how much should a kitten weigh at 5 weeks? The average weight for a 4 week old kitten is 2 lbs. 1 oz and up to 3 lbs. 1 oz., depending on the breed, heredity and environment of the cat . It usually takes until 6 months or older before they can be considered full grown (around 20 – 40 pounds). What will your kitten eat? If you want to know what your new feline friend eats, go to our Cat Food section for information about feeding kittens properly.