Diagonal Calculator — Squaring Frames & Cabinets

Measure both diagonals. If they are equal, the frame is perfectly square. If one diagonal is longer than the other, the frame is racked (out of square). Adjust by pushing the corners of the longer diagonal closer together until both diagonals match.

The 3-4-5 method uses a right triangle: measure 3 units along one edge, 4 units along the adjacent edge, and the diagonal should be exactly 5 units if the corner is square. In metric, use 300-400-500mm or 600-800-1000mm for better accuracy.

For furniture, both diagonals should be within 1mm of each other. For cabinet carcasses, within 2mm is acceptable. For rough framing, within 3mm is usually fine. The smaller the project, the more important precision becomes.

Yes, measure the internal width and height of the door frame opening and calculate the diagonal. Measure both diagonals of the actual frame. If they match the calculated value, the frame is square and the correct size.