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Squaring and Diagonal Calculator

The diagonal that proves a corner is square.

Your measurementsEvery assumption editable01

One leg of the right angle — a wall, a form edge, a deck side.

DiagonalCalculated live02

Result

20ft diagonal

20 ft 0 in — square when the diagonal matches

First side
12 ft
Second side
16 ft
Diagonal240 in
20 ft
In feet and inches
20 ft 0 in
Angle at the first side
36.87°
Angle at the second side
53.13°

This is how you square anything without a large square. Measure the two sides, calculate the diagonal, and adjust until the measured diagonal matches — at which point the corner is exactly 90°. It works on a deck frame, a wall layout or a concrete form.

For a rectangle, check both diagonals. Equal diagonals mean the shape is a true rectangle rather than merely having one square corner, which is the failure mode of checking only one.

The 3-4-5 rule is this same relationship at a convenient ratio. Any multiple works, and larger is more accurate — 12-16-20 on a deck is far more precise than 3-4-5 on the same frame.

Measure two sides, calculate the diagonal, then adjust until the measured diagonal matches. At that point the corner is exactly 90°. It works on a deck frame, a wall layout or a concrete form, and needs nothing but a tape.

Why use this tool?What it does differently03

Why use this tool?

Squares anything

No large square needed — just a tape and two people.

Feet and inches

Given in the form you will actually measure to, plus decimal feet.

Both angles

For layouts where the diagonal itself is a member rather than a check.

3-4-5, generalised

Any multiple works, and bigger is more accurate on a large frame.

How this worksThe method04

How this squaring and diagonal calculator works

The diagonal of a rectangle is the square root of the sum of the squares of its sides — Pythagoras, applied to a job site. If the measured diagonal matches the calculated one, the corner between those sides is exactly 90 degrees.

For a full rectangle, check both diagonals. Equal diagonals prove the shape is a true rectangle rather than a parallelogram with one square corner, which is the failure mode of checking only one.

The 3-4-5 rule is this same relationship at a convenient ratio. Larger multiples are more accurate because a fixed measuring error is a smaller proportion of the total — 12-16-20 beats 3-4-5 on the same frame.

How to use itStep by step05

How to use it

  1. Step 1: Measure the two sides

    The two legs meeting at the corner you want square.

  2. Step 2: Calculate the diagonal

    The figure your tape should read corner to corner.

  3. Step 3: Adjust until it matches

    Rack the frame until the measured diagonal equals the calculated one.

  4. Step 4: Check the other diagonal

    On a full rectangle, both must match for the shape to be true.

Example usageWorked figures06

Example usage

A deck frame
12 by 16 ft gives a diagonal of exactly 20 ft. That is 3-4-5 scaled by four, which is why those numbers appear so often.
A large slab
A 24 by 32 ft form has a 40 ft diagonal. Using the full size rather than a small 3-4-5 triangle in one corner is considerably more accurate.
An awkward size
A 13 ft 6 in by 9 ft 4 in room has a diagonal of 16.415 ft, or 16 ft 4.98 in — which is why calculating beats looking for a convenient ratio.
Frequently asked questionsCommon questions07

Frequently asked questions

How do I square a deck or foundation?

Measure the two sides, calculate the diagonal, then adjust the frame until the measured diagonal matches. At that point the corner is exactly 90 degrees.

What is the 3-4-5 method?

A right triangle with sides in a 3:4:5 ratio. Any multiple works — 6-8-10, 12-16-20 — and larger multiples are more accurate on a big frame.

Why check both diagonals?

Because one square corner does not make a rectangle. Equal diagonals prove the whole shape is true rather than racked into a parallelogram.

How accurate does the diagonal need to be?

Within an eighth of an inch is comfortable on a domestic frame. The longer the sides, the more forgiving a given error is in angular terms.

Does this work for non-rectangular layouts?

It squares any single corner between two known sides. Complex layouts are best broken into rectangles and squared one at a time.

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