How much should a 11year old that is 4feet 9inch tall weigh?

for a 11 year old you have a formula to calculate height...its age*6+77...in your case its 11*6+77=143cms should be your ideal and correct height for the age and the formula for weight is age *3...but this is only for people of age 7 to 12...thus in ur case 11*3=33...thus 33kgs should be the ideal weight for your age....if you have less than this that means its not a big problem...only if the difference is alot compared to the normal weight for age and height for age given above...then you are suffering from malnutrition and you must eat alot of proteins...