At what age is a Border Collie fully grown?

Most reach close to their adult height by around 12 months and finish filling out in muscle and frame by around 18 months, though this varies between individual dogs. Growth charts are most useful for comparing your own puppy's pattern over time rather than pinning down an exact finishing age.

How much should a Border Collie puppy weigh at 6 months?

There is a wide normal range at six months because Border Collies vary a fair amount as adults, so a chart comparison against your puppy's own earlier weigh-ins is more useful than a single target figure. If you are concerned your puppy seems noticeably off track, your vet can check its condition properly.

Why has my Border Collie puppy's growth suddenly slowed?

A slowdown after the fastest early growth phase is completely normal and expected as a puppy moves into adolescence. A sudden plateau or weight loss that breaks from your puppy's previous steady pattern is different and worth mentioning to your vet rather than assuming it is the normal slowdown.

Do working-line and show-line Border Collies grow at different rates?

There can be some variation in typical adult size between lines bred for different purposes, but the overall growth pattern — fastest early on, levelling off by around a year to eighteen months — holds across the breed generally. Comparing your puppy against its own earlier weigh-ins matters more than which line it comes from.

Border Collie puppy growth chart: reading the curve

A Border Collie puppy typically does most of its growing in the first six to eight months, reaching close to adult height by around a year old and settling into its final weight and build by around eighteen months. Individual dogs vary a fair amount either side of that, so a growth chart is most useful as a shape to compare your own puppy's curve against, rather than a target weight to hit on a specific week.

What a typical curve looks like

Border Collies are a medium-sized, athletic breed, and most reach somewhere around 14 to 20kg as adults, with plenty of individual and line-to-line variation either side of that. Growth is fastest in the first few months, slows through adolescence, and tends to level off by around 12 to 18 months, by which point most of the height gain is done even if some muscle and frame filling out continues.

Because the breed covers a reasonably wide weight range even when healthy, a single number on a chart matters less than the shape of the curve. A puppy that is steadily gaining and tracking its own line week to week is usually doing fine, even if it sits toward the lighter or heavier end of what is typical for the breed.

How to use a growth chart properly

Weigh your puppy at roughly the same time of day, on the same scale, every couple of weeks, and plot the numbers rather than relying on memory. A steady upward curve with no sudden jumps or drops is the thing to look for, not whether any single weigh-in matches an average for the breed.

Growth charts are a comparison tool, not a diagnosis. If your puppy's weight suddenly plateaus, drops, or shoots up unexpectedly compared with its own previous pattern, that is worth mentioning to your vet at the next check-up rather than waiting to see if it corrects itself.

What actually changes as a Collie grows

Coat, coordination and energy levels all shift noticeably through the growing months. The double coat thickens and the working-line drive that Border Collies are known for tends to become obvious well before the dog is physically full grown, which is part of why exercise needs to be managed carefully for a growing joint structure rather than matched to the dog's apparent energy.

Feeding amounts should be adjusted as growth slows rather than kept level throughout, since a Collie eating puppy-level portions well past its main growth phase can put on weight it does not need. Your vet or the feeding guide on the food you use is the right place to check current portion sizes as your puppy's growth curve flattens out.

Common questions

At what age is a Border Collie fully grown?: Most reach close to their adult height by around 12 months and finish filling out in muscle and frame by around 18 months, though this varies between individual dogs. Growth charts are most useful for comparing your own puppy's pattern over time rather than pinning down an exact finishing age.
How much should a Border Collie puppy weigh at 6 months?: There is a wide normal range at six months because Border Collies vary a fair amount as adults, so a chart comparison against your puppy's own earlier weigh-ins is more useful than a single target figure. If you are concerned your puppy seems noticeably off track, your vet can check its condition properly.
Why has my Border Collie puppy's growth suddenly slowed?: A slowdown after the fastest early growth phase is completely normal and expected as a puppy moves into adolescence. A sudden plateau or weight loss that breaks from your puppy's previous steady pattern is different and worth mentioning to your vet rather than assuming it is the normal slowdown.
Do working-line and show-line Border Collies grow at different rates?: There can be some variation in typical adult size between lines bred for different purposes, but the overall growth pattern — fastest early on, levelling off by around a year to eighteen months — holds across the breed generally. Comparing your puppy against its own earlier weigh-ins matters more than which line it comes from.

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.