Your dog measures between two harness sizes. Which one do you buy?

You measured, you checked the chart, and your dog's chest sits in two sizes at once. This is not a mistake in the chart and it is not a mistake in your measuring. It is how harness ranges are built, and almost every dog lands in an overlap somewhere.

The instinct is to pick whichever number the measurement is closer to. That instinct is wrong often enough to be worth unlearning.

Why the ranges overlap on purpose

A harness is adjustable. The published range is not the size of the dog it fits — it is the span the straps can be let out and pulled in to cover. Manufacturers overlap consecutive sizes so that no dog falls into a gap between two products, which would be a returned order every time.

That means a 65cm chest genuinely is a Julius-K9 Mini and a Julius-K9 0. Both will close around the dog. They will not fit the same way.

The question that actually decides it

Not "which size is closer?" but "where in each range does my dog sit, and which direction is he going?"

A measurement near the bottom of a range means the straps will be near their tightest to close. There is room to grow into, and almost no room to take in.

A measurement near the top of a range means the straps are near their loosest. There is adjustment left to tighten, and nothing left to let out.

So:

• Puppy, or a dog still filling out — take the larger size. You want the room to grow, and a harness at the bottom of its range still fits properly today.
• Adult at a settled weight — take the size where the dog sits nearer the middle. That is the one with adjustment available in both directions, which is what you want when a coat thickens in winter or the dog gains a kilo.
• Adult that has been losing weight, or a dog on a diet — take the smaller size. You need room to take in, not let out.
• Dog that has already escaped a harness once — take the smaller size and fit it properly. Escapes are a fit-too-loose failure, and starting near the top of a range means you have nothing left to tighten.

The one case where you should ignore all of that

Deep-chested breeds. Greyhounds, Whippets, Dachshunds, Basset Hounds, Dobermans — their chest is disproportionately large relative to their weight, and the girth measurement is telling you the truth while the weight column on the chart is not.

If the chart's weight guidance and its girth guidance disagree for your dog, the girth wins. Weight is a sanity check that manufacturers publish because it is easy for owners to know. Girth is what the product physically has to go around.

Short-faced, barrel-chested breeds have the same problem in the other direction. A French Bulldog or a Pug carries far more chest than its weight suggests, which is why weight-based guidance under-sizes them more often than any other build.

Do not average the two sizes

Worth saying because people try it: if your dog is between a Medium and a Large, there is no half-step. Buying the Medium and hoping the straps stretch is how a harness ends up loose at the shoulder, which is exactly where a dog reverses out of one.

Check it with the tape, not the box

Once it arrives, the fit test is the same regardless of which size you chose: two fingers flat under any strap, no more and no less. If you can get a whole hand under it, it is too loose whatever the label says. If you cannot get two fingers in, it is too tight and it will rub behind the leg.

That test overrides the chart, the calculator and this article. The chart gets you to the right size to order. Your hands confirm it.

Let the tool do the comparison

The reason the between-sizes question is annoying is that you are reading one brand's chart in isolation. The sizing tool takes your measurement once and shows every brand at the same time, including where in each range your dog falls and which sizes overlap — so you can see whether the brand you were looking at is the one that fits your dog most centrally, or whether a different brand puts him comfortably in the middle of a size instead of on the edge of two.

Sometimes the answer to "which of these two sizes?" is "neither, because this other brand fits him properly."