A rectangle is square only when both diagonals are equal. Squaring a single corner with 3-4-5 is not enough on its own, because the opposite sides might not be parallel or equal. Measuring both diagonals checks all four corners at once. This calculator gives the ideal diagonal, the difference between your two measurements, and whether that difference is inside a typical layout tolerance.
How to check a layout
- Confirm opposite sides are equal first. If the lengths differ, diagonals can be equal and the shape is still a trapezoid.
- Measure corner to corner both ways, from the same reference point on each corner (outside of the form, centre of the string intersection, etc.).
- If the diagonals differ, push the frame along the longer diagonal, or pull along the shorter one, by about half the difference. Re-measure.
Formula
Ideal diagonal = √(length² + width²). For a 24 × 16 ft slab that is √832 ≈ 28.844 ft, or 28' 10-1/8".
How square is square enough?
Common layout practice for residential work is about 1/8" for runs under 10 ft and 1/4" from 10 ft to 100 ft. Cabinet and tile work want tighter numbers; rough site grading can live with more. Our tolerance guide covers this in detail.
Watch: how it is done
Video by RR Buildings on YouTube. More in our video library.
Frequently asked questions
Why must opposite sides be equal before checking diagonals?
A shape with unequal opposite sides can still have equal diagonals (an isosceles trapezoid). Equal sides plus equal diagonals guarantees a rectangle.
Which way do I push the frame?
Push the corners that sit on the longer diagonal toward each other. That shortens the long diagonal and lengthens the short one until they match.
What tolerance should I use for a deck?
Most deck builders aim for diagonals within 1/4" on a typical 12–20 ft deck. Tighter is better for decking that runs parallel to the house.
Results are planning estimates. For structural work, confirm requirements with your local building authority and a qualified professional. See our disclaimer.