Most harness returns aren't a quality problem. They're a five-minute measuring step that got skipped in favor of guessing a size from a photo. Measuring a dog properly takes less time than the return process does, and it's the difference between a harness that fits right out of the box and one you're re-ordering next week.
What you need
A soft fabric tape measure works best. No tape measure, no problem - a piece of string or ribbon and a ruler does the same job; measure the string afterward instead of trying to read a curved tape against fur. You'll also want a few treats, since a dog standing still and relaxed gives you a far more accurate number than one twisting away from a stranger-feeling tape around their body.
Step 1: Chest girth
This is the measurement that matters most for a harness. Wrap the tape around your dog's body at the widest point behind the front legs - roughly where the elbows sit when your dog is standing normally. Keep the tape snug against the fur but not compressed into it; you want the natural girth, not a squeezed one. As a working guide, you should be able to fit two fingers flat between the tape and your dog's body.
Step 2: Neck girth, if the harness calls for it
Some harness styles, and matching collar-and-leash sets, also size off neck circumference. Measure where a collar would naturally sit, using the same snug-but-not-tight standard as the chest measurement. A plain 19mm-wide leash and collar is sized this way rather than off chest girth, so the neck number is the only one that matters for it.
Step 3: Check the number against a real size chart
Once you have your measurements, compare them against the specific product's size chart rather than a generic "small/medium/large" guess - the same label means different things across different harness styles. Our adjustable no-pull harness, for example, is offered across sizes from small through 2XL specifically so you can match your dog's actual chest girth number to a listed size rather than eyeballing it against their breed name.
Personalised harnesses are worth measuring twice for, because the patch is made up against the size you order rather than something you can swap later. The personalised no-pull harness with a custom patch and the custom patch reflective harness are both listed for small through large dogs, so the girth reading decides which you order.
Large-breed owners in particular benefit from a listing that publishes the actual range. Our padded-handle vest harness in L gives a chest circumference of roughly 80-105cm (31.5-41.3 inches), so you can hold your tape reading against a number instead of a breed name. The same padded-handle vest in black is the option to order if you would rather not take whatever colour the main picture shows.
Step 4: When you're between two sizes
If your measurement falls between two listed sizes, which way you round depends on the dog. Size up for a puppy still growing, a thick or double-coated breed where fur adds bulk to the reading, or if you want extra buckle range to spare. Size down for a lean or deep-chested breed - Greyhounds, Whippets, and similar builds - where a harness sized generously tends to sit too loose across the chest.
For a puppy specifically, the adjustment range matters more than the size on the label — our guide to the best harness for a fast-growing puppy covers what to look for.
Step 5: The two-finger fit-check, once it's on
Buckle the harness at what looks like the middle adjustment setting and run this check before you head out the door:
- Slide two fingers flat under every strap. If you can't get two fingers under, it's too tight. If you can fit a whole hand, it's too loose.
- Watch your dog walk a few steps. A harness that twists to one side or slides toward the neck isn't adjusted correctly yet, even if the girth number was right.
- Check behind the front legs for any rubbing after a short walk, particularly with a new harness on a puppy or a dog with thin fur in that area.
Measuring at home takes five minutes and removes almost all the guesswork that leads to a return. Browse our full range of adjustable harnesses once you've got your numbers, and check what 1.2m and 5ft mean on a walk before you pick the lead to go with it.