There is no standard dog size. The same dog can be a small in one brand and a medium in the next, which is why so many collars and harnesses go back. Measure your dog once, then use the chart of the brand you are actually buying. The tool below gives you the collar length to order and shows how your dog maps onto two real manufacturer charts.
Around the base of the neck, where the collar sits.
The widest part of the ribcage, just behind the front legs.
Sizes are taken from each manufacturer’s published chart. Always check the chart of the product you are buying before ordering.
How to measure your dog
You need two numbers, and a soft tape measure. A piece of string marked with a pen and held against a ruler works just as well.
- Neck: around the base of the neck, where a collar naturally sits. Start near the withers, the ridge between the shoulder blades, and wrap round to the front of the chest.
- Chest girth: around the widest part of the ribcage, usually just behind the front legs. This is the measurement harnesses are built around.
- Back length, if you are buying a coat: along the spine from the base of the neck to the base of the tail.
Keep the tape snug but not tight, and slide two fingers underneath as you measure so you are building in breathing room from the start.
The two-finger rule
The fit test every vet and trainer repeats: once the collar is on, you should be able to slide two fingers flat between the collar and your dog’s neck. One finger is enough on a toy breed. Tighter than that restricts breathing and rubs; looser and the collar slips over the ears, which is how dogs get loose in traffic.
In practice that means ordering a collar about 1 inch larger than the neck measurement on a small dog, and 2 inches larger on a medium or large one.
Why brand size charts disagree
There is no industry standard, and the charts are not even built on the same measurements. Here are two real ones:
| Ruffwear size | Chest girth |
|---|---|
| XXXS | 9 to 13 in (23 to 33 cm) |
| XXS | 13 to 17 in (33 to 43 cm) |
| XS | 17 to 22 in (43 to 56 cm) |
| S | 22 to 27 in (56 to 69 cm) |
| M | 27 to 32 in (69 to 81 cm) |
| L | 32 to 36 in (81 to 91 cm) |
| XL | 36 to 42 in (91 to 107 cm) |
| Kurgo size | Neck | Chest | Weight |
|---|---|---|---|
| X-Small | 9 to 13 in | 12 to 18 in | 5 to 10 lb |
| Small | 12 to 20 in | 16 to 22 in | 10 to 25 lb |
| Medium | 16 to 25 in | 18 to 28 in | 25 to 50 lb |
| Large | 18 to 30 in | 24 to 34 in | 50 to 80 lb |
| X-Large | 24 to 35 in | 28 to 44 in | 80 to 110 lb |
A dog with a 26 inch chest is a Ruffwear S and a Kurgo Medium. Neither chart is wrong; they simply draw the lines in different places, and Ruffwear sizes on girth alone while Kurgo wants neck, chest and weight. This is the single biggest reason dog gear gets returned.
Getting a harness fit right
A harness that fits badly does more harm than a collar that fits badly, because it sits against moving joints.
- The neck opening belongs near the top of the breastbone, not pressing on the throat.
- The chest strap sits just behind the front legs, not back on the soft belly and not jammed up into the armpits.
- Two fingers under every strap.
- Watch the shoulders. Your dog should be able to reach the front legs forward and out to the side freely. A harness that cuts across the shoulder blades shortens the stride.
- Check for rubbed fur or raw skin behind the elbows after walks. That is the harness telling you it does not fit.
Puppies: measure again, and again
A puppy’s neck can gain inches in a month, and a collar that fitted at 12 weeks can be dangerous by 16. Check the two-finger gap weekly while they are growing, buy adjustable, and expect to replace it at least once before adulthood. If you want to know how much growing is left, our puppy weight calculator estimates adult size.
Frequently asked questions
How tight should a dog collar be?
Where do I measure for a harness?
My dog is between two sizes. Which do I buy?
Why is my dog a small in one brand and a medium in another?
Is a harness better than a collar?
Where these numbers come from
- How to measure and the two-finger rule: the American Kennel Club’s guide to measuring your dog for gear (neck at the base where the collar sits, girth at the widest part of the ribcage behind the front legs, back length along the spine).
- Harness fit and the damage a bad one does: the AKC on well-fitting harnesses (neck opening near the top of the sternum, girth strap clear of the ribcage edge and elbows, two fingers under the straps, shoulder movement unrestricted).
- Size bands: the published charts from Ruffwear and Kurgo. Checked October 2026; manufacturers revise these, so confirm before ordering.