Construction
Square-Up Calculator (Diagonal Check)
Enter your length and width — get the exact diagonal measurement that confirms the corners are perfectly square (90°). Works for any rectangular layout: pools, slabs, decks, sheds, foundations, framed walls.
Diagonal should measure
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When both diagonals match this number, your layout is perfectly square.
Diagram
Measure corner-to-corner both ways. Both diagonals must equal the number above.
How to use this in the field
- Set your four corner stakes at the planned length and width.
- Pull a tape corner-to-corner in both directions (across the X).
- Both diagonals should equal —.
- If one diagonal is too long: pull that corner IN toward the center until both match.
- Verify all four sides are still correct length, then secure.
The math
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Common builder shortcuts
| Length × Width | Diagonal | Notes |
|---|---|---|
| 3 ft × 4 ft | 5 ft | 3-4-5 triangle (the classic) |
| 6 ft × 8 ft | 10 ft | Scaled 3-4-5 |
| 9 ft × 12 ft | 15 ft | Scaled 3-4-5 |
| 10 ft × 10 ft | 14 ft 1 7/16" | Perfect square = side × √2 |
| 12 ft × 16 ft | 20 ft | Scaled 3-4-5 |
| 16 ft × 20 ft | 25 ft 7 7/16" | Common deck size |
| 20 ft × 40 ft | 44 ft 8 5/8" | Typical pool |
| 30 ft × 40 ft | 50 ft | Scaled 3-4-5 |
FAQ
Why both diagonals? +
A 4-sided shape with equal sides can still be a parallelogram (skewed). Equal diagonals are the only thing that guarantees 90° corners. Both diagonals must equal the same number from this calc.
What tolerance is OK? +
1/4" for general construction (pools, slabs, decks). 1/8" for framed walls, cabinetry, and foundations. 1/2" for rough grading and excavation.
What if my diagonals are equal but off the target? +
Then the shape is a true rectangle (90° corners), but your length or width is wrong. Re-check the sides — one is too long or too short.
Which corner do I move when out of square? +
The one on the LONGER diagonal. Push it toward the center along its short side. The long diagonal will shorten and the short one will lengthen until they meet at the target.
Can I do 3-4-5 instead? +
Yes. 3-4-5 (or 6-8-10, 9-12-15, etc.) is the same math at one corner. The diagonal check verifies the whole rectangle at once — most pros use both.
Does this work for non-rectangular shapes? +
Only for rectangles. For freeform pools, mark a rectangular envelope first, square it up using this calc, then mark curves inside it.