For size reference I bought 3T for my almost 5 year old - he is 4 but in a month he will be 5 and he is less than 4. I don't know his specific measurements but basically he usually wears top 4T/4 (not tight) and 3T/36m. floors. The 3T set fit our son perfectly - definitely not even a tad loose like the three model boys on the product description picture but a lot tighter which is what we wanted as a base layer. The sleeves are a bit short for our boy as he normally wears a size 4 top, but the waistband and length of the pants were perfect. They were fantastic for cold weather vacations as a base layer. I was worried they would be too bulky/too itchy to wear under other clothes but they are not bulky at all - actually quite thin but surprisingly very warm! It's kind of a very tight weave so it holds in heat rather than having tiny sewing holes all over it like "waffle weave" I imagined. And super soft and comfortable at the same time! I really liked that they were thin enough to take up very little packing space when we were trying to travel light. We ordered 2 sets of these (both in 3T). My son wore one or the other every day during our 9 day vacation in 30-45 F weather. I'm glad they worked.
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