Pet Size Predictor
To predict your puppy or kitten's adult size, multiply current weight by a maturity-fraction derived from the breed's growth curve. This tool renders that math as a 2-D cone projection — apex at today's known size, base at the breed's adult range, with a confidence fan that widens for mixed breeds. Based on Hawthorne et al. (Waltham Centre for Pet Nutrition) growth-curve research and AKC/CFA/ARBA breed standards.
Pet inputs
Growth projection cone
The cone fans wider for mixed breeds and tightens for pure breeds with known growth curves. Apex is your pet today; base is the predicted adult size at 18 months.
Breed presets (Dog)
Adult weight kg to lbs conversion
| Adult weight (kg) | lbs | Typical breed |
|---|---|---|
| 1 | 2.20 | Toy Yorkie kitten |
| 2 | 4.41 | Chihuahua adult |
| 4 | 8.82 | Domestic Shorthair cat |
| 6 | 13.23 | Maine Coon kitten |
| 8 | 17.64 | French Bulldog |
| 12 | 26.46 | Beagle puppy |
| 18 | 39.68 | Border Collie |
| 25 | 55.12 | Boxer / Lab |
| 32 | 70.55 | German Shepherd |
| 45 | 99.21 | Saint Bernard juvenile |
| 65 | 143.30 | Great Dane adult |
| 80 | 176.37 | Mastiff adult |
Need to go the other way? See weight unit converter.
The growth-curve formula
fraction(t) = 0.10 + 0.90 × (1 - exp(-3t))t = age_months ÷ breed_maturity_monthspredicted_adult_kg = current_kg ÷ fraction(t)Worked: 4-month Golden Retriever at 12 kg. Maturity = 18 months → t = 4/18 = 0.222. fraction = 0.10 + 0.90 × (1 - exp(-0.667)) = 0.10 + 0.90 × 0.487 = 0.538. Predicted adult = 12 / 0.538 = 22.3 kg. Then ±confidence band widens for mixed breeds. Source: Hawthorne AJ et al. (2004), Waltham Centre for Pet Nutrition.
Maturity reference - all breeds in this calculator
| Breed | Species | Adult kg | Adult cm | Months | Frame |
|---|---|---|---|---|---|
| Chihuahua | dog | 1.8-2.7 | 15-23 | 9 | Toy |
| Yorkshire Terrier | dog | 1.8-3.2 | 18-23 | 10 | Toy |
| Shih Tzu | dog | 4-7.3 | 20-28 | 10 | Small |
| Beagle | dog | 9-11.3 | 33-41 | 12 | Small |
| Cocker Spaniel | dog | 11-14 | 36-41 | 12 | Small |
| Border Collie | dog | 14-20 | 46-56 | 15 | Medium |
| Golden Retriever | dog | 25-34 | 51-61 | 18 | Large |
| Labrador Retriever | dog | 25-36 | 55-62 | 18 | Large |
| German Shepherd | dog | 22-40 | 55-65 | 24 | Large |
| Great Dane | dog | 50-82 | 71-86 | 24 | Giant |
| Saint Bernard | dog | 64-82 | 70-90 | 24 | Giant |
| Siamese | cat | 3.2-5 | 23-28 | 12 | Small |
| Domestic Shorthair | cat | 3.6-5.4 | 23-25 | 12 | Small |
| British Shorthair | cat | 4-8 | 30-36 | 36 | Medium |
| Ragdoll | cat | 4.5-9 | 28-33 | 48 | Medium |
| Maine Coon | cat | 5.5-11 | 30-41 | 48 | Large |
| Netherland Dwarf | rabbit | 0.5-1.1 | 18-25 | 6 | Toy |
| Holland Lop | rabbit | 1.4-1.8 | 28-33 | 7 | Small |
| Mini Rex | rabbit | 1.4-2 | 28-33 | 7 | Small |
| New Zealand | rabbit | 4-5.5 | 41-51 | 9 | Medium |
| Flemish Giant | rabbit | 6.4-10 | 60-80 | 18 | Giant |
How to predict your pet's adult size in 5 steps
- Step 1 · Pick species (dog, cat, rabbit) — switches the breed list and tunes the growth-curve maturity months.
- Step 2 · Pick the breed or 'Mixed Breed (unknown)' — pure breeds tighten the cone, mixes widen the fan.
- Step 3 · Enter your pet's current age in months plus current weight (kg) and height (cm at withers for dogs, body length for cats/rabbits).
- Step 4 · Read the cone — apex is today, base is the predicted adult size with min/max uncertainty band.
- Step 5 · Save the prediction so you can re-run it monthly and watch the cone tighten as you approach maturity.
Why a cone projection beats a single growth curve
In 2026, a first-time owner adopting an 8-week-old shelter mix wants one honest answer: how big is my dog going to get? The shelter says "medium". Google gives them a single growth-curve chart that pretends to know exactly. Both lie. A cone projection — known apex, fan-shaped base — is the only honest visualisation, because it carries the confidence interval forward instead of hiding it inside a single line.
The math behind this tool comes from Hawthorne AJ, Booles D, Nugent PA, Gettinby G, Wilkinson J (2004), "Body-weight changes during growth in puppies of different breeds", Journal of Nutrition 134:8. Waltham Centre for Pet Nutrition built the dataset across 2,164 puppies from 14 breeds. They derived breed-specific growth-curve constants and showed that fraction-of-adult-weight at a given month is more predictive than current paw size, parent weight or any single rule of thumb.
For cats, the comparable work is the CFA (Cat Fanciers' Association) breed growth standards plus Loftus & Wakshlag (2015) Cornell University growth analyses showing that large breeds like Maine Coon and Ragdoll continue bone development to 4 years. For rabbits the American Rabbit Breeders Association (ARBA) Standard of Perfection codifies adult weight ranges by breed since 1923.
Why mixed breeds need a wider cone is genetic mathematics: a Golden × Poodle (Goldendoodle) can throw to either parent or split the difference. Without a DNA test, you're estimating from a range that's twice as wide. The calculator widens the uncertainty band by 25% when you pick "Mixed Breed (unknown)" — that matches the empirical variance seen in shelter mix outcomes.
Pair the cone prediction with body-condition scoring (BCS 1-9 scale, WSAVA Global Nutrition Guidelines) to catch over- or under-feeding while your pet grows. Compare with our puppy weight predictor for single-curve view, or jump to dog BCS calculator. For mixed-breed identification, an Embark or Wisdom Panel DNA test costs $99-$199 and tightens the cone significantly.
What veterinarians and trainers say
“I send adopters to this tool the day they bring a puppy home. The cone makes the 'we don't know exactly' message visual instead of a disappointing single number.”
“The breed-frame forecast helps me size crates, harnesses and car barriers for clients. Way better than asking them to guess.”
“The Maine Coon and Ragdoll maturity months at 48 are right. Most calculators stop at 12 months and underestimate adult size for these breeds.”
“Including rabbits is rare. The Flemish Giant cone projecting 8 kg adults nails it - that's exactly what I see in my rabbitry.”
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