Page Proportion Calculator
Sometimes I want to create sketches at a small scale before do start a watercolor on a large sheet of paper, and I want the proportion of the sketch to match the proportion for the large sheet. Here's a simple calculator that will figure out the dimensions you should use in your sketchbook.
For example, suppose you are planning to do a 22 x 30 painting, and your sketchbook is 8.5 x 11. Enter the numbers in the form, click the "Calculate!" button, and you will see that you will need to crop your sketchbook page to 8.07 x 11 to keep the proportion the same as your full sheet.