This post describes how to trim a piece of A4 paper to match the aspect ratio of U.S. letter size paper. This is needed for many paper airplane plans.

U.S. Letter size paper has a width of 220 mm (millimeters) and a length of 279.4 mm, giving an aspect ratio of 0.7874. A4 paper has a width of 210 mm and a length of 297 mm.

To adjust a piece of A4 paper to match the aspect ratio of U.S. letter size paper, multiply the width of the A4 sheet by the aspect ratio of of the U.S. Letter paper:

210 / .07874 = 266.7

To trim the A4 paper, cut off 30.3 mm from the height:

297 - 266.7 = 30.3

30.3 mm is very close to 1 3/16 inches, in case you have only that kind of ruler.