Find the exact diagonal a rectangular layout should measure to confirm its corners are square.
The 3-4-5 Rule, Explained
Squaring a layout comes down to the Pythagorean theorem: for a right triangle, the diagonal equals the square root of each side squared and added together. Measure two perpendicular sides (A and B); if the actual corner-to-corner diagonal matches the calculated one, the corner is square.alue, your corners are perfectly square at exactly 90 degrees.
Worked Example
For a 20 ft by 30 ft foundation, the correct diagonal works out to 36'-0 11/16" (36.06 ft). If both corner-to-corner measurements match that, the layout is square.
Common Mistakes
- Only checking one diagonal. Both corner-to-corner measurements need to match for a layout to be truly square.
- Measuring from the wrong reference point, like the outside of batter boards instead of the actual foundation line.
- Assuming "close enough" on a diagonal that's off by more than about 1/4" over a typical foundation run.
Frequently Asked Questions
- What is the 3-4-5 rule?
- A simplified version of this same math: if one side is 3 units and the other is 4, the diagonal will be exactly 5 units when the corner is square.
- How far off is too far off?
- For most residential work, keep it within about 1/4" over a normal-sized foundation. Slab-on-grade work with strict framing plans needs tighter tolerances.
- Does this work for any rectangle, not just squares?
- Yes, any rectangular layout. The two sides don't need to be equal.