How to make almost perfect square out of any rectangle

[Numerlor]

The first thing you need to do is Isosceles right triangle which is fairly easy, after that fold points with 45° to the one with 90°.  It will be perfect as long as you do it perfectly (So almost no time) But it should only differ 2mm at max.
Side of square will be the lenght of shorter side of triangle /2 (a leg?)
Hope some of you don't know this already

Picture of where you should fold thr triangle (A and B to C)
anyone who has idea how to properly calculate differences in surface area (from my creations it is something around 5,6 times smaller)

Step by step

 

Edited May 5, 2017 by Numerlor

[Bill Phil]

Try the circle next. /squaring the circle joke

[Numerlor]

1 minute ago, Bill Phil said:

Try the circle next. /squaring the circle joke

First I'll need a perfect circle 

[0111narwhalz]

Easier: Fold one edge onto an adjacent edge. Then excise the parts with exactly one layer. Unfold and get a square exactly as perfect as your rectangle.