What size harness for a Border Collie?
Most adult Border Collies fall somewhere in the medium-to-large range on a standard harness chart, but this is a breed with real variation between a lean working-line dog and a stockier show-line one, so measuring your own dog properly matters more than assuming a size from the breed name.
Measure the chest, not the breed name
Harnesses are sized by chest girth — the measurement around the widest part of the ribcage, just behind the front legs, not the neck or the length of the body. Stand your Collie squarely, wrap a soft tape measure around that widest point, and leave room for two fingers to slide comfortably underneath.
That single measurement matters far more than your dog's breed, age or even their weight in isolation, because it reflects their actual body shape rather than a general assumption about what a Border Collie 'should' measure.
Where most Border Collies land on a standard chart
Adult Border Collies typically stand 46 to 56cm at the shoulder and weigh 14 to 20kg, which on a typical sizing chart tends to put many dogs around the medium-to-large boundary — roughly where a chart's medium band (around 50 to 68cm chest) meets its large band (around 65 to 85cm).
Working-line Collies, bred for speed and endurance on a farm, often run leaner and narrower through the chest than the stockier build sometimes seen in show lines, which is exactly why 'most Border Collies are a medium' isn't reliable enough to skip measuring your own dog.
Getting the fit right beyond just the size letter
Once you have a size that matches your dog's measurement, check the harness has enough strap adjustment to fine-tune the fit, since a chest girth on the edge of two sizes benefits from a harness with generous adjustment either way.
Watch for rubbing just behind the front legs after the first few walks, particularly on a breed that runs, turns and stops hard during exercise — a harness that seemed fine standing still can shift and chafe once your Collie is properly moving. Re-measure a growing puppy every few weeks rather than assuming last month's size still fits.
The sizes we stock
Everything below is in stock with us, priced with delivery worked out, and packed and posted by hand in the UK.
Padded No-Pull Dog Harness Vest
A soft padded vest harness that spreads pressure evenly across the chest for comfortable, no-pull control. Adjustable straps, breathable mesh lining and reflective trim make everyday walks safer.
From£35.99
Faux-Leather Dog Harness & Lead Set
A smart faux-leather harness and matching lead set with gold-tone hardware and quick-release buckles. The padded, adjustable harness spreads pressure comfortably for stylish, secure walks. Suits small dogs and cats.
From£13.99
Dog Car Seat Belt Safety Tether
An adjustable nylon car safety tether that clips into your seatbelt buckle and onto your dog's harness, keeping them safely restrained while you drive. Strong metal clasp and durable webbing.
£24.99
Common questions
- How do I measure my Border Collie for a harness?
- Wrap a soft tape measure around the widest part of their chest, just behind the front legs, while they're standing squarely. Leave room for two fingers to fit comfortably underneath the tape for a snug rather than tight fit.
- What size harness fits an average adult Border Collie?
- Many adult Border Collies fall around the medium-to-large boundary on a standard chart, given their typical 14 to 20kg weight and 46 to 56cm height, but working-line and show-line dogs can differ noticeably. Measuring your own dog's chest is more reliable than assuming from these figures alone.
- Why does my Border Collie's harness rub?
- This often shows up just behind the front legs once a dog is running and turning properly rather than standing still, which is where a harness that looked fine in the shop can start to chafe. Checking the fit again after a proper walk, not just at home, helps catch this early.
- How often should I re-measure a growing Border Collie puppy?
- Every few weeks while they're still growing, since a puppy can outgrow a harness surprisingly quickly. A snug fit today doesn't mean it will still fit comfortably a month from now.
Sources
Written against guidance published by UK veterinary and welfare organisations. Always confirm specifics with your own vet.
Written by Matt Garnett, who runs Giddy Pets and is a lifelong pet owner — not a vet. This is general information, not veterinary advice, and it cannot replace an examination by the vet who knows your animal. How we write these guides.