Foundation Squaring Calculator
Calculate the diagonal measurement needed to confirm a rectangular foundation or deck layout is square, using the 3-4-5 method.
How this calculator works
A rectangle has one property a parallelogram doesn’t: its two corner-to-corner diagonals are exactly equal, and both match a value set by the side lengths. This calculator takes your planned length and width and returns that diagonal distance, so you can measure your actual layout and confirm the corners are true.
Stake out your rectangle using the length and width, then measure from one corner straight across to the opposite corner, and again for the other pair of corners. If both measured diagonals match the calculated number, and match each other, all four corners are square.
Formula: Diagonal = √(length² + width²) — the Pythagorean theorem. Measure both diagonals of your rectangular layout; if they match this calculated number (and match each other), the corners are square.
Worked example
A 24 ft × 36 ft foundation:
- Length² + width²: 24² + 36² = 576 + 1,296 = 1,872
- Diagonal: √1,872 ≈ 43.27 ft
Notes
This is the same math as the classic “3-4-5 triangle” trick carpenters use to square a single corner, just scaled up to check the whole rectangle at once. Instead of checking one corner with a 3 ft and 4 ft leg, you’re checking the full layout by measuring corner to opposite corner, in both directions.
If both diagonals match this calculated number and match each other, all four corners are true 90 degrees. If they don’t match — even if every side length is correct — the layout has drifted into a parallelogram, and the corners need adjustment before you dig or pour.
How to use
Enter the planned length and width of your rectangular layout in feet. Use the calculated diagonal as your target, then physically measure both corner-to-corner diagonals on your staked-out layout and compare. Adjust stakes until both diagonals match the calculated value and match each other.
A layout-checking calculation, not a substitute for a survey on a property-line-critical foundation. Re-measure after any stake adjustment.
Frequently asked questions
What does it mean for a foundation layout to be 'square'?
Square means every corner of the rectangle is a true 90-degree angle. A layout can have the correct side lengths and still not be square if the corners are skewed, which turns the shape into a parallelogram. Checking the diagonals is how you confirm the corners themselves, not just the sides, are correct.
How is this related to the 3-4-5 triangle method?
The 3-4-5 method checks a single corner: measure 3 ft down one side and 4 ft down the other, and the distance between those two points should be exactly 5 ft if the corner is 90 degrees. This calculator applies the same Pythagorean relationship to the full rectangle, checking corner-to-corner across the whole layout at once instead of one corner at a time.
What if my two diagonal measurements don't match each other?
If the two diagonals aren't equal to each other, the layout isn't square even if it's close to the calculated number, and at least one corner needs to be adjusted. Move the stakes at one end until both diagonals come out equal and match the calculated value, then re-check every side length to make sure it hasn't shifted.
Can I use this method for a non-rectangular layout?
No — this diagonal check only works for layouts intended to be rectangles (four corners, opposite sides equal and parallel). L-shaped, angled, or irregular foundations need to be broken into rectangular sections and each section squared separately, or checked with a different layout method.
Estimates only. Verify quantities with your supplier before purchasing.