I was very confused by "on-point" dimensions when I first started quilting in 2002. When I wanted to know how many
blocks would fit across a certain size quilt I didn't have the more handy info that bearisgray listed above. I ended up calculating what width each size block would be and keeping that as a chart on my computer. Here it is if anyone needs it
On-Point Squares: width

straight-set on-point-set
1” 1.4
1.5” 2.12”
2” 2.83”
2.25” 3.18”
2.5” 3.5”
3” 4.25”
3.5” 4.95”
4” 5.66”
4.25” 6”
4.5” 6.36”
5” 7.07”
5.5” 7.78”
6” 8.5”
6.5” 9.2”
7” 10”
7.5” 10.6”
8” 11.3”
8.5” 12”
9” 12.73”
9.5” 13.44”
10” 14”
10.5” 14.85”
11” 15.6”
11.5” 16.3”
12” 17”
13” 18.38”
14” 19.8
15” 21.2
16” 22.63”
17” 24”
18” 24.5”
19” 26.87
22” 31.1”
24” 34”
24.5” 34.6”