A harness that fits is invisible. Your dog forgets it is there, it does not shift on the walk, and you stop thinking about it. A harness that does not fit shows up as a bald patch behind the elbow, a dog who freezes when you bring it out, or the sickening moment on a pavement when they reverse out of it entirely.
Almost all of that is sizing, and almost all of it is fixable.
The short answer
To size a dog harness, measure your dog's chest girth with a soft tape measure at the widest point just behind the front legs, and measure the base of the neck where a collar sits. Match the chest measurement to the maker's chart first, because chest girth is the measurement that decides the size. If your dog falls between two sizes, size up. Then check the fit: two fingers should slide flat under any strap, and the chest strap should sit a few centimetres behind the armpit, never in it.
Why you cannot go by breed or weight
Sizing charts based on breed are a rough marketing shorthand and nothing more. Two cockers of the same weight can be four centimetres apart at the chest. A whippet and a pug can weigh the same and need entirely different shapes. Even within one brand, sizes are not transferable: our Small Dog Harness from My Chi and Me comes up larger than several well-known small-dog brands, which is exactly why its own chart is written as XXS fits like a small, XS fits like a medium, and so on.
Measure the dog. It takes ninety seconds and saves a return.
How to measure your dog for a harness
Use a soft dressmaking tape. If you only have a rigid one, use a piece of string and lay it flat against a ruler afterwards.
- Stand your dog square, on all four legs, on a flat floor. Not sitting, not on a sofa. A sitting dog reads two to three centimetres larger at the chest.
- Chest girth. Pass the tape around the deepest part of the ribcage, just behind the front legs, and back up to meet itself. Snug against the coat, not pulled tight. This is the number that matters most.
- Neck. Measure around the base of the neck where a collar naturally sits, low, at the shoulders, rather than high under the jaw.
- Back length, if the chart asks for it. From the base of the neck to the base of the tail.
- Write the numbers down in centimetres and compare them to the specific chart for the specific harness. Not a generic one.
A worked example
The Rose Wylie Harness is sized like this, and it is a good illustration of how much the ranges overlap:
| Size | Neck | Chest |
|---|---|---|
| XS | 20 to 28cm | 30 to 38cm |
| S | 25 to 35cm | 35 to 56cm |
| M | 30 to 46cm | 46 to 71cm |
| L | 40 to 53cm | 61 to 97cm |
A dog measuring 36cm at the chest sits inside both XS and S. The rule that settles it is the same one nearly every maker prints and nearly every owner ignores: if your dog is between sizes, size up. A harness with adjustment left in it can be brought in. A harness that is already on its last hole cannot be let out.
The two-finger rule, properly explained
Everyone has heard it. Most people apply it wrongly.
Two fingers should fit flat, side by side, lying against the dog's body, under any strap. Not two fingers stacked. Not two fingers turned sideways and forced. If you can only get one finger in, it is too tight and will rub. If your whole hand slides under, it is too loose and your dog will eventually back out of it.
Check at three places: the neck opening, the chest strap under the body, and where the strap crosses the shoulder. A harness can be correct at one and wrong at another, which is the usual reason an otherwise well-sized harness still rubs.
Where the straps should actually sit
- The chest strap runs behind the front legs with a clear gap, ideally two to three finger widths back from the armpit. Sitting in the armpit is the single most common fitting fault, and it is what causes the bald patch and the sore.
- The front piece sits across the breastbone, in front of the chest, not up against the throat.
- The neck opening sits at the base of the neck, on the shoulders.
- The back D-ring should sit centrally between the shoulder blades and stay there. If it drifts to one side within a few minutes of walking, the harness is too loose or the wrong shape for your dog.
- Nothing should restrict the shoulder. Watch your dog trot away from you. The front legs should extend fully forward. If the stride looks shortened or choppy, the harness is cutting across the shoulder joint.
Adjusting a harness step by step
- Loosen every strap fully before the first fitting. Fighting a harness onto a dog while it is still set for someone else's dog teaches them to dread it.
- Put it on with the dog standing, rewarding calmly throughout.
- Tighten the neck first, to the two-finger fit.
- Tighten the chest second, again to two fingers, checking the strap has stayed behind the armpit.
- Walk the dog around the room, then re-check both. Fabric settles and dogs breathe out.
- Re-check after the first proper walk, and again after the first wash.
