How to square a foundation or deck
- Set the two long sides to the same length and the two short sides to the same length. A shape with equal opposite sides is a parallelogram. It may not be square yet, but it's close.
- Enter side A and side B above. The diagonal when square is what corner-to-corner should measure.
- Measure both diagonals. If one is long and the other short, rack the frame by pushing the long diagonal's corners toward each other until both diagonals match the calculated number.
The math
A square corner makes a right triangle, so the diagonal is √(A² + B²). That's the Pythagorean theorem. For a 24' × 16' deck: √(288² + 192²) = 346.13", or 28'-10 1/8". Rounding matters here. Lay out to the nearest ⅛" or 1/16", not the nearest inch.
The 3-4-5 method
When you only need one square corner, such as a wall layout or a deck ledger, a 3-4-5 triangle is faster. Measure 3' along one line and 4' along the other from the same corner. When the distance between those two marks is exactly 5', the corner is square. Scale it up for accuracy on bigger layouts: 6-8-10, 9-12-15, or 12-16-20. A bigger triangle means a small tape error throws the angle off less.
Tips from the field
- Pull diagonals from the same reference on each corner (outside corners of the form, string intersections, or nail marks) and keep the tape tight and level.
- On a batter-board layout, square the strings first, then drop plumb to the footing.
- For walls, check the diagonals of the wall frame before you nail on sheathing. Once the first sheet is on, the wall is locked.
FAQ
How do I calculate the diagonal of a rectangle?
Square each side, add them, and take the square root: diagonal = √(A² + B²). A 12' × 16' rectangle has a 20' diagonal.
How do you square a foundation with diagonals?
Make opposite sides equal, then measure both diagonals. If they are not equal, shift the corners until they are. Equal diagonals with equal opposite sides means the rectangle is square.
What is the 3-4-5 rule in construction?
A triangle with sides of 3, 4 and 5 (in any unit) always has a 90° corner. Mark 3 on one line and 4 on the other from the corner. When the marks are 5 apart, the corner is square. Multiples like 6-8-10 work the same way.
How close to square is close enough?
For framing, diagonals within ⅛" to ¼" of each other is typical on a house-size layout. Concrete forms and finish work call for tighter. This calculator flags anything within ⅛" as square.