Free construction calculator
Square corner calculator
Two sides in, the diagonal for a perfect 90° corner out, in feet, inches and sixteenths. Measure the diagonal you have and it tells you how far out of square you are, and which way to move.
Result
| Diagonal for a square corner | 17' 0-3/8"Real: 204.353 in, a hair under 3/8" · 17.029 ft |
|---|---|
| Your sides | 11' 0" and 13' 0" |
| Biggest 3-4-5 that fits | 9-12-15 ftMark 9 ft on one side, 12 ft on the other; they should be 15 ft apart |
- Measure both sides from the same point, and keep the tape tight and level.
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How it works
A square corner makes a right triangle, so the diagonal follows Pythagoras: diagonal = √(A² + B²).
The 3-4-5 method is the same rule with whole numbers: 3 ft along one side, 4 ft along the other, and they are 5 ft apart when the corner is square. Multiples work too (6-8-10, 9-12-15), and the bigger the triangle, the more accurate the corner.
If you measure the diagonal you actually have, the calculator works the corner’s real angle out with the law of cosines: angle = acos((A² + B² − D²) ÷ 2AB). A diagonal that is too long means the corner is open (over 90°); too short means it is closed.
Results are rounded to the nearest 1/16". When the real number sits between two sixteenths, it says so ("a hair under").
Worked example
Sides of 11 ft and 13 ft. √(11² + 13²) = √290 = 17.029 ft, which is 17' 0-3/8" to the nearest sixteenth (0.353" past 17 ft, a hair under 3/8").
The biggest 3-4-5 that fits along 11 ft and 13 ft is 9-12-15: mark 9 ft on the short side and 12 ft on the long side, and the marks should be exactly 15 ft apart.
Checking it. If you pull 17' 1" across instead, the corner is open by about a third of a degree (90.37°); pull the end of the 13 ft side in until the diagonal reads 17' 0-3/8".
Questions people ask
How do I check if a corner is square?
Measure along both sides from the corner, mark them, and measure between the marks. Compare that diagonal with the one this calculator gives you. Equal diagonals across a rectangle (corner to corner both ways) is the same check for the whole layout.
What is the 3-4-5 rule?
A triangle with sides of 3, 4 and 5 always has a 90° corner between the 3 and the 4. Use any multiple: 6-8-10 or 9-12-15 ft for a foundation, 3-4-5 in feet for a small deck.
What diagonal makes 11 by 13 square?
17' 0-3/8" (17.029 ft). It is 0.353" past 17 ft, a hair under 3/8".
Can I enter feet and inches?
Yes. 11' 6", 11 6, 11 ft 6 in and 11.5 all mean the same. Fractions work: 11' 6-1/2" or 11 6 1/2.
How close is close enough?
For framing and forms, most crews aim for the diagonals within ⅛" on a small room and ¼" on a foundation. Tighter is better for tile and cabinets.
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Built by Northdesk
These calculators are made by Northdesk, construction business software with Dex, an AI assistant, built in.
- Concrete contractorsPours that move with the weather, footings measured off the plans, finishers on the right site at 6 AM, and an inspector who signs in before you pour.
- General contractorsSubs to bid, insurance certificates to chase, inspectors to meet, owners to keep happy and a budget to protect.
- Masonry contractorsBlock, brick, stone and mortar on the right site, the saw and the mixer where they should be, and a crew that speaks two languages on the same page.
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