How fast do kittens grow? I weighed Boo two weeks ago and she was 4lbs 8oz. I just bagged and weighed her and she is 6lbs 7oz. That’s awfully fast. She does seem bigger.How do I weigh her? She loves paper bags so I put an empty bag on the electric kitchen scale and zero it. Then I put the bag on the floor and let Boo wander into it. I pick up the bag with Boo in it – she doesn’t mind – and put it on the scale. Presto – she gets weighed!