Question 1

My two diagonal measurements don't match. What do I do?

Accepted Answer

This means your panel is a parallelogram, not a rectangle. To fix it, you need to gently push or pull the two longest corners together. For example, if one diagonal is 100" and the other is 99.5", you need to rack the frame to shorten the 100" diagonal and lengthen the 99.5" one until they match.

Question 2

What's the best way to measure a long diagonal accurately?

Accepted Answer

For very large panels, a standard tape measure can sag, introducing error. It's best to have a helper hold one end. For maximum precision, use a steel tape measure, as fiberglass tapes can stretch slightly. Make sure the tape is held flat against the panel.

Question 3

Why is checking diagonals more accurate than using a framing square?

Accepted Answer

A framing square only checks the corner it's in. A small error in one corner might not be obvious. Measuring the diagonals checks the squareness of the entire assembly at once. An error of just 1/16" in a corner can translate to a much larger, more obvious difference in the diagonal measurements.

Question 4

Does the 3-4-5 method work for checking square?

Accepted Answer

Yes, the 3-4-5 method is just a practical application of the Pythagorean theorem. You measure 3 units along one side and 4 units along the adjacent side; the diagonal between those two points should be exactly 5 units. While this is great for layout, measuring the full diagonals is generally faster and more holistic for checking a large, already-assembled panel.

Question 5

What if my opposite sides aren't the same length?

Accepted Answer

If your opposite sides are not equal (e.g., one side is 96" and the other is 95.5"), then your panel is a trapezoid, not a rectangle. In this case, the diagonals will never be equal. You must first re-cut your parts so that opposite sides match before you can attempt to square the assembly.

WoodCalc Pro

Large Panel Diagonal Calculator

Unlock 16,000+ Woodworking Plans