Photo: Unsplash (free licence).
Quick answer: measure the chest just behind the front legs and the neck with a flexible tape, compare the centimetres with the size chart for that specific model and, when fitting, check that two fingers fit between the strap and the body. Weight or breed are only a guide: the measurement decides.
A badly fitted harness rubs, slips off or restricts movement. The good news is that getting the size right takes less than five minutes if you follow a set order. You'll need a flexible (sewing) tape measure, a piece of paper to note things down and a few treats to keep your dog calm.
Step 1. Measure the chest
Wrap the tape around the widest part of the ribcage, just behind the front legs. It should be snug but not tight. Measure twice with your dog standing and note the larger figure.
Step 2. Measure the neck
Measure around the base of the neck, where a wide collar would sit, not the narrow part next to the head. Many H- and Y-shaped harnesses have a strap here, and the manufacturer's chart asks for this measurement.
Step 3. Compare with that model's size chart
The figures in the illustration are an example: every brand uses its own ranges, and a size S in one model may be an M in another. If your measurement falls on the boundary between two sizes, check how much adjustment each one allows before deciding.
Step 4. Put it on and use the two-finger rule
With the harness on, you should be able to slide two flat fingers between any strap and the body. If your whole hand fits, it's too loose and your dog could slip out; if your fingers don't fit, it's too tight. Also check that the straps don't rub the armpits.
Step 5. Test it at home before the first walk
Let your dog walk, sit and lie down in the harness for a few minutes, rewarding calm behaviour. Watch whether its gait changes or it tries to get the harness off. If all is well, you're ready to go out; with puppies, measure again every few weeks because they grow fast.
Checklist
Tick it off as you go:
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Frequently asked questions
Can I choose the size by weight or breed alone?
It's not recommended. Two dogs of the same weight can have very different chest sizes. Use weight only as a guide and decide based on the measurement.
Should I buy a size up so it lasts longer?
No. A harness that's too big shifts, rubs and makes it easier for your dog to slip out. Choose the right size and check it again as your dog grows.
Harness or collar?
It depends on the dog and the walk. If your dog pulls a lot or has neck discomfort, ask your vet or a dog trainer which equipment suits it; a harness alone won't correct behaviour.