Puppy Growth & Adult Weight Calculator

Puppy growth prediction works backwards from a single fact: at any given age, a puppy has already put on a fairly predictable fraction of its adult weight. Weigh the puppy, look up the fraction for its age and breed size, divide, and you have the projection.

The formula: Adult Weight ≈ Current Weight ÷ Fraction of adult weight already reached

You will see a tidier-looking rule quoted everywhere: current weight divided by age in weeks, times 52. Do not use it. It assumes a puppy gains weight at a constant rate for a year, and puppies do nothing of the sort. They grow explosively for four months, then taper. Run a normal 16-week Labrador at 14 kg through it and you get a 45 kg adult, which is Newfoundland territory rather than Labrador.

Percentage of adult weight already reached:

Small breeds (under 10 kg adult) such as the Chihuahua, Pomeranian, Toy Poodle:

• Half their adult weight at around 12 weeks
• Essentially finished by 9 to 10 months

Medium breeds (10–25 kg) such as the Beagle, Border Collie, Cocker Spaniel:

• Half their adult weight at around 14 to 15 weeks
• Finished around 12 months

Large breeds (25–40 kg) such as the Labrador, Golden Retriever, Siberian Husky:

• Half their adult weight at around 18 weeks
• Still filling out at 14 to 16 months

Giant breeds (over 40 kg) such as the Great Dane, St. Bernard, Mastiff:

• Half their adult weight at around 21 to 22 weeks
• Not finished until 18 to 24 months

The shortcut worth remembering: for a medium or large dog, weigh at 16 weeks and double it. That falls straight out of the table (55% and 45% are either side of half) and it is close enough for choosing a crate or a harness. Giant breeds need the 16-week weight multiplied by roughly 2.7.

Factors that affect adult size:

• Genetics: The single biggest factor. If you know the parent weights, average them for a rough estimate.
• Gender: Males are typically 10–20% heavier than females of the same breed.
• Nutrition: Underfeeding slows growth; overfeeding a large breed puppy can cause joint problems.
• Neutering age: Neutering before full growth can slightly increase adult height in some large breeds.

Growth plates and development: Puppy bones have soft growth plates at the ends that harden as they mature. Over-exercising a puppy on hard surfaces before growth plates close (around 12–18 months for large breeds) can cause lasting joint damage. Avoid forced running and jumping until your vet confirms it is safe.

Note: This calculator gives an estimate only. A DNA breed test or veterinary assessment will give the most accurate adult size prediction for mixed-breed dogs.