds |
| 14–20 in | Medium | Spaniels, Border Collies, Bulldogs |
| 18–26 in | Large | Labradors, German Shepherds |
Treat this as orientation only. Ranges vary between brands, sometimes considerably — always check the specific chart on the collar you are buying, and trust your measurement over the label.
When to re-measure
- Puppies: every two weeks. They grow faster than anyone expects, and a collar that has become tight is a genuine welfare problem. Budget for at least two collars before adulthood.
- Adults: monthly, by feel. You do not need the tape every time — just run two fingers under the collar. If they no longer fit flat, adjust it.
- After a coat change. A double-coated dog can shed enough in spring to change its effective neck size by a full hole. Check after a heavy molt or a groom.
- After weight change. Any noticeable gain or loss means re-checking.
One thing measurement will not fix
If your dog backs out of a correctly fitted collar — and sighthounds and many rescues do, because the head is narrower than the neck — the answer is not a tighter collar. It is a martingale, which limits how far the collar can be pulled open, or a harness for walks. The AKC’s guidance on collars and harnesses is worth reading alongside this if that describes your dog.
