See BMI calculator. Bmi is calculated by entering height and weight (pounds or kg, not in stones) into a bmi calculator - you can find one at http://pediatrics.About.Com/cs/usefultools/l/bl_bmi_calc.Htm. For children, bmi is classified as "normal" (below 85th percentile), overweight (bmi 85th to 94th percentile) and obese (bmi at 95th percentile or above). Overweight and obese classes were changed several years ago.
You're at a good BMI. Bmi only takes height and weight into consideration and so at 5'4" and 9stone3 (129lbs) you have a bmi of 22.1 normal bmi weight range is 18.5 to 24.5. Online calculator: http://www.nhlbisupport.com/bmi/ bmi is not always the best measurement. This flickr set tries to illustrate that bmi categories can be misleading as well. http://www.flickr.com/photos/77367764@n00/sets/72157602199008819/.