Pencil and paper.

Specifically graph paper, a measuring tape, and some ability to do simple math.

I have been using "Floorplanner" software for several years. It takes time to get past the learning curve. Over the years, they have improved their features. There is a "free" version if you want to try the basics. A couple months ago, they upgraded their software, and then I had to pay extra to get their basic features working again. One drawback is that it is European based -- a search for "hand towel bar" yields nothing, but a search for "porte" gives me a selection of towel bars.

And the comment above is the reason I prefer pencil and paper!