Puppy Weight Predictor

About this calculator

One of the first questions every new puppy owner asks is: how big will my puppy get? This calculator uses a logistic growth model — the same mathematical shape that researchers used to analyse 50,000+ puppy growth records — to estimate your puppy's likely adult weight from three simple inputs: its age in weeks, its current weight, and its breed-size class (small, medium, large, or giant). The result is a point estimate with a ±10% confidence band, because puppy growth is influenced by genetics, nutrition, sex, and neuter status in ways no single formula can fully capture. Treat the number as a useful planning guide, not a guarantee, and always involve your veterinarian for health decisions.

How to read your results

The hero number is the predicted adult weight — the weight your puppy is likely to reach when fully grown. Directly below it you will see a low–high range (±10%) that reflects the real-world variability in growth rates within a size class. The growth curve below the result shows the predicted weight trajectory from birth to roughly full maturity: the solid dot marks your puppy's current age and weight, so you can see where it sits on the curve. The "Maturity age" stat shows the typical age at which puppies of this size class reach their adult weight. Because this is a statistical estimate, mixed-breed puppies, unusually large or small individuals within a size class, and puppies with health or nutrition issues can fall well outside the band.

Worked example

A medium-breed puppy (e.g. a Cocker Spaniel or Beagle) weighing 10 lb at 16 weeks of age.

The logistic model puts a medium puppy at about 44.7% of adult weight at 16 weeks (inflection point x₀ = 18.26 weeks, k = 0.095). Predicted adult weight = 10 ÷ 0.447 ≈ 22.4 lb, with a range of roughly 20–25 lb. Maturity is typically around 12 months for medium breeds.

Frequently asked questions

How accurate is this prediction?

For purebred dogs of a known size class, the estimate is typically accurate within 10–20% of the true adult weight. The ±10% band shown here reflects typical variability within a size class; individual dogs can fall slightly outside it. Factors like genetics, sex (males tend to be larger), neuter status, diet quality, and health history all affect final size. Use the prediction for planning purposes — not for medical, dietary, or breeding decisions — and confirm with your vet.

How should I classify a mixed-breed puppy?

For a mixed-breed puppy, use the best estimate of its likely adult size. If you know the breeds of both parents, use the size class that matches their average adult weight. If parentage is unknown, a DNA breed test can help; alternatively, a veterinarian can often give a rough estimate based on paw and body proportions. Classifying a puppy one size class off can shift the prediction by 20–40%, so a vet's guidance is especially valuable for mixed breeds.

When do different sizes of dogs stop growing?

Small and medium breeds typically reach their adult weight between 9 and 13 months. Large breeds continue growing until around 18 months, and giant breeds (Great Danes, Mastiffs, Saint Bernards) may not reach full adult weight until 22–26 months. Growth plates close later in larger dogs, which is why their skeletal development takes longer. The growth curve in this calculator extends to the typical maturity age for the selected size class.

Why does the prediction seem high for a very young puppy?

The logistic model assumes the puppy is on a normal growth trajectory. At very young ages (under 6–8 weeks), a puppy may weigh less than typical for its size class due to litter competition, early illness, or simply being at the smaller end of normal variation. This can make the model overshoot the adult weight. For puppies under 8 weeks, treat the prediction with extra scepticism and re-run it once the puppy is a bit older and more settled into its growth trajectory.

What is the logistic growth model?

The logistic model describes growth that starts slowly, accelerates in the middle, then slows again as the animal approaches its mature size. Mathematically, the fraction of adult weight at any age follows the formula f(t) = 1 / (1 + e^(−k(t − x₀))), where t is age in weeks, x₀ is the age at which the puppy is at 50% of its adult weight (the inflection point), and k controls how quickly growth happens. The inflection-point values used here — 15.11 weeks for small, 18.26 weeks for medium, 22.85 weeks for large — come from a peer-reviewed logistic analysis of dogs of different breeds (Orjuela-Leal et al., 2014). The giant-breed inflection point (28 weeks) is extrapolated from the size-class trend in that study.

How it's calculated

Adult weight is predicted using a logistic growth model: fraction_of_adult(t) = 1 / (1 + exp(−k × (t − x₀))), where t is the puppy's current age in weeks. The predicted adult weight = current weight ÷ fraction_of_adult. A ±10% band is applied to the central estimate. Inflection points (x₀, the age at 50% of adult weight): small = 15.11 weeks, medium = 18.26 weeks, large = 22.85 weeks (Orjuela-Leal et al., 2014, Revista MVZ Córdoba); giant = 28.0 weeks (extrapolated). Growth-rate constants (k): small = 0.160, medium = 0.095, large = 0.076, giant = 0.055. Maturity ages: small and medium = 12 months, large = 18 months, giant = 24 months (consistent with WALTHAM Petcare Science Institute growth-chart ranges). Sources: (1) Orjuela-Leal et al. (2014), https://www.researchgate.net/publication/289573995; (2) WALTHAM Puppy Growth Charts, https://www.waltham.com/resources/puppy-growth-charts.