Why is a rug needed under the bed?

We have one. I remember reading you should have enough rug to be able to walk on the rug on the three sides, so that's what we did. We had carpeting bound since I wanted a non-standard size.
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The floors are going to be on a concrete slab, so we wanted to be able to stand on the rug while putting on slippers lol! At least during the winter months! We are going to do a bound carpet that's why I am wondering how far to come out! I am thinking to do a 10 by 12!Thoughts?

I just checked ours. I was wrong; it's not a non-standard size. It's 8 by 10 with a king sized bed. I wouldn't want it any larger, but maybe your room is larger.

Thank you for letting me know! My room is a 17 by 15 !

Ours is smaller, about 14 by 16. I also wanted to make sure that, as I walked through the room to get to the MB bath and closet, I wouldn't have to walk on the rug in order to minimize dirt. You might want to add that consideration before choosing a size.

How about three smaller rugs...one for the foot of the bed and two for the sides?

DD2 has a queen with an 8 x 10 rug that looks nice. The rug goes long way from side-to-side of the bed.

Here's a good article:

Here is a link that might be useful: How to Choose a Bedroom Rug

Thank you so much!