I've tried multiple training pants for my daughter and these contain accidents the best.  I ordered the 3T for my 2 year old, who is 36" tall and weighs 30 pounds.  They fit perfectly.  The size chart on the Carter's packaging reads as follows:

18 months  24-27 lb
2t toddler  29-31 lb
3t toddler  31-34 lb
4t toddler  34-38 lb

According to the packaging my 30 pound daughter should be in a 2t, but there's no way.  The 3t fits snuggly around her skinny thighs.