Hello,

My cat is about 3 years old and weighs 5.1k the vet said he was a bit over weight but then could not say what the average weight was?

Average weight will depend on the build for the cat so there's no real answer I'm afraid.

I've got a cat who's a little over 7kg but not significantly overweight- he's a very large tom with enormous bone structure. When he came in to the Shelter one of the staff was heard to utter the phrase 'that's not a cat, it's a shetland pony!'. I've also got a 5 kg cat who's carrying more fat than is healthy (but she's getting on a bit and isn't really into the idea of exercise at 15).

it does depend on the cats breed and build, and what sex it is because males are usually bigger built, therefore weigh more.
but the averagish weight for an adult cat would be around 9lbs to 11lbs though.