The calculator uses a percentage of your width. If you had a square monitor, then 100% would be identical to 16:9's 56.25%.
Just to add, these are the equivalent values for other aspect ratios:
1:1 - 100%
5:4 - 80%
4:3 - 75%
16:10 - 62.5%
16:9 - 56.25%
21:9 - 42.1875%
32:9 - 28.125%