Pro Tips: How To Square a Large Project Using the 3-4-5 Rule
Whether you are tiling a floor or laying out a foundation, measuring for square is an important technique to ensure that and area is square. A carpenter's square or framing square just isn't large enough to square two long lines. To make certain that two lines are 90° to each other, use the simple 3-4-5 technique.
To ensure that two lines are 90° to each other, measure down one line 3 feet from the corner and make a mark. Then measure down the other line 4 feet and make a mark. Now measure the diagonal distance between those two marks, it should measure 5 feet. If so, the lines are square. If the marks are more than 5 feet, then the lines are more than 90° and if it is less than 5 feet, the angle is less than 90°.
You can increase the accuracy of your square by using longer lines and measure using 6-8-10 or 9-12-15. The rule is a basic rule of geometry called the Pythagorean Theorem. A squared plus B squared equals C squared, thus 3*3 + 4*4 = 5*5 (9+16=25).