Puppy Growth in Mixed Breeds: How Big Will They Get?

Mixed breed puppies are adorable bundles of joy—and sometimes a total mystery when it comes to how big they’ll grow. Without knowing the exact parentage, it can feel like a guessing game. Will your pup stay lap-sized or turn into a couch-hogging giant?

While there’s no way to predict your mixed breed puppy’s adult size with 100% accuracy, there are helpful clues you can look for. From paw size to parent breeds, this guide will walk you through the signs that can help you make an educated guess.

What Influences Puppy Size?

There are a few key factors that influence how big your mixed breed dog might get:

Parent Breeds
If you know who mom and dad are, you already have a good starting point. Most puppies will fall somewhere between the size of their two parents, though it’s not always a perfect average.

Genetic Wildcards
Even within a single litter, mixed breed puppies can vary a lot. One pup might take after a smaller grandparent, while another inherits the frame of a larger ancestor. That’s the beauty—and challenge—of mixed breeds.

Growth Rate Differences
Small breeds typically reach their full size faster than large breeds. So if your pup is growing steadily past the 6-month mark, they might be on the larger side.

Signs to Help Predict Adult Size

Paw Size
One of the classic indicators of a puppy’s future size is their paws. If your mixed breed pup has big, clumsy paws they haven’t grown into yet, it often means they’ve still got a lot of growing to do.

Leg Length and Body Proportions
Puppies with long legs and lanky builds are often still in the middle of a big growth phase. If your dog seems a little uncoordinated or “all legs,” they might be destined for a larger adult size.

Weight Milestones by Age
Generally, a puppy’s weight at certain ages can be doubled to get a rough idea of their adult weight—especially around the 4-month mark (more on that below).

Coat and Muscle Development
Muscle tone and coat fullness can also give you hints. Puppies who are slow to “fill out” might still be growing, especially if they’re mixed with a larger or working breed.

Use the “Double at 16 Weeks” Rule

A common method to estimate a mixed breed puppy’s adult size is to double their weight at 16 weeks (4 months). So if your pup weighs 20 pounds at 4 months, you can expect them to land around 40 pounds as an adult.

This works best for medium and large dogs. Small breeds often finish growing before the 6-month mark, and giant breeds can grow well past a year, so this trick isn’t always reliable at the extremes.

DNA Testing: Is It Worth It?

Dog DNA tests have come a long way, and they can be a fun way to understand your pup’s genetic background. Knowing which breeds make up your dog can help you estimate adult size based on typical breed standards.

Keep in mind, though, that DNA tests aren’t a guarantee. Even if your pup is 50% Labrador, they might not inherit the Lab’s size. But it’s one more tool that can help fill in the puzzle.

Common Growth Patterns by Size Group

Every dog is different, but here’s a general breakdown of growth timelines based on adult size:

Small Mixes (under 25 lbs):
Often reach full size by 9 to 10 months. They may have a quick growth spurt early on and then level off.

Medium Mixes (25–50 lbs):
Usually grow steadily until about 12 months. These pups benefit most from the “double at 16 weeks” rule.

Large Mixes (50+ lbs):
Can keep growing up to 18 months or even 2 years, especially in muscle and chest width. Don’t be surprised if they’re still filling out after their first birthday.

Real-Life Examples from Mixed Breed Owners

With my own Border Collie mix, I was convinced she’d stay under 30 pounds. At 4 months she was only 15 pounds—but she kept growing past a year and ended up closer to 50! Her big paws and long legs should’ve clued me in.

Another friend had a Terrier-Chihuahua mix who looked like a tiny fluffball—and stayed that way. She was basically full-grown at 6 months.

The takeaway? Mixed breed puppies can surprise you, but the signs are often there if you know where to look.

What If You Still Can’t Tell?

If your puppy’s size still feels like a mystery, that’s okay! Focus on keeping them healthy and happy, and let their growth unfold naturally. In the meantime:

• Make sure your home and gear (crate, harness, food portions) are adjustable
• Prepare for different sizes if possible—medium crates and collars usually offer the most flexibility
• Embrace the mystery! Part of the charm of mixed breeds is not knowing exactly what you’re going to get

Conclusion

Guessing your mixed breed puppy’s adult size is part science, part observation, and part fun. While it’s impossible to know for sure, paying attention to things like paw size, growth rate, and breed background can give you a pretty good idea.

Whether your dog tops out at 20 pounds or grows into a 70-pound snuggle buddy, one thing’s for sure—they’ll be the perfect size in your heart.

FAQ

Are big paws a sign my puppy will be big?
Often, yes! Oversized paws usually mean your puppy still has growing to do, but it’s not a perfect predictor.

Can DNA tests tell me my dog’s adult size?
They can give you a general idea based on breed mix, but there’s still variation depending on which traits your dog inherits.

When do mixed breed puppies stop growing?
It depends on their final size. Small dogs may be done growing by 9 months, while large breeds can take up to 18 months or longer.