At what age is a Border Collie fully grown?

Most reach close to their adult height by around 12 months, though they can continue filling out in muscle and frame until 15 to 18 months. Individual variation is normal, so don't be concerned if your dog takes a little longer to finish developing.

Why is my Border Collie smaller than the average weight?

Plenty of healthy Border Collies sit below an average figure, particularly females and dogs from a lighter-framed line. As long as your dog is active, eating well and their body condition looks right, a smaller-than-average weight usually isn't a concern on its own.

Should I weigh my Border Collie regularly?

Weighing every few weeks during the growth period, then every few months as an adult, is a sensible routine. It's a useful way to spot a trend early, rather than something that needs doing constantly.

Can diet affect how fast my Border Collie puppy grows?

Yes, an overly energy-dense diet can push growth faster than is ideal, which is part of why a proper puppy food fed to the guide on the pack matters. If you're concerned about your puppy's growth rate, your vet can advise based on their actual condition.

Border Collie weight chart

A healthy adult Border Collie typically weighs somewhere between roughly 12 and 20kg, with males usually at the heavier end and females lighter, though build and line vary. Growth is fastest in the first six months and generally levels off by 12 to 15 months of age.

How growth typically progresses

Border Collie puppies gain weight quickly in their first few months, then growth slows as they approach their adult size, usually somewhere between 12 and 15 months, sometimes a little later for larger-framed dogs. Exact figures vary a lot between individual dogs, so a chart is a general guide rather than a target to hit.

Working-line and show-line Border Collies can differ noticeably in build, and littermates themselves can end up quite different sizes as adults, so comparing your puppy to a sibling or to a chart figure too closely isn't especially useful on its own.

Reading a growth chart properly

A weight chart is most useful for spotting a trend rather than judging a single weigh-in. A puppy tracking steadily along roughly the same curve over several weeks is generally doing fine, even if their exact weight sits above or below an average figure for their age.

Body condition, being able to feel but not see the ribs, and a visible waist from above, is a better everyday check than weight alone, since it accounts for your individual dog's frame rather than comparing them to an average.

When weight is worth raising with a vet

A puppy who suddenly stops gaining weight, loses weight, or who is gaining noticeably faster than expected is worth a vet check, since either extreme can point to something worth investigating rather than just individual variation.

Routine weigh-ins at vaccination and check-up appointments are a good way to track this properly over time, since a vet can plot your dog's actual growth against expected patterns far more accurately than a generic chart can.

Common questions

At what age is a Border Collie fully grown?: Most reach close to their adult height by around 12 months, though they can continue filling out in muscle and frame until 15 to 18 months. Individual variation is normal, so don't be concerned if your dog takes a little longer to finish developing.
Why is my Border Collie smaller than the average weight?: Plenty of healthy Border Collies sit below an average figure, particularly females and dogs from a lighter-framed line. As long as your dog is active, eating well and their body condition looks right, a smaller-than-average weight usually isn't a concern on its own.
Should I weigh my Border Collie regularly?: Weighing every few weeks during the growth period, then every few months as an adult, is a sensible routine. It's a useful way to spot a trend early, rather than something that needs doing constantly.
Can diet affect how fast my Border Collie puppy grows?: Yes, an overly energy-dense diet can push growth faster than is ideal, which is part of why a proper puppy food fed to the guide on the pack matters. If you're concerned about your puppy's growth rate, your vet can advise based on their actual condition.

Sources

Written against guidance published by UK veterinary and welfare organisations. Always confirm specifics with your own vet.

PDSA — Pet Health Hub

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.