Measure one leg over 3'.
Measure the other leg up 4'.
The hypotenuse will be 5'.
Use the same edge of the tape on both marks when you go to
check the 5'.
You can also double/triple/quadruple this (6', 8', 10') for
a larger building and to check that your wall isn't "running
And who says that public schools aren't doing their job? Apparently, it's
not just a problem in the US.
If you want a right angle, if you create a triangle with sides of 3 units, 4
units, and 5 units, the angle between the 3 and 4 unit sides is a right
If you have a floor or wall laid out and want to check it for square, make
sure the diagonals are equilateral (oops...I mean the same length).
How about a(squared) + b(squared) = c(squared)
where 'a' and 'b' are the sides next to the (hopefully) 90 deg corner, and
'c' is the hypotenuse (diagonal). It works regardless of the length of
either short side.
If opposite walls are the same length, your method will get
the walls parallel and thus right angles. It doesn't work if
one of an opposite pair is shorter, i.e. you don't have a
rectangle. You use geometry for squaring a base. I'm
curious about your "square the walls with the floor tho."
Nobody does that. You want the floor level and you want the
walls to be perpendicular. To do that you use gravity. To
get something level you use water (absent flow and wind,
water surfaces are always level)(most people use a tube
since any surfaces connected will be level) and to get walls
perpendicular you use a plumb bob. Those methods are basic
and the standard. Of course most people just use a level
which is a derivative method, i.e., the level has to be
adjusted to something else to be sure the bubbles indicate
level and perpendicular.
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