Dog Collar Size Guide

A collar is sized on neck girth — measured where the collar actually sits, high on the neck near the head — plus a strap width matched to the dog's size. Get the girth right and the collar is safe; get the width right and it is comfortable. The two mistakes that cause most problems are buying to the collar's stated size rather than its adjustment range, and assuming a collar measurement is the same as a harness neck measurement. It is not: harness neck straps sit several centimeters lower, at the shoulders, and give a bigger number. This page covers both measurements, the charts, the two-finger rule and the breeds that need something non-standard.

Where Do You Measure for a Collar?

Position first, tension second.

Measure high on the neck, in the spot the collar will actually rest — just below the ears and the jawline, above the point where the neck broadens into the shoulders.

Wrap a soft tape all the way round, snug against the coat without compressing it.

Then add the fit allowance. Slide two fingers under the tape as you measure, or add roughly 2 cm for a small dog, 3–4 cm for a medium one and 5 cm for a large one.

Read it twice. Dogs move and coats compress differently each time. Two consistent readings beat one.

No tape measure? String and a ruler work exactly as well. Wrap, pinch, lay flat, measure.

And the key distinction: this is a different number from the neck measurement you take for a harness. A harness neck strap sits low, at the base of the neck where it meets the shoulders, and that measurement is several centimeters larger. Using one number for both produces either a loose collar or a harness the dog cannot get its head into — the harness method is in how to measure a dog for a harness.

Buy the Adjustment Range, Not the Size Label

The part of collar buying most people miss.

Collars are sold with a size name and an adjustment range. A "medium" might adjust between 35 and 50 cm. The name tells you very little; the range tells you everything.

Aim to land in the middle of the range, not at either end. A collar at its last hole has nowhere left to go if the dog gains weight or grows a winter coat. A collar at its first hole is bunched and untidy, and the excess strap flaps.

Which means a dog measuring 36 cm should probably buy the size whose range is roughly 30–45 cm rather than the one running 35–50 cm, even though both technically fit.

Check the range is printed before buying. Listings that give only a size name and a weight bracket are guessing on your behalf.

Typical Collar Size Chart

Orientation only — measure your own dog and check the specific product's range.

Weight brackets are not on this chart deliberately. They appear on most packaging and they are unreliable for the same reason they are unreliable for harnesses: two dogs of identical weight can have very different necks. A Whippet and a Staffie at 15 kg are nowhere near the same collar size.

How Wide Should the Strap Be?

A separate decision from girth, and one that affects comfort more than safety.

Wider spreads pressure. On a large or strong dog, a wide strap distributes force across more of the neck, which is more comfortable and less likely to mark the coat.

Too wide restricts movement. On a small dog, an oversized strap sits stiffly, interferes with head movement and is simply heavy. A 38 mm collar on a Yorkie is uncomfortable regardless of the girth being correct.

Rough matching: 10 mm for toy breeds, 15 mm for small, 20 mm for medium, 25 mm for large, 38 mm for giant.

Long-necked, fine-coated breeds — Greyhounds, Whippets, Salukis — do better on wider, padded collars despite being light dogs, because the neck is slender and the skin is thin. Sighthound collars are made wide for exactly this reason.

Heavy hardware counts too. A chunky buckle and D-ring add weight a small dog will notice all day.

The Two-Finger Rule

The check that decides whether the fit is right, once the collar is on.

Slide two flat fingers between the collar and the neck. They should go in with mild resistance.

One finger, with difficulty: too tight. Look for flattened or worn fur under the strap, dandruff or redness on the skin, or coughing when the lead is attached.

Three or more fingers: too loose. A loose collar can be backed out of, and it can also catch on things — a genuine hazard rather than a fit preference.

Check it against the head, too. A correctly fitted collar should not slide over the ears when pulled forward firmly. If it does, tighten it a hole.

Re-check after a week. New collars, particularly leather ones, stretch and settle.

Which Dogs Need Something Non-Standard?

Standard buckle collars suit most dogs. Some shapes and situations do not.

Sighthounds — Greyhounds, Whippets, Salukis, Lurchers. The head is narrower than the neck, so a collar loose enough to be comfortable is loose enough to slip straight off. These dogs need a martingale (limited-slip) collar, which tightens to a set point when pulled and no further, or a wide sighthound collar.

Broad-headed breeds — Bulldogs, Pugs. The opposite problem: the head is wider than the neck, so a collar that goes on easily is often too loose once in position. Check the two-finger rule rather than assuming.

Flat-faced breeds. Any pressure on the neck is a bigger concern for dogs with already-restricted airways. A harness is generally the better default for walking these breeds, with the collar reserved for identification.

