Dog Size Calculator

Predict a puppy's adult weight from its current weight, age, and breed size category using the growth-percentage method.

Quick Facts

Method
Adult weight = current weight ÷ growth fraction at current age
Growth fraction comes from a size-category growth curve (small, medium, large, giant).

Your Results

Calculated
Projected adult weight
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Most likely mature weight
Likely range
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Roughly ±15%
Growth completed
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% of adult weight so far
Growth remaining
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Weight still to gain

Ready

Enter your puppy's weight, age, and size category, then calculate.

How the Dog Size Calculator works

This calculator predicts a puppy's adult weight using the growth-percentage method. Puppies do not grow at a steady rate — they gain weight fastest in the first few months, then slow down as they approach their mature size. If you know roughly what fraction of its adult weight a puppy has already reached at its current age, you can project the finish line:

Adult weight = current weight ÷ (fraction of adult weight reached at current age)

The fraction depends heavily on breed size. A toy breed is nearly full-grown by 10–12 months, while a Great Dane or Mastiff keeps growing until 18–24 months. This tool stores a separate growth curve for four size categories — small, medium, large, and giant — and interpolates between age points to find the fraction for your puppy's exact age.

A worked example

Suppose a medium-breed puppy weighs 15 lb at 12 weeks. Medium-breed puppies have typically reached about 50% of their adult weight by 12 weeks, so:

Adult weight ≈ 15 lb ÷ 0.50 = 30 lb

The same 15 lb puppy at 12 weeks projected as a large breed (about 44% of adult weight at 12 weeks) would instead finish near 15 ÷ 0.44 ≈ 34 lb — which is why picking the right size category matters.

Growth benchmarks by size category

  • Small (adult under 20 lb): about 50% of adult weight by roughly 11–12 weeks and essentially full-grown by 10–12 months.
  • Medium (20–50 lb): about 50% of adult weight by roughly 4 months (16 weeks); mature around 12–14 months.
  • Large (50–90 lb): about half-grown by 4.5–5 months; mature around 15–18 months.
  • Giant (over 90 lb): about half-grown by 5–6 months; not fully mature until 18–24 months.

Quick rule-of-thumb formulas

Breeders and vets use several shortcut rules that this calculator's curves are consistent with:

  • Small/medium breeds: weight at 6 weeks × 4, or weight at 8 weeks × 3, approximates adult weight.
  • Medium/large breeds: double the weight at 14–16 weeks (about 4 months) to estimate adult weight.
  • Linear check: (current weight ÷ current age in weeks) × 52 gives a rough medium-breed estimate — useful only as a sanity check, since growth is not truly linear.

Frequently Asked Questions

How is a puppy's adult weight predicted?
The growth-percentage method divides the puppy's current weight by the fraction of adult weight a dog of that size category has typically reached at that age. For example, a medium-breed puppy weighing 15 lb at 12 weeks (about 50% grown) projects to roughly 15 ÷ 0.50 = 30 lb as an adult.
How accurate is a puppy weight predictor?
Accuracy is highest for purebred dogs of a known size category and lowest for very young puppies and mixed breeds. Real adult weight commonly lands within about 10–20% of the projection, which is why the tool shows a ±15% range. Genetics, nutrition, spay/neuter timing, and body condition all shift the final number.
What if I don't know the breed?
For a mixed-breed puppy, pick the size category that matches the expected adult size — paw size relative to the body, parent sizes (if known), and the breed of any purebred parent are useful clues. If unsure, run the estimate for two adjacent categories and treat the two results as the plausible span.
At what age is the estimate most reliable?
Estimates get steadily more accurate as a puppy ages. Before about 12 weeks, growth is fast and variable, so early figures are loose. By 4–6 months a puppy has locked in much of its frame, and the projection tightens considerably. By the age of expected maturity for its size, the current weight is essentially the answer.

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