A clever little site

There are times when I'm working in on a quilt and I know what I want one dimension to be but haven't got the heart to do the geometry to find the others. This site http://www.mathopenref.com/squarediagonals.html seems to do it all for me. Not only that, it's Java based so everything is done on a click and drag basis, live on screen:

The link that I've listed goes straight to the square diagonals page but there is an extensive subject list available.