3-4-5 Rule Calculator
Square any corner with the 3-4-5 rule: enter one side to get the other and the diagonal to measure, or use the real lengths of any two sides.
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- Diagonal to measure
- 5 m
What it does
To set out a perfect right angle, mark a distance along each side of the corner, then measure between the two marks. If that diagonal matches the figure here, the corner is exactly 90 degrees.
The 3-4-5 rule is the classic version: mark 3 along one side and 4 along the other, and the diagonal should be exactly 5, in any unit. Enter either short side and the other follows in the 3 : 4 ratio, with the diagonal worked out for you. Or switch the method to Any two sides to square up an extension, deck or room from its real sizes.
How to use it
- Method
3-4-5 ratio: enter either short side and the other follows (3 and 4, 30 and 40, 1.2 and 1.6). Any two sides: use the real lengths of what you're building, such as a 2.4 × 3.6 m extension.
- The "3" side
Measure this far along one side from the corner and mark it. Typing here sets the "4" side for you. Bigger is more accurate: use the largest that fits.
- The "4" side
Measure this far along the other side from the same corner. Typing here sets the "3" side for you.
- First side
Measure along one side of the corner from the corner point and mark it. Any length works; longer is more accurate.
- Second side
Measure along the other side from the same corner point and mark that too.
Common uses
- Setting out foundations or a slab for an extension, shed or garage. Try the Concrete Calculator
- Squaring up a patio, deck or tiled floor before laying it. Try the Tile Calculator
- Checking a room or stud wall is square before fitting floors or kitchens.
- Working out any right-angled triangle from two sides. Not built yet: request it
- Working out a roof's pitch and rafter length. Try the Roof Pitch Calculator
The formula
Diagonal = √(first side² + second side²)3-4-5 ratio: "4" side = "3" side × 4 ÷ 3, and diagonal = "3" side × 5 ÷ 3Where:
- √
- square root
This is Pythagoras' theorem. The 3-4-5 rule is the whole-number case: 3² + 4² = 9 + 16 = 25, and √25 = 5.
Worked example
Mark 3 m along one side of the corner and 4 m along the other:
- 3 m squared is 9, and 4 m squared is 16
- Added together: 25
- The square root of 25 is 5 m
When the two marks are exactly 5 m apart, the corner is square. Any multiple works the same way: 30, 40 and 50, or 1.2, 1.6 and 2.0.
Questions people ask
What is the 3-4-5 rule?
A way to make a right angle with just a tape measure. Mark 3 units along one side of a corner and 4 along the other; when the distance between the marks is exactly 5, the corner is square. Builders have used it for thousands of years because it needs no square or laser.
What sizes can I use?
Any multiple works: 0.6, 0.8 and 1.0 m; 0.9, 1.2 and 1.5 m; 1.8, 2.4 and 3.0 m; or 3, 4 and 5 ft. Bigger triangles are more accurate, because a small error in a short diagonal makes a bigger difference to the angle. For a whole building, use its actual sizes here instead.
What if my diagonal doesn't match?
If it's longer than the figure here, the corner is more than 90 degrees, so move the second mark in towards the first side. If it's shorter, the corner is less than 90 degrees, so move it out. Adjust and measure again until it matches.
Last checked 28 October 2026 · Last rev. 31 October 2026 · e8b0