The Five Harness Sizing Questions I Answer Every Week at Work

Short answer: girth in centimetres is the measurement that picks the size, taken with a soft tape around the widest part of the ribcage just behind the front legs. Two fingers should slide flat under every strap without you having to push, and between sizes you go up unless your dog is deep-chested and narrow like a whippet or a greyhound. If the harness slides the size is wrong; if it stays put and still rubs, the shape is wrong.

I answer customer messages for a pet gear store, which means I have read more harness sizing questions than any human should. The pattern in them never changes: people measure the wrong place, or they measure correctly and then doubt the number, or the harness arrives and does not behave the way the chart promised. None of this is the customer's fault. Sizing charts are written by people who know the product too well to explain it, so this is my attempt at the version I would send a friend: five questions, answered properly, with the returns data behind each answer.

Why sizing goes wrong more often than anything else

A harness is the only piece of gear we sell that has to fit a moving, breathing, opinionated animal in three dimensions at once. A bed can be slightly wrong and nobody notices. A harness that is slightly wrong slides, rubs, or gets escaped from, and the owner blames the harness rather than the size. In our returns data, fit-related returns outnumber every other reason combined, and most of them share one root cause: the size was chosen from the dog's weight or breed, not from a tape measure. Weight tells you nothing about girth. Two kelpies of the same weight can differ by eight centimetres around the chest, and the harness will only fit one of them.

Question one: which measurement actually decides the size?

Girth, always. Take a soft tape around the widest part of the ribcage, just behind the front legs, where the harness's chest strap will sit. That number, in centimetres, is the one you match to the chart. The neck measurement matters second, and only if the harness has a separate neck strap. Length down the back matters least for most designs, which surprises people, because back length is the number breed guides list first. If you only have thirty seconds and one hand free, measure the girth twice and trust the larger number. Fur, especially a winter coat, compresses under a tape, so measure snug but not tight, then check it again the following week when the tape is no longer a novelty.

Question two: how do I know it is too loose versus too tight?

The two-finger test is the standard and it works, with one refinement nobody mentions: the fingers have to slide flat, not squeeze in. If you are pushing, it is too tight. If your whole hand slides through, it is too loose, and looseness is the more common mistake. A loose harness rotates in a stop, and a rotating harness rubs in places the product photos never show: behind the front legs, along the armpit, and across the point of the shoulder. The other check is the pull test on a lead-attachment point: have someone hold the lead and walk the dog a few steps. If the chest strap migrates toward the throat, the harness is either too loose or the wrong shape for your dog, and no amount of adjusting fixes a shape mismatch.

Question three: my dog is between sizes — size up or down?

Size up, with one exception. A harness has adjustment in the straps, usually a few centimetres each way, so the larger size has room to come in and the smaller one has nothing left to give. The exception is a dog at the very bottom of a size range with a deep, narrow chest, such as a whippet or a greyhound: those shapes need a harness built for them, and neither size on a general chart will sit right. If you are truly between sizes, the honest answer is often that this particular harness is not the one, which is exactly the kind of thing a customer service person can tell you and a size chart cannot.

Question four: why did the same size fit my last dog differently?

Because the number that matched was a coincidence. I see this most with people who owned one dog for twelve years and then got another of a similar build. Girth differs with age, weight, coat and condition, and it changes over a dog's life: an adolescent filling out, a senior losing muscle over the spine, a dog who has spent six months on a different food. The practical version of the answer is to re-measure every time, keep the tape in the same drawer as the leads, and treat each new harness as a fresh purchase rather than a repeat order.

Question five: when is it a sizing problem and when is it a shape problem?

This is the question that saves the most money, so here it is plainly. If the harness slides, the size is wrong. If the harness stays put and still rubs, the shape is wrong. Sliding is fixed by measuring properly and adjusting straps. Rubbing means the harness's geometry disagrees with your dog's anatomy, and the fix is a different cut: a different chest strap angle, a wider yoke, or a design built for deep-chested dogs. Return it and tell the seller what happened. Any decent customer service team can steer you to the shape that suits your dog's build once they know it is a shape issue rather than a sizing one, and you will save yourself a second and third purchase.

What I tell people before they buy

Measure the girth, twice, with a soft tape. Write the number down and keep it in your phone, because you will need it again. Look at the harness's adjustment range rather than its headline size, and check where your dog's measurements fall inside that range: aiming for the middle is far more forgiving than arriving at the edge. Then ask us the shape question if your dog is deep-chested, narrow, long-backed, or shaped like a loaf of bread. We answer it every day, and there is nothing on the message that has not been asked before.

• Measure the girth twice with a soft tape and keep the number in your phone.
• Judge the adjustment range rather than the headline size, and aim for the middle of it.
• Ask the shape question if your dog is deep-chested, narrow or long-backed.
• Treat the first harness on a growing puppy as a training tool rather than an investment.

Frequently Asked Questions