Common problems, and what is actually causing them
My dog escapes the harness
Usually a chest strap set too loose, or a harness one size too big bought because the dog is still growing. A dog reverses out by backing up and dropping the shoulders through the front opening. Tighten to the two-finger fit, and for a determined escape artist choose a design with a strap that sits properly behind the ribcage rather than one that relies only on the neck.
It rubs behind the front legs
The chest strap is too far forward, too tight, or both. Move it back, loosen to two fingers, and give the skin a few days to recover. Wet fabric against skin makes rubbing far worse, which is why this turns up so often in autumn.
It spins sideways on every walk
Too loose overall, or the wrong shape for the dog's build. Deep-chested dogs and barrel-chested dogs need different geometry, and a harness that fits the girth number can still be wrong in shape.
My dog freezes or refuses to move in it
Check for pinching at the shoulder and armpit first, because discomfort looks like stubbornness. If the fit is genuinely good, it is a training question, not a sizing one: harness on, treat, harness off, repeated over a few days, before you ever attach a lead.
The harness fitted last month and does not now
Puppies. Expect to re-measure monthly until they are done growing, and to replace at least once. Winter coats also add real girth on double-coated breeds, so a harness fitted in August can genuinely need letting out by November.
Harness or collar?
Both, for most dogs. A harness takes the pressure off the neck and is the better choice for walking, for dogs who pull, for flat-faced breeds and for anyone with a tracheal or neck issue. A collar is where the ID tag lives, and UK law requires a dog in a public place to wear a collar with the owner's name and address on it, so the collar does not become optional just because you use a harness. Our guide on harness versus collar goes into the trade-offs.
What to look for when you buy
- Adjustment in at least two places, neck and chest. One-point adjustment fits very few dogs well.
- A published measurement chart in centimetres, not just breed names.
- Wide, soft webbing or padding at the points that bear load.
- A back attachment ring as standard. A front ring as well is genuinely useful for dogs who pull.
- Washability, because a harness worn through a British winter gets filthy. Check whether it is machine washable or hand-wash only before you buy, not after.
Two examples from our own range, to show the range of approaches. The Rose Wylie Harness is stretch webbing with three lead attachment points, adjustable at neck and chest, hand-wash and air-dry, and it comes with a matching lead and poo bag holder. The Buddy's Peruvian Handmade Harness is a hand-braided woven cotton body with naturally tanned leather detailing, made in small batches in Valencia, sized XXS to L and spot-clean only. Different dogs, different lives, different right answers. Both live in the Harnesses collection.
Caring for a harness so the fit lasts
A stiff, salt-crusted harness fits worse than a clean one and rubs more. Follow the maker's instructions rather than a general rule: some are machine washable at 30 degrees and dried flat, some are hand-wash with mild soap and air-dry, and anything with leather detailing should be spot-cleaned with a damp cloth and kept away from the machine entirely. Never tumble dry any of them. Heat shrinks webbing and hardens leather, and both change the fit.
Frequently asked questions
How do I measure my dog for a harness?
Stand your dog square and measure the chest girth with a soft tape at the widest point just behind the front legs, plus the neck at the base where a collar sits. Compare those numbers to the chart for that specific harness, since sizes are not consistent between brands.
What if my dog is between two harness sizes?
Size up. Every maker we stock says the same, and the reason is simple: you can tighten a larger harness into a good fit, but you cannot make a small one bigger.
How tight should a dog harness be?
Two fingers should slide flat underneath any strap, lying against the body rather than stacked or forced. Tighter than that will rub, looser than that and your dog can back out of it.
Where should the chest strap sit?
Behind the front legs with a gap of two to three finger widths from the armpit. A strap sitting in the armpit is the most common cause of rubbing and hair loss.
Can a dog wear a harness all day?
It is better not to. Take it off at home so the skin and coat can breathe, especially if the harness is damp from a walk. A collar with an ID tag is what your dog wears day to day.
How often should I re-check the fit?
Monthly for a growing puppy, seasonally for an adult, and always after a wash or after a coat change. It takes thirty seconds with two fingers.
The last word
Getting a harness right is one of those quiet things nobody notices, least of all the dog, which is exactly the point. Ninety seconds with a tape measure now is a winter of walks where nothing pinches, nothing slips and nobody has to think about it again.
Browse the Harnesses collection or the wider Walkies range. Free UK delivery on orders over £30.