How tight should a dog's collar be?

Slide two fingers flat under the collar at the side of your dog's neck. If they slip in and out easily but you couldn't fit a third, the fit is right. Battersea and Dogs Trust both teach the same check, and it works for puppies and giant breeds alike because it scales with the dog.

Do the check standing, not while your dog is lying down. Necks change shape when a dog lowers its head, which is exactly the position they are in when they back out of a collar.

Most collars that fail are too loose, not too tight. A frightened dog drops its head, plants its front feet and reverses, and a collar with three or four fingers of slack slides straight off over the ears. It happens most often at the vet, at fireworks and when a lead is pulled tight at a kerb.

If your dog has a narrow head compared with its neck — greyhounds, whippets, lurchers and many collies — a normal collar will always be a poor fit. A wide martingale or sighthound collar tightens a fixed amount under pressure and is the safer answer.

A collar left too tight for a long time can grow into the skin, which is a serious welfare problem and a vet job. Any smell, discharge or broken skin under a collar means a vet appointment, not a loosened buckle.

• You can't get two fingers under without forcing them.
• A flattened line or bald patch in the coat where the collar sits.
• Damp, pink or sore skin under the collar.
• Coughing or a change in breathing when the lead goes tight.
• Your dog scratches at the collar or tries to get it off.

• Puppies: every week or two. They change shape faster than you expect.
• After a clip, moult or a big coat change — a spaniel in August is a different size from the same spaniel in February.
• After weight gain or loss, including a diet your vet has put them on.
• When you swap between a winter coat and a summer one.
• Any time the collar has taken a hard pull.

Fit is only half the job. In the UK, the Control of Dogs Order 1992 says a dog in a public place must wear a collar with the owner's name and address on it, or on a tag attached to it. A phone number is optional but sensible, and a microchip does not replace the tag. The details are in our UK dog tag law guide.

Check the tag with the same hands-on habit as the collar. Engraving wears smooth, split rings work loose, and a tag that has fallen off is the most common reason a found dog can't be reunited quickly. Our QR code dog tags sit flat against the collar and can't be rubbed blank, and many owners keep an engraved tag on the other ring as a backup — the case for both is in QR tag or engraved tag.

A harness spreads pressure across the chest and is kinder for dogs that pull, have breathing trouble or a delicate neck. It does not replace the collar in law, though: the ID still has to be on a collar worn in public. Most owners walk on a harness and keep a light collar with the tag on it underneath.

Harness fit uses one finger, not two, under the strapping — tighter than a collar, because a loose harness twists and rubs.