As an Amazon Associate I earn from qualifying purchases. Links to Amazon are affiliate links. Prices are shown on Amazon, not here.
Quick answer
An adult Border Collie usually measures 55–75 cm around the chest and weighs 12–20 kg1.
These are the sizes most adult Border Collies need. Your dog may differ, so measure to be sure.
Get an exact size for your dogHow a Border Collie is built, and why it matters for fit
The breed standard describes the Border Collie as "athletic in appearance", with well-sprung ribs, a chest that is "deep and rather broad", and deep, muscular loins that are not tucked up2. Compared with a Lab of the same height, a Collie is lighter, with a narrower but still deep chest.
Because the loins are not tucked up, the body stays fairly even in width from chest to waist. That helps a harness stay in place, but it also means the girth strap can drift backwards on a dog that is moving fast unless it is fitted snugly.
The shoulders do a lot of work in a breed that sprints and turns. In one small treadmill study of nine dogs, both of the harness styles tested reduced how far the shoulder extended, and the one marketed as "non-restrictive" reduced it slightly more3. The study was small, so treat it as a reason to check how your dog moves in its harness rather than a verdict on any one style.
How to measure your Border Collie, step by step
- Get a soft tape measure, or a piece of string and a ruler. Stand your dog on a level floor with all four feet square underneath it.
- Find the measuring point: the widest part of the ribcage, just behind the front legs4. Julius-K9 asks you to measure a set distance behind the front legs, which depends on the size: two finger-widths for the Baby sizes, three for Mini-Mini and Mini, and four for size 0 and up5.
- Wrap the tape all the way round. Keep it flat and untwisted, snug against the body but not squeezing. Read the number in centimetres where the end meets the tape.
- Measure twice4. If the two readings differ, use the larger one.
- Weigh your dog too. Julius-K9 gives a weight range for every size as a second guide5; the other brands here size by chest only.
Border Collie measuring tips
- Border Collies rarely stand still. Measure after exercise, and use a "stand" or "wait" cue with a treat.
- If your Collie has a longer coat, press the tape through it so it sits against the body.
- Check whether your dog falls in Julius-K9 Mini or size 0 territory: Mini is measured three finger-widths behind the front legs, size 0 four5. If unsure, measure at both points.
Want every detail, including neck, back length and weighing at home? Read our complete guide to measuring your dog.
Border Collie harness size by brand
Each brand cuts its sizes differently, so the same dog can be a different size, or a different letter, in each. Here is how a typical adult Border Collie fits each chart, and why.
Julius-K9
IDC Powerharness
Julius-K9 lists a chest range and a weight range for every size5. Chest girth is the main guide; we use weight to choose between two sizes when the chest fits both.
- A smaller Border Collie (chest 55 cm, 12 kg): size Mini
- A typical Border Collie (chest 65 cm, 16 kg): size 0 (also fits Mini, 1)
- A larger Border Collie (chest 75 cm, 20 kg): size 0 (also fits 1, 2)
Overlaps in the Border Collie range: 58–67 cm: sizes Mini and 0 both fit; 63–75 cm: sizes 0 and 1 both fit; 71–75 cm: sizes 1 and 2 both fit.
Sizes overlap by design; chest girth is the main guide, weight is secondary.
| Size | Chest | Weight |
|---|---|---|
| Baby 1 | 29–36 cm | 0.8–3 kg |
| Baby 2 | 33–45 cm | 2–5 kg |
| Mini-Mini | 40–53 cm | 4–7 kg |
| Mini | 49–67 cm | 7–15 kg |
| 0 | 58–76 cm | 14–25 kg |
| 1 | 63–85 cm | 23–30 kg |
| 2 | 71–96 cm | 28–40 kg |
| 3 | 82–115 cm | 40–70 kg |
| 4 | 96–138 cm | 70–90 kg |
Highlighted rows are the sizes typical adult Border Collies need.
Check price on AmazonRuffwear
Front Range Harness
Ruffwear sizes the Front Range Harness by chest girth only6, so your tape measure is all you need.
- A smaller Border Collie (chest 55 cm): size XS
- A typical Border Collie (chest 65 cm): size S
- A larger Border Collie (chest 75 cm): size M
Sized by chest girth only.
| Size | Chest |
|---|---|
| XXS | 33–43 cm |
| XS | 43–56 cm |
| S | 56–69 cm |
| M | 69–81 cm |
| L/XL | 81–107 cm |
Highlighted rows are the sizes typical adult Border Collies need.
Check price on AmazonBailey & Coco
Trail & Glow Harness
Bailey & Coco sizes the Trail & Glow Harness by chest girth only7, so your tape measure is all you need.
- A smaller Border Collie (chest 55 cm): size M (also fits S, L)
- A typical Border Collie (chest 65 cm): size L (also fits S, M)
- A larger Border Collie (chest 75 cm): size L (also fits XL)
Overlaps in the Border Collie range: 55–66 cm: sizes S and M both fit; 55–71 cm: sizes M and L both fit; 71–75 cm: sizes L and XL both fit.
Very adjustable, so neighbouring sizes overlap a lot. Sized by chest girth only.
| Size | Chest |
|---|---|
| XXS | 22–41 cm |
| XS | 26–50 cm |
| S | 35–66 cm |
| M | 40–71 cm |
| L | 55–83 cm |
| XL | 71–99 cm |
Highlighted rows are the sizes typical adult Border Collies need.
Check price on AmazonPuppies and growing dogs: when to re-measure
Adult Border Collies in our data weigh 12–20 kg, which is in the middle of the size groups used in research on puppy growth. Medium dogs reach adult weight sooner than large breeds, but later than toy breeds8. Collie puppies often go through a lanky stage, so re-measure the chest rather than judging by eye.
