How to measure a dog that will not stand still

Every harness chart on the internet sizes by chest girth, and every one of them assumes you can get an accurate number off a live dog. If your dog treats a tape measure as a toy, a threat, or an invitation to roll over, that assumption is doing a lot of work.

Here is the measurement, then the ways round the dog.

The measurement, precisely

Chest girth is the circumference of the ribcage at its deepest point, immediately behind the front legs.

Not the neck. Not the belly. Not the widest point of the shoulders. The spot where a girth strap would naturally sit, roughly two to three centimetres behind the elbows.

Three rules that matter more than they sound:

1. Dog standing, on all fours. A sitting dog's ribcage spreads and reads a few centimetres large.
2. Snug against the coat, not pulled tight. You are measuring the dog as the harness will meet it. Compressing the coat gives you a number the harness will be loose against.
3. Level, all the way round. A tape that rides forward under the chest and back over the spine is measuring a diagonal, which reads long.

When the dog will not cooperate

Measure during a meal. The single most effective trick. A dog with its head in a bowl is standing still, on all fours, in a predictable place, thinking about something else entirely. You have about twenty seconds.

Use string, not tape. A tape measure is stiff, cold, and makes a noise when it retracts — three things a nervous dog objects to. A bootlace or a phone charging cable is silent and soft. Wrap it, pinch where it meets, take it off the dog, and measure the string against a ruler with the dog somewhere else.

Get a second person. One holds a treat at nose height to keep the dog standing and facing forward, the other measures. This turns a wrestling match into a five-second job.

Measure a harness that already fits instead. If you have one that fits well, do up the girth strap to its current setting, take it off, lay it flat, and measure the loop. That is your girth. It bypasses the dog entirely and it is often more accurate than measuring a wriggling animal.

Take three measurements and use the largest. Rushed measurements are almost always short, because a tape that slips reads small. If you get 62, 64 and 63, use 64.

Do it after a walk. Not before. A dog that has just run for forty minutes is much easier to persuade to stand still than one that has been indoors since breakfast.

No tape and no string

Use a piece of A4 paper. The long edge is 29.7cm, the short edge 21cm. Wrap it round, mark where it overlaps, and add. Two long edges plus a bit is about 65cm, which is enough to get you into the right size on most charts.

Or start from the breed. A breed's published weight range narrows the answer to a shortlist before you measure anything — the breed pages work that range through each manufacturer's own chart, which is enough to tell you whether you are shopping for a Medium or a Large even if it cannot give you the final answer.

The mistakes that cost you a return

Measuring the neck and calling it the chest. Common, and it produces a harness several sizes too small. If your number is smaller than your dog's collar setting plus a bit, you have measured the wrong thing.

Measuring a puppy and buying for now. Puppies change fast. Measure, then buy the size that puts the current measurement near the bottom of its range, so there is somewhere to grow into.

Measuring through a thick winter coat and not re-checking. A double-coated dog can read four or five centimetres larger in February than in July. Measure at the coat length the dog is usually kept in, and re-check after a groom.

Rounding down. If you get 63.5cm, use 64. A harness with adjustment left is safe; a harness at the end of its adjustment is one hard reverse away from coming off.

Then use the number once

The reason to get this right in one go is that a single accurate girth answers the question at every brand simultaneously. Letter sizes are not comparable — the same dog is a Julius-K9 "1" and a Ruffwear "M" — so a measurement is portable in a way that "he's a Medium" is not.

Put the number in once and read the answer for every brand, including which sizes overlap for your dog and where in each range he sits. That last part is what decides whether the harness that arrives has adjustment left in it.