How Big Will My Mixed-Breed Puppy Get? How to Estimate

The best clues to a mixed-breed puppy's adult size are its parents' sizes, the growth line your vet plots over its first few months, and its genetics. None of them gives an exact number, and littermates can end up noticeably different sizes, but together they usually narrow things down to a likely size range.

The short version

• The parents are the first clue, if you know them.
• The growth line is the second: healthy puppies tend to stay on their line on a growth chart.
• Genetics is the third: a handful of genes explain much of the size difference between breeds, and a DNA test can estimate adult size.
• Littermates can differ, because each puppy inherits its own mix of size genes.
• Giant mixes are harder to estimate, because the standard growth charts stop at 40 kg.

Clue 1: the parents

If you know both parents, their adult sizes are the most direct clue. A mix often ends up somewhere between its parents' sizes, though not necessarily in the middle, because size depends on which size genes each puppy happens to inherit.

For shelter and rescue puppies, the parents are usually unknown, and breed labels are often guesses. Studies comparing shelter labels with DNA have found they frequently miss breeds, as our guide to DNA testing a rescue dog explains, so treat a label like "Lab mix" as a loose hint at best.

Clue 2: the growth line

Dogs have growth charts, much like children do. A 2017 study built growth standards from the records of more than 6 million young dogs, in five size categories based on predicted adult weight: under 6.5 kg, 6.5 to 9 kg, 9 to 15 kg, 15 to 30 kg, and 30 to 40 kg. The charts cover puppies from 12 weeks to 2 years old.

Healthy puppies tend to stick to their line. A 2020 follow-up study found fewer than 5% of healthy dogs moved across more than two centile lines while growing, while dogs that became overweight or underweight often did. So once your vet has plotted a few weigh-ins, the line your puppy follows is a good hint at the size category it's heading for. Our guide to when dogs stop growing covers how long each size takes to fill out.

Clue 3: genetics

Size is strongly genetic. A 2007 study found a single version of the IGF1 gene common to all small breeds and nearly absent from giant ones, and a 2013 study found that variants near six genes explain about half of the size differences between breeds, with smaller dogs tending to carry more of the small-size variants.

A DNA test can turn that into an estimate. The PuppyGenes report shows your puppy's estimated breed ancestry, and its temperament and size predictions section includes an estimated adult height and weight based on its own genetics: useful context, not a promise. Our guide to whether DNA can predict adult size explains what that estimate can and can't do.

Why littermates end up different sizes

The 2013 study found that breeds aren't fixed for these size variants: dogs within the same breed carry different combinations. Mixed breeds carry even more variety, and each puppy inherits a random half of each parent's genes. So two puppies from the same litter can inherit quite different sets of size variants, and end up noticeably different sizes as adults.

Food and health matter too. The growth standards were built from healthy dogs kept in ideal body condition, so a puppy that's overfed or underfed won't follow the same line. Our guide to checking whether a dog is overweight shows how to check body condition at home.

Putting the clues together

1. Note the parents' adult sizes, if you know them, or treat any breed label as a loose hint.
2. Ask your vet to plot your puppy's weight on a growth chart at each visit, and watch which line it follows.
3. If you want a genetic estimate, a DNA test adds estimated breed ancestry and a predicted adult height and weight. Our guide to DNA testing a puppy covers when to swab.