The largest study of puppy growth so far, based on records from over 6 million young dogs, grouped dogs by adult weight: under 6.5 kg, 6.5–9, 9–15, 15–30 and 30–40 kg8. An adult Border Collie at 12–20 kg falls in the 9–15 kg and 15–30 kg groups.
Size a puppy on its chest measurement today, not on the size you expect it to reach4. A harness bought to "grow into" is loose now, and a loose harness slips, rubs and is easier to back out of.
While your puppy is growing fast, we suggest measuring its chest every two weeks, then monthly until it stops changing. A quick check in between: if you can no longer slide two fingers under the straps, measure again.
Most harnesses adjust over a good part of their range, so a growing puppy may stay in one size for a while. When your puppy's chest reaches the top of its size's range, move up.
If your Border Collie is between two sizes
Brand charts often overlap, so a chest measurement can fall inside two sizes. Here is where that happens for a typical adult Border Collie:
- Julius-K9: 58–67 cm: sizes Mini and 0 both fit; 63–75 cm: sizes 0 and 1 both fit; 71–75 cm: sizes 1 and 2 both fit.
- Ruffwear: no overlapping sizes in this range.
- Bailey & Coco: 55–66 cm: sizes S and M both fit; 55–71 cm: sizes M and L both fit; 71–75 cm: sizes L and XL both fit.
How to choose:
- Our size finder picks the size where your dog's measurement sits nearer the middle of the range, so the harness can be adjusted either way.
- Ruffwear says that if your dog is right on the border between two sizes and fully grown, choose the larger size6. For a growing puppy that is truly between sizes, Ruffwear also suggests the larger one4.
- For Julius-K9, check the weight ranges too: if your dog's weight only fits one of the two sizes, go with that one5.
- Deep-chested dogs with a narrow waist can slip a loose harness, so if the larger size would only just fit on its tightest setting, the smaller one is usually safer.
Fit checks when the harness arrives
Two-finger rule
You should be able to slide two fingers flat under every strap, and no more6. Tighter than that can rub; looser can slip.
Strap behind the front legs
The strap around the body should sit a little way behind the front legs, not pressed up into the "armpit", where it will rub as your dog walks.
Nothing on the throat
Every strap at the front should sit on the breastbone or low on the chest, not across the throat.
Free shoulders
Watch your dog walk and trot. Its stride should look the same as without the harness. A small study found harnesses can shorten how far the shoulder reaches forward3, so if the stride looks shorter, adjust the fit or try another style.
Check for rubbing
After the first few walks, look behind the front legs, under the chest and around the neck for redness or broken hairs. If you find any, loosen or reposition the harness. If the skin is sore, stop using it and ask your vet.
Escape test
With the harness on, gently pull the lead forwards, then backwards and upwards. The harness shouldn't slide over the shoulders or ribs. If it does, tighten it, or choose a design with an extra strap behind the ribcage.
Re-check after a week
Webbing and buckles can settle in the first few walks. Check the two-finger fit again after a week.
Harness styles that suit a Border Collie
Y-front harness with the chest strap clear of the shoulder
Keeps the strap off the throat and away from the point of the shoulder. Watch your dog trot in it: the stride should look the same as without the harness.
Lightweight, low-profile harness
Less bulk means less heat and less to catch on brambles for an active dog.
Step-in or clip-on design
Useful for an excited dog that won't stand still while you pull something over its head.
Be careful with
- Any harness where a strap runs straight across the front of the shoulders. Check how your dog moves in it before you commit.
- Walking harnesses used for running sports. If you do canicross or bikejoring, use a harness designed for pulling; it is sized differently and isn't covered here.
Common mistakes
- Fitting the harness loosely "so it doesn't restrict". A loose harness moves around at speed and rubs more.
- Judging the fit only when the dog is standing. Watch it walk, trot and turn.
- Measuring the neck instead of the chest. Harness sizes are set by chest girth.
- Assuming a size letter means the same thing in every brand. A Medium in one brand can be a Large in another, so check each chart in centimetres.
- Buying a size up "to grow into" for a puppy.
Collar and ID tag: the UK rule
In the UK, a harness doesn't replace a collar. A dog in a public place must wear a collar with the owner's name and address on it, or on a tag or plate attached to it9. Many owners clip the lead to the harness and use the collar for ID.
Border Collie harness questions
What size harness does a Border Collie need?
An adult Border Collie usually has a chest of 55–75 cm and weighs 12–20 kg1. That usually means Julius-K9 IDC Powerharness: sizes Mini or 0; Ruffwear Front Range Harness: sizes XS to M; Bailey & Coco Trail & Glow Harness: sizes M or L. Measure your dog's chest to get the exact size.
How do I measure my Border Collie for a harness?
Stand your dog square, then wrap a soft tape around the widest part of the chest just behind the front legs4. Keep it flat and snug and read it in centimetres. Measure twice. See our full guide to measuring your dog.
What size harness for a Border Collie puppy?
Size a puppy on its chest measurement today, not its expected adult size4. Measure every couple of weeks while it grows fast and move up a size when its chest reaches the top of the current size.
What if my Border Collie is between two harness sizes?
What is the best harness type for a Border Collie?
Most owners choose a lightweight Y-front harness that keeps straps clear of the shoulder joint. There isn't strong evidence that any one style is best, so the most useful check is to watch your dog move: its stride should look the same with and without the harness3.
My Border Collie hates having a harness put on. What can I do?
Try a step-in or clip-on design so nothing goes over the head. Pair the harness with treats: show it, treat, touch it to the dog, treat, then fasten one clip at a time over a few short sessions.
Can I use a normal harness for canicross?
Walking harnesses are not built for a dog pulling hard for long periods. For canicross or bikejoring, use a purpose-made running harness and follow that brand's own measuring guide.