Most cats reach adult size by 12-18 months.
Kittens grow rapidly: 1 lb/month is typical for the first 6 months. Most domestic cats reach adult size by 12-18 months, but large breeds like Maine Coons and Ragdolls continue growing until 3-4 years. At 16 weeks, a kitten is approximately 25-30% of adult weight. At 6 months, about 60-75%. A 4-lb kitten at 16 weeks will likely be 10-12 lbs as an adult. Factors affecting size: genetics (parent size is the best predictor), nutrition, spay/neuter timing (early spaying may slightly increase final size due to delayed growth plate closure), and overall health. Male cats are typically 1-2 lbs heavier than females of the same breed.