Dogs that pull. A collar concentrates all the force on the throat. A front-clip harness redistributes it and is both kinder and more effective — sizing in no-pull harness size guide.

Puppies and young dogs, where growth makes any fixed size temporary.

Long-coated breeds. Collars mat the fur underneath. A rolled leather collar reduces this considerably compared with a flat one.

Collars for Puppies

Growth turns a correct collar into a tight one faster than most owners expect.

Check the fit weekly, not monthly. During a growth spurt a puppy's neck can gain a centimeter in two weeks, and a collar that was correct becomes genuinely dangerous.

Buy cheap and adjustable, and expect two or three collars before adult size. Spending heavily on a puppy collar is spending on something with a short life.

Start light. A young puppy's first collar should be as light as possible — a soft, narrow, lightweight strap. Weight and stiffness are why puppies scratch at collars.

Introduce it gradually. On for a few minutes with a distraction, then off. A puppy that learns to hate the collar in week one is harder work for months.

Watch for the warning signs: scratching at the neck, worn or flattened fur under the strap, coughing when the lead is attached, or a collar at its last hole. Any of those means resize now.

Adult size arrives at different times. Small breeds are close to final neck girth by nine months; large breeds keep thickening in the neck well past a year, sometimes to eighteen months.

Collar vs Harness: Which Should the Dog Wear?

Not an either-or in practice, and the sizing differs for each.

A collar's primary job is identification. In many places it is a legal requirement for a dog in public to wear a tag with the owner's details. That alone is a reason for every dog to have a well-fitting collar, regardless of what it walks on.

A harness's job is control and comfort on the lead. It spreads force across the chest rather than concentrating it on the throat, which matters for dogs that pull, flat-faced breeds, small dogs with delicate tracheas, and dogs with any neck or spinal issue.

Most owners end up with both, and that is a reasonable answer: collar on for ID, harness on for walks. The dog harness size guide covers the second half.

Don't attach the lead to both at once unless using a purpose-made double-ended lead — the two pull against each other.

And remember the measurements are different. Collar girth is taken high on the neck; harness neck girth is taken low at the shoulders. Two numbers, on the same dog.

Common Collar Sizing Mistakes

Using the harness neck measurement. Different position, bigger number, loose collar.

Buying on weight. Necks vary far more than weight suggests.

Buying to sit at the end of the adjustment range. Aim for the middle.

Choosing a strap width by looks. Too wide on a small dog is uncomfortable; too narrow on a large dog concentrates pressure.

Leaving it too loose "so it's comfortable". A collar that can be backed out of is a safety problem, and one that catches on things is worse.

Never re-checking. Coats thicken in winter, dogs gain and lose weight, leather stretches. Check the two-finger rule monthly.

Using a standard collar on a sighthound. Get a martingale.

Ignoring a weight change. If a collar suddenly does not fit and the dog is fully grown, check against healthy weight range for dogs by size, and weigh properly using the method in how to weigh your dog at home.

Forgetting other gear uses other numbers. Coats need three measurements, covered in dog sweater and coat size guide; life jackets use girth and length, in dog life jacket size guide.

FAQ

How do I measure my dog for a collar?

Wrap a soft tape around the neck where the collar will sit — high, just below the ears and jaw — snug against the coat. Then add about 2 cm for a small dog, 3–4 cm for a medium and 5 cm for a large one.

Is collar size the same as harness neck size?

No. A collar sits high on the neck; a harness neck strap sits low, at the base of the neck where it meets the shoulders. The harness measurement is several centimeters larger.

How tight should a dog collar be?

Two flat fingers should slide between the collar and the neck with mild resistance. One finger means too tight; three or more means the dog could back out of it.

What width collar does my dog need?

Roughly 10 mm for toy breeds, 15 mm small, 20 mm medium, 25 mm large and 38 mm giant. Slender-necked breeds like Greyhounds and Whippets do better on wider, padded collars despite being light.

Why does my Greyhound slip out of its collar?

Because a sighthound's head is narrower than its neck, so any collar loose enough to be comfortable can slide straight off. These breeds need a martingale collar, which tightens to a fixed point when pulled.

How often should I check a puppy's collar?

Weekly. A puppy's neck can gain a centimeter during two weeks-long growth spurt, and a collar that fitted correctly can become tight enough to cause harm in that time.

Girth Sets the Size, Width Sets the Comfort

Collar sizing is two decisions taken together: measure the neck girth high on the neck where the collar sits, add the fit allowance, and buy the product whose adjustment range puts you near its middle — then match the strap width to the dog's build. Confirm with two flat fingers under the strap, check the collar cannot pull over the ears, and re-check monthly, weekly for a puppy. Then size the harness separately, because that measurement is taken somewhere else entirely: dog harness size guide.