How do I check if a frame is square?

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.

What is the 3-4-5 method for squaring?

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.

How accurate do my diagonals need to be?

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.

Can I use this for checking door frames?

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.

WoodCalcs

Diagonal Calculator — Squaring Frames & Cabinets

Calculate diagonal measurements for squaring up frames, cabinets, and carcasses.

Width

Shortest dimension of the area

Height

Measure floor to ceiling or top of area

How We Calculate This

This calculator uses the Pythagorean theorem to find the diagonal of a rectangle.

The formula

Diagonal = sqrt(Width² + Height²)

When building a frame or carcass, both diagonals should be equal. Clamp the assembly lightly, measure both diagonals, and adjust until they match before tightening.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify measurements and material quantities before cutting or purchasing.