Assembly checks

Compare a box with its target diagonals

Calculate the diagonal of an intended rectangle and compare two measurements without inventing a pass tolerance.

A rectangular footprint has a diagonal determined by its width and length. Comparing that target with both measured diagonals helps separate a size error from skew during a dry fit. However, matching diagonals are only one check. An isosceles trapezoid also has equal diagonals, so the numbers must be considered with opposite edge lengths and actual corner angles.

How to use the result

1. Use the same kind of points for every measurement: all inside corners or all outside corners. Enter the corresponding target width and length.
2. Keep the assembly flat and measure each diagonal consistently. Record the readings before moving a clamp.
3. Compare the signed differences with the target as well as the difference between diagonals. Recheck opposite sides and use a square at the corners.

Worked example

A 300 by 400 mm rectangle has a 500 mm diagonal. Readings of 500 and 502 mm differ by 2 mm, with one 2 mm above the target. Two readings of 510 mm would match each other but still disagree with the intended rectangle. Neither situation is automatically accepted by this tool.

Before making

• Choose any permitted tolerance from the actual design and measurement method, not from this calculator.
• Do not force a twisted assembly flat and then mistake the forced readings for a good fit.
• If opposite lengths differ, resolve that first; equalizing only the diagonals can hide a trapezoid.

Continue your plan

• frame miters
• open desktop tray