Check Square Calculator (3-4-5 Method)

The check square calculator uses the Pythagorean theorem to find the diagonal a square corner should have, from the two distances you measure along each side. It also reports how far off your measured diagonal is when you enter one, so you know whether the corner needs adjusting.

Last reviewed September 2026Method & sources Report an error

Not to scale.

Expected Diagonal (if square)

5.000 ft

Breakdown
Expected Diagonal (if square)	5.000 ft
Your Measurement Is Off By

How to Calculate Check Square

The 3-4-5 method is a direct application of the Pythagorean theorem: for a true right angle (90°), the diagonal connecting two points measured along each side satisfies `diagonal = √(legA² + legB²)`. The classic version measures 3 ft along one side and 4 ft along the other, giving a diagonal of exactly 5 ft (3² + 4² = 5²) if the corner is truly square. The same relationship works at any scale. Doubling to 6-8-10, or any other leg lengths, works too, so this calculator computes the expected diagonal for whatever distances you choose to measure.

Example: The classic 3-4-5 check

The classic 3-4-5 check: 1. Expected diagonal: √(3² + 4²) = √(9 + 16) = √25 = 5 ft exactly. If your actual measured diagonal comes out to 5.125 ft instead, the corner is 0.125 ft (1.5 in.) out of square. The corner needs adjusting until the measured diagonal matches 5 ft. Scaling up to 6 ft and 8 ft legs instead: √(6² + 8²) = √(36 + 64) = √100 = 10 ft. That is the same 3-4-5 ratio doubled, giving more precision over a longer span.

What Is Check Square?

A check square calculator confirms whether a corner is truly a 90° right angle. That corner might be a foundation layout, a deck frame, or a wall. It uses nothing more than a tape measure and the Pythagorean theorem. Measure a known distance along each side from the corner, then measure the diagonal between those two points. It should match the calculated expected diagonal exactly if the corner is square.

Check Square: Pros and Cons

Pros

Works at any scale, not just the classic 3-4-5. Larger leg lengths (like 6-8-10 or 9-12-15) give more precision over a longer wall or foundation run
Shows exactly how far off-square a measured corner is. You know which direction and how much to adjust, rather than getting a plain pass or fail
Needs only a tape measure, with no framing square or other specialized tool required

Cons

Doesn't tell you which specific side needs to move. It only shows that a discrepancy exists between the measured and expected diagonal
Larger structures benefit from larger leg lengths for better precision. A small 3-4-5 check on a large foundation may not catch a subtle out-of-square condition as reliably as a longer measurement would
Assumes both legs are measured perfectly straight and starting exactly from the corner point. A measuring error in either leg throws off the expected diagonal calculation

Check Square: What a Pro Checks

Use bigger leg lengths on a large foundation or deck frame. Scaling up to 6-8-10 or 9-12-15 spreads the same measuring error over a longer distance, making an out-of-square corner easier to catch.
Start both leg measurements from the exact same corner point. An error in either leg throws off the expected diagonal, since the check is only as accurate as the two straight lines it depends on.
If the measured diagonal comes out long, the corner is more than 90 degrees. If it comes out short, it's less than 90. Use that direction to know which way to adjust, along with the size of the difference.

Frequently Asked Questions

What is the 3-4-5 method for squaring a corner?

Measure 3 ft along one side of the corner and 4 ft along the other, then measure the diagonal distance between those two points. If the corner is a true right angle, that diagonal will measure exactly 5 ft. That is a direct result of the Pythagorean theorem (3² + 4² = 5²).

Can I use different measurements than 3-4-5?

Yes. Any two leg lengths work, since the underlying relationship (diagonal = √(legA² + legB²)) holds for any right angle, not only the specific 3-4-5 ratio. Larger measurements (like 6-8-10) are often preferred for bigger projects since they're more precise over a longer span.

Why use 6-8-10 instead of 3-4-5?

It's the same 3-4-5 ratio doubled. It spreads any small measuring error over a longer distance, making an out-of-square condition easier to detect and more precisely correctable. That's useful for a larger foundation or deck frame, where a small check near the corner might miss an error that shows up further out.

What if my measured diagonal doesn't match the expected diagonal?

The corner isn't quite square. If the measured diagonal is longer than expected, the angle is slightly more than 90°. If it's shorter, the angle is slightly less. Adjust the corner and re-measure until the diagonal matches the expected value from this calculator.

Do I need a framing square if I use the 3-4-5 method?

No. The 3-4-5 method is specifically useful because it only requires a tape measure. That makes it practical for checking large layouts, like a foundation or deck footprint, that are far too big for a standard framing square to check directly.

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