Square-up / diagonal check

What this calculator does

Give it the width and height of a frame, a base plate, a stair stringer pair, anything rectangular, and it returns the diagonal in decimal inches and to the nearest sixteenth. Add the diagonal you actually measured and it tells you how far out of square the piece is. A rectangle is square when both diagonals match.

Worked example

A 240" × 120" frame has a diagonal of 268.33" = 22'-4 5/16". If the tape reads 268 1/2", the frame is 0.17" out of square — pull the long corner back.

Common questions

Which diagonal do I measure?

Both. The tool gives the theoretical length; measure corner to corner each way. If they differ from each other, the frame is racked; if they both differ from the theoretical, the overall dimensions are off.

How much out of square is acceptable?

That comes from the spec, not the tool — AISC Code of Standard Practice and the project tolerances. The tool tells you the number; the spec tells you whether it passes.

Does it work for a tank shell or a curved piece?

No — straight-sided rectangles only. For a rolled member use the curved beam rolling calculator.