As children grow at different rates, the best thing to do is get a healthy weight range, that also takes into account the height of the baby.
This site gives a range of between 20 and 30.6 pounds. (9.1-13.9 Kg.)
http://www.eirpharm.com/search_weight_height.php?age_months=18+Months&submit.x=28&submit.y=9
This site gives average weight for boys and girls at different ages:
http://www.babybag.com/articles/htwt_av.htm
Parent
Mother of 6 kids ages 5-22 yrs; other-health impaired, normal and gifted kids