A carpet underlay should be around 12 mm thick. Of course, this goes for the thicker underlays that provide sound isolation and a softer feel. If you need a simple non-slip carpet underlay, you can go with a thinner design. These measurements will go perfect if you plan on putting the carpet in your living room, kitchen or bedroom. But, for trickier areas such as stairs, you should go for an underlay around 9mm thick. It’s easier to fit on the stairs, and it won’t stand out so much as a thick underlay.