The calculation is based on the Pythagorean theorem (a² + b² = c²), which relates the sides of a right-angled triangle.

1. Identify Sides: The length and width of your rectangular panel form the 'a' and 'b' sides of a right-angled triangle.
2. The Diagonal as Hypotenuse: The diagonal of the rectangle is the hypotenuse ('c') of that triangle.
3. Apply the Formula: The calculator squares the length and width, adds them together, and then finds the square root of the sum to determine the diagonal length.
   Diagonal = √(Length² + Width²)

Example 1: Squaring a 4×8 cabinet back panel.
Length 96″, width 48″ → √(96² + 48²) = √11520 ≈ 107.33″. Set both diagonals to 107 5/16″ and the case is square before you glue the carcass.

Example 2: A 24″ × 36″ door panel.
√(24² + 36²) = √1872 ≈ 43.27″. Clamp a diagonal brace to that length while the glue cures so the frame sets square.

Example 3: Knowing how far off you are.
If the target is 107.33″ and one diagonal reads 107.75″ while the other reads 106.90″, the frame is racked. Push the two corners of the long diagonal toward each other until both settle on 107.33″.

• Measuring outside-to-outside inconsistently: Both diagonals must run corner-to-corner at the same reference point (outside edges or centerlines), or the comparison is meaningless.
• Squaring before matching opposite sides: Diagonals can only ever agree if the opposing lengths are already equal. Fix part sizes first, then square.
• Tape sag and parallax: On long runs a floppy tape reads short. Pull tight, keep it flat, and sight square down to the mark.
• Chasing the diagonal on a warped panel: A twisted panel can read square diagonally yet still be out of flat. Check with a straightedge too.
• Rounding the target number: Squaring to 107″ instead of the true 107.33″ leaves a visible gap. Carry the full decimal to the tape and split the difference between the two corners.

"On anything over four feet, I'd rather trust two equal diagonals than a square's corner. The longer the measurement, the more a tiny corner error magnifies into an obvious diagonal difference — it's the same reason a tape reads a bigger miss across a full sheet." — Cabinet Shop Foreman

"Mark the target number on a piece of battened stock once, then use that story stick against both corners. It's faster and more repeatable than reading a tape to a hundredth on every unit you build."

• Square: In this context, having four corners that are perfect 90-degree angles.
• Diagonal: A straight line connecting two opposite corners of a rectangle.
• Pythagorean Theorem: A fundamental principle in geometry stating that in a right-angled triangle, the square of the length of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the other two sides.