How this calculator works
A rectangle is square when its two diagonals are equal. The diagonal of a rectangle is the hypotenuse of a right triangle, so its length is the square root of the length squared plus the width squared. The 3-4-5 rule is the simplest case: a triangle with sides of 3, 4 and 5 units has a right angle between the 3 and the 4, which is a quick way to square a corner on site.
Worked example: a 16 × 12 ft frame must have diagonals of 20 ft 0 in (20.000 ft), because the diagonal of a 12-16-20 triangle is exactly 20 ft. A 10 × 10 ft square has diagonals of 14 ft 1 11/16 in. If you measure 20.02 and 19.98 ft on the 16 × 12 frame, the difference is 0.48 inches: out of square by 0.48 in: adjust and measure again.
Diagonals of common rectangles
The diagonal a square frame of this size must have.
| Size | Diagonal (feet) | Diagonal (ft and in) |
|---|---|---|
| 8 × 8 ft | 11.314 | 11 ft 3 3/4 in |
| 10 × 10 ft | 14.142 | 14 ft 1 11/16 in |
| 10 × 12 ft | 15.620 | 15 ft 7 7/16 in |
| 12 × 12 ft | 16.971 | 16 ft 11 5/8 in |
| 12 × 16 ft | 20.000 | 20 ft 0 in |
| 16 × 20 ft | 25.612 | 25 ft 7 3/8 in |
| 20 × 24 ft | 31.241 | 31 ft 2 7/8 in |
| 24 × 30 ft | 38.419 | 38 ft 5 in |
Tips before you buy
- Measure the diagonals from the same points on each corner, such as the outside edges, and on the same level.
- If the diagonals differ, nudge the corners of the longer diagonal inward a little and measure again until they match.
- Use the biggest 3-4-5 triangle you can fit (6-8-10, 9-12-15, 12-16-20), since small errors in a small triangle become big ones over a long wall.
- A tape that is hooked on a nail or held by a helper at the same spot each time gives repeatable readings.
- Check the layout again after you set the batter boards or the forms, since they move as you work.
Tools & materials for this project
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Frequently asked questions
How do you check if a frame is square?
Measure both diagonals, corner to corner. If they are equal the frame is square. For a 16 × 12 ft frame they should both be 20 ft 0 in.
What is the 3-4-5 rule?
A triangle with sides of 3, 4 and 5 units has a right angle between the shorter sides. Mark 3 ft along one side and 4 ft along the other from the corner, and the distance between the marks must be 5 ft. Bigger multiples (6-8-10, 9-12-15, 12-16-20) are more accurate.
How square does a deck or a foundation need to be?
Within about 1/4 inch on the diagonals for a deck or framing, and 1/8 inch for finish work. Large foundations are usually checked to within 1/2 inch.
How do I fix a frame that is out of square?
Find the longer diagonal and push the two corners it joins inward, or the other corners outward, by about half the difference, then measure again.
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Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.