Check Square Calculator

1. Enter the length of the rectangle (the longer side) in feet.
2. Enter the width (the shorter side) in feet.
3. Read the expected diagonal and the 3-4-5 check values.
4. Optionally enter your measured diagonal to see whether your layout is square, close or out of square.

Here is what each result line means:

For wall framing dimensions, see the stud calculator.

Checking square means verifying that a rectangular layout has true 90-degree corners. Builders, carpenters and concrete workers check square before setting forms, laying plates or nailing decking. A rectangle with four right angles has two diagonals of exactly the same length. If the diagonals differ, at least one corner is not 90 degrees.

The two most common methods are the diagonal method (compare measured diagonals to the Pythagorean value) and the 3-4-5 method (check one corner at a time using a right-triangle ratio).

The calculator computes the expected diagonal from the Pythagorean theorem. If you enter a measured diagonal, it subtracts the expected value to show how far off square the layout is.

The 3-4-5 values are scaled so the short leg fits within the shorter wall. For example, on a 12-foot-wide layout the calculator uses 12-16-20 (k = 4).

1. Square the length and the width, add them, and take the square root.
2. Compare the result to your measured diagonal.
3. If the difference is less than 1/8 inch, the layout is square for most construction purposes.

A foundation measures 30 feet long and 40 feet wide. You measure one diagonal at 50.02 feet.

Step 1. Expected diagonal = sqrt(30^2 + 40^2) = sqrt(900 + 1600) = sqrt(2500) = 50.000 feet.

Step 2. 3-4-5 check: shorter side is 30 ft, k = 10, so measure 30 ft along one side, 40 ft along the other, and the diagonal should be 50 ft.

Step 3. Measured diagonal = 50.02 ft. Difference = 0.020 ft = 0.24 inches.

Verdict. The layout is close to square but off by about 1/4 inch. For most residential work this is acceptable, but for finish carpentry or tile work you may want to adjust further.

Tape Sag

A long tape measure sags under its own weight, making the reading slightly short. For diagonals over 50 feet, add tension or use a laser distance measurer for better accuracy.

Temperature

A steel tape expands about 1/16 inch per 100 feet for every 15-degree Fahrenheit rise. This is negligible for most residential work but matters on large commercial slabs.

Slope

If the ground or slab is not level, the measured diagonal will be longer than the true horizontal distance. Level the tape or measure on a flat surface.

Stake and String Stretch

Nylon string stretches more than steel tape. Pull string lines tight and re-check after staking. Prefer a dry line or wire for long spans.

Corner Rounding

If the corner is a rounded form or a batter board, hook the tape to the exact intersection point, not the outside of the board.

Both methods confirm 90-degree corners, but they work differently and suit different situations.

Use the diagonal method when you can reach both corners. Use the 3-4-5 method for a single corner, such as checking a wall plate before nailing.

Foundation Layout

Check diagonals before you dig or pour. Moving string lines costs nothing; moving a poured footer costs a lot.

Deck Framing

Square the rim joist and ledger before installing the joists. An out-of-square deck is visible along the fascia and railing.

Wall Framing

Rack the wall on the subfloor until the diagonals match before standing it up and nailing the bottom plate.

Concrete Forms

Measure diagonals of the form before pouring. Concrete is permanent; plywood forms are not.

Fence Layout

Square the first corner post to the house or property line so the rest of the fence runs straight.

1. Measuring from Outside of Boards Instead of Centers

Hook the tape to the same reference point (inside corner, center of stake, nail head) on every measurement. Switching between inside and outside edges adds a board thickness of error.

2. Using Too Short a 3-4-5 Triangle

A literal 3-4-5 feet check has 1/4 inch of uncertainty at best. Scale up to 12-16-20 or 15-20-25 to cut the percentage error.

3. Forgetting to Check Both Diagonals

One correct diagonal does not prove all four corners are square. A parallelogram (opposite sides parallel but not 90 degrees) can have one diagonal match the Pythagorean value while the other does not.

4. Ignoring Slope

On sloped ground, level the tape before reading the diagonal. A 2-percent slope adds about 1/4 inch per 50 feet.

5. Rounding Too Aggressively

Report the diagonal to at least three decimal places in feet (thousandths) or to the nearest 1/16 inch. Rounding to the nearest inch hides whether the layout is truly square.

The calculator gives the mathematically exact diagonal from the Pythagorean theorem. The only source of error is in your measurements, not in the formula.

Measure both diagonals corner to corner. If they are the same length, all four corners are 90 degrees. The expected diagonal is the square root of the length squared plus the width squared.

Measure 3 feet along one side of a corner and 4 feet along the adjacent side. The diagonal between those two marks should be exactly 5 feet. If it is, the corner is 90 degrees. Multiples like 6-8-10 or 12-16-20 also work.

Accuracy depends on the size of the triangle. A literal 3-4-5 foot check is accurate to about 1/4 inch. Scaling up to 12-16-20 improves accuracy to about 1/16 inch because the percentage error shrinks with longer sides.

Yes. The Pythagorean formula works in any consistent unit. Enter your measurements in meters and the expected diagonal will be in meters.

For most residential framing and concrete work, 1/4 inch is acceptable. For tile work, cabinetry or finish carpentry, tighten the tolerance to 1/8 inch or less.

No. It assumes a rectangle with four intended 90-degree corners. Trapezoids and parallelograms need different methods.

At least one corner is not 90 degrees. The layout may be a parallelogram. Adjust the corners until both diagonals match the expected value.

Loosen the stakes on one end and push the layout diagonally. If one diagonal is too long, push that end inward. Re-measure after each adjustment until both diagonals match.

A tape measure long enough to reach the diagonal. For layouts over 50 feet, a laser distance measurer is more accurate than a steel tape because it eliminates sag.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• InchCalculator, Squaring Calculator (diagonal formula, squareness check method and worked example for rectangular layouts).
• Instructables, Handy Tricks to Find Square (3-4-5 rule, scaled multiples, practical corner-checking technique for construction).
• Bunnings Workshop, How to Square a Concrete Slab (diagonal comparison method for slab layout, adjustment technique).