i bought a size 2T for my 3 year old son. hes 40 lbs and 40 inches.. taller and bigger for his age but i had a feeling if i bought him a 3T that hed be drowning in them.. plus i hate super baggy boy pants. the 2T fits great, not tight or restricting at all, and are the perfect length. i need to buy a couple more pairs to have in rotation, but these seem to be great quality, well made, and should last a long time.