How to Size a Dog Collar | Neck First, Not Weight

Getting the size right comes down to one measurement, not the dog’s weight or breed. A soft tape measure around the neck at the collar position gives you the number you need, and a two-finger gap between collar and skin confirms the fit. Here’s the exact process and the mistakes that throw it off.

Measure the Dog’s Neck Where the Collar Sits

Use a soft tape measure and wrap it around the neck at the spot where the collar will actually rest — usually the lower part of the neck, just above the shoulders. Keep the tape snug but not tight, the way a properly fitted collar should feel.

If you don’t have a tape measure, take a piece of string, wrap it the same way, then lay the string flat against a ruler to get the length. That number is your starting point for selecting a size.

Watch the position carefully. Measuring too high on the neck or too loosely gives you a number that’s off, and a collar sized from that will either slip over the head or squeeze. The tape should sit exactly where the collar will live every day.

Sizing an Existing Collar or Choosing a New One

If the dog already has a collar that fits well, you can size from it. Lay it flat and measure from the buckle’s center bar to the hole currently in use — do not include the buckle itself. For a side-release buckle collar, measure from the female end to the opposite end, excluding the male part that clicks in. PetSmart’s sizing chart uses this method to match you to the right product line.

When you buy a new collar, look for one whose adjustable range contains your dog’s neck measurement. Most collars list a range like 12–18 inches; your dog’s number should fall comfortably inside it, not right at the edge. If the dog’s measurement sits between sizes, several manufacturers’ guides say to round up to the next larger size — you can always tighten it, but you can’t add length to a collar that’s too short.

The Two-Finger Rule and Common Sizing Mistakes

With the collar on, you should be able to slide two fingers comfortably between the collar and the dog’s neck. This gap confirms the collar is snug enough that it won’t slip off over the head but loose enough that it never presses on the throat. Too tight causes discomfort and chafing; too loose risks the collar sliding right off.

One of the most common sizing mistakes is relying on the dog’s weight instead of an actual neck measurement. DogIDs’ sizing guidance notes that weight alone doesn’t predict neck size — two dogs at the same weight can have very different necks. Another frequent error is including the buckle when measuring an existing collar, which adds length you don’t need and throws the whole fit off.

Confirming the Fit Before You Buy

Once you have the neck measurement and know the two-finger rule, you’re ready to shop. The collar should be snug but never restrictive, and the buckle or adjustment should have room to move in both directions so you can fine-tune it as the dog grows or gains weight.

For a young dog that’s still growing, remember the collar won’t fit forever — check the fit every few weeks and loosen or replace it as needed. If you’re shopping for a puppy and want to know which styles hold up best during the growth phase, our roundup of the best collar for a puppy compares the top options side by side.