Skills · skill
Squaring with the 3-4-5 Rule
Measuring · Beginner
- Difficulty
- Beginner
- Safety
- Trip hazards from string lines; nothing else.
- Done When
- The two diagonals of the finished rectangle agree to within a few millimetres over several metres.
- Tools
- A tape, string line, pegs, a pencil; two people make it easier.
- How It Fails
- Measuring from the wrong mark. A tape with a bent hook that adds a few millimetres. Diagonals checked to the pegs rather than the string.
- Category
- Measuring
- Summary
- Laying out a right angle over any distance with nothing but a tape — a shed base, a wall, a fence line.
Steps
Set the first line with string and pegs.
Mark 3 units (say 3 m) along it from the corner.
Swing a 4-unit arc from the corner and a 5-unit arc from the 3 m mark; where they cross is the second line's direction.
Run the second line through that point.
For a rectangle, measure both diagonals; adjust until they are equal.
Use the biggest multiple the site allows.
The Science
A triangle with sides 3, 4 and 5 has a right angle opposite the 5, by Pythagoras: 3² + 4² = 5². Any multiple works, and a bigger triangle is more accurate because a millimetre of tape error matters less over five metres than over fifty centimetres. The same theorem checks a rectangle: if both diagonals are equal, its corners are square.
Open on the Skills shelf — the interactive card, the filters, and everything alongside it.