How tight should a dog harness be?

Snug enough that it cannot move around the dog, loose enough that it never presses. In practice that means two fingers, laid flat, under every strap, with the dog standing — and a harness that stays where you put it when you tug it backwards. Most fitting problems come from doing the first check wrongly or skipping the second.

The two-finger check, done properly

Everyone has heard of it. It goes wrong in four ways:

• Fingers on edge instead of flat. Two fingers stacked or turned sideways need far more slack than two fingers lying flat against the dog. Flat is the test.
• Dog sitting or lying down. A sitting dog's ribcage spreads and its shoulders move forward, so the same harness reads tighter than it will be on the walk. Check with the dog standing square on all fours.
• Checking one strap. Owners check the strap they can reach — usually across the back, which carries little load. The strap that matters most is the girth strap behind the front legs; check the neck strap too, and any belly strap.
• Checking once. Webbing beds into the coat and adjusters can creep. Check again after the first ten minutes of the first walk, and again after a wet walk.

What you are feeling for: two fingers slide in with light contact on both sides. If they will not go in without pushing, it is too tight. If a third finger fits, or your fingers can turn over, it is too loose.

The check most people skip: does it travel?

A harness can pass the finger test and still be loose in the way that matters. Take hold of the girth strap and try to slide it forwards along the ribcage, towards the front legs. It should stop almost at once against the back of the elbows. If it walks forwards several centimetres, the harness is too big for this dog — a dog reversing in a panic will do exactly what your hand just did, and the harness comes off over its head.

Then hold the harness at the back and pull gently backwards and upwards, roughly the way the dog's own weight would if it threw itself into reverse. A good fit moves the dog, not the harness. If the chest piece climbs towards the throat, tighten the girth strap or go down a size.

Where each strap should sit

The neck strap

On the base of the neck, resting on the breastbone at the front — not up on the throat. When the lead goes tight it should press on bone, not on the windpipe. If the dog coughs when it pulls, the neck strap is sitting too high or the harness is riding forward.

The girth strap

Around the ribcage, about a hand's width behind the front legs on a large dog and two or three fingers behind on a small one. Too close and it rubs the soft skin behind the elbow with every stride; too far back and it sits on the belly rather than the ribs, where it can slip.

Anything across the front of the chest

A strap running straight across the front of the shoulders limits how far the front legs can reach. If the harness has a chest piece, it should sit on the breastbone in a Y and leave the point of each shoulder free. Watch the dog trot: a shortened, choppy front stride in the harness that disappears off-lead is the strap talking.

Adjusting a new harness from scratch

1. Let every strap out before the first fitting. Fitting a harness that arrived pulled tight usually ends with one strap doing all the work.
2. Put it on and fasten it, then set the neck strap first so the front sits on the breastbone.
3. Set the girth strap next, until two flat fingers fit with light contact.
4. Do the travel check and the backwards pull.
5. Look at the buckles and sliders: none should sit in the armpit, and none should be at the very end of its strap. A slider at its last few centimetres means the harness is the wrong size, not that it needs pulling harder.
6. Walk ten minutes, then check again.

When the right tension is impossible

If you cannot get the two-finger fit without running out of strap — or if the girth strap is right but the neck is loose — the harness is the wrong size or the wrong shape for the dog, and no amount of adjusting will fix it. Two common cases:

• The size is on an edge. If the chest measurement sits at the very top or bottom of the size's published range, there is no adjustment left in one direction. Look for a size, or another brand, that puts the dog nearer the middle. Between two sizes covers how to choose.
• The shape does not suit the build. Deep-chested, narrow-headed dogs often need a neck and girth that adjust independently, and sighthounds often need a third strap around the waist. Sizing a deep-chested dog goes through it.

The size has to be right before the tension can be. Measure the chest behind the front legs, standing, and the size finder shows where that measurement sits inside every brand's sizes — a dog in the middle of a size is a dog that can be fitted properly.