I have a 9 m/o staffy bull terrier he weighs 20 kg is this too light?
How much do they weigh when they are fully grown

I wouldn't say that is too light as my Elkhound bitches weight that, and they are a stocky breed, but taller than a staff.

The only way to tell if weight is correct is tooo feel.  You should be able to see a definate waist and see the last ribs, and easuily feel but not see the others.  Many Staffs are kept far too heavy and you hear them breathing hard, fatness is not fitness, and a young dog like yours should definately be kept lean for it's health.

oh k sweet as, how old are u dogs?

Hi staffy_01, my female staff is 1 1/2 years old and weighs 16 kilos which I think is about 2 1/2 stone.  she is quite lean leggy and muscular rather than the shorter squat looking staffs but like all staffs is absolutely adorable.

My girls are 14 months, 5 years, 7 years, nearly 10 years and 12 1/2 years, and I also part own a male of 3 years. 

The girls are five generations being Puppy, mother, Grandmother, Great Grandmother and Great Great Grandmother, and the boy is the youngest ones Dad.