Nice and thin, as it is only one layer of muslin. It is also light and breathable, so I can see us being able to use it even in the summer! Bought this after my friend recommended trying a sleep sack to keep my 15-month-old son from kicking off the blankets at night. And it works!!! We have used it every single night, and will continue to do so! Really good quality, too, only wish maybe the zipper part was slightly thicker because the thin zipper on this can be a bit tricky to put on a cranky, wiggling baby. However, the fact that it's so small keeps it from being uncomfortable if baby sleeps on their tummy so I guess it's a fair trade off! Will be buying another one of these, soon. Maybe a cozy one for winter!