Carpenter how-to
How to Check a Layout for Square Using the 3-4-5 Method
The 3-4-5 rule uses basic geometry to confirm a perfect 90-degree corner.
Quick answer
The 3-4-5 rule uses basic geometry to confirm a perfect 90-degree corner. In short: from; measure; measure, then complete the remaining steps below.
Step by step
From the corner, measure 3 feet along one line and mark it.
Measure 4 feet along the other line and mark it.
Measure the diagonal between the two marks.
If the diagonal is exactly 5 feet, the corner is square.
If not, adjust one line until the diagonal reads 5 feet.
For larger layouts, use multiples such as 6-8-10 or 9-12-15 for more accuracy.
Safety reminders for carpenters
- Use fall protection and inspect scaffolds and ladders before use
- Wear eye and hearing protection with power tools
- Keep blade guards in place and never remove safety features
- Lift with your legs and get help with heavy materials
Key terms
Stud · On center (OC) · Top and bottom plates · Header · Joist · Rafter: see the full carpenter glossary.