Why do you need to know this?
There are pediatric WEIGHT and HEIGHT charts that they use to figure out hwo a kid is doing. So if an 18 month old girl is in the 75th percentile for height and weight, she is fine. The key here is also how they have been running - for example if a baby has always been in the 25th percentile, that is fine, but if the go from the 75th to the 25th, that raises all sorts of red flags.
However if the kid is in the 50th percentile for height and the 99th for weight, the kid is probably a bit chunky. But my oldest used to be in the 75th for height and the 90th for weight but that is fine; my youngest is the opposite and has always been tall and skinny.
Do you think the kid is underweight? Overweight? It can be hard to get kids to eat the right thing (I have a friend with a five year old who will only eight WHITE food). At 18 months, however, they need a full fat diet (whole milk, not skim) to help their brains develop properly. It is pretty complex.
