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LAYOUT & GEOMETRY • LESSON 05

3-4-5 NEVER LIES
SQUARING CHART & MASTERCLASS

Use the 3-4-5 rule to square any corner on the job site with 100% accuracy. If the 3-side and 4-side are true, the 5-diagonal will always guarantee a perfect 90° right angle.

10 MIN READ + INTERACTIVE TOOL
LEVEL: ALL FRAMERS & BUILDERS
INCLUDES COMPLETE SQUARING CHART
INTERACTIVE 3-4-5 SQUARING CALCULATOR & CORNER TESTER

Calculate required diagonal lengths or check if your current wall/deck corner is perfectly square.

Required Diagonal Length (5-Side)
20' 0"
12² + 16² = 20² (SQUARE)
A: 12' B: 16' C: 20'
Corner Verification Result
90.0°
PERFECT 90° SQUARE CORNER

Your measurements form an exact right angle. Corner is ready for plates and sheathing!

Marcos De Mello - The Master Framer
MARCOS DE MELLO
@THEMASTERFRAMER • INSTAGRAM REEL LESSON
WATCH ORIGINAL REEL →
"If your walls aren’t square, nothing after this will be right. This is one of the easiest ways to check layout on site."
1. Wall Plates & Layout
A 1/4" error at the bottom plate compounds into 1"+ out of plumb at the top wall plate.
2. Roof Rafters & Trusses
If building corners aren't square, hip rafters, valley jacks, and ridge lines will twist.
3. Finish Trades & Cabinets
Drywall, flooring tile, kitchen cabinets, and trim will have visible gaps if walls aren't 90°.
? HOW TO USE THE 3-4-5 RULE
1
Measure 3 units along one wall, plate, or string line (e.g. 3 feet or 9 inches). Make a precise mark.
2
Measure 4 units along the adjoining wall or plate (e.g. 4 feet or 12 inches). Make a precise mark.
3
Measure across the diagonal between your two marks. It should equal exactly 5 units (e.g. 5 feet or 15 inches).
4
Check & Adjust: If diagonal equals 5 units, your corner is perfectly square. If not, push wall in/out and re-test.
JOB SITE TIPS & PRO HACKS
✔
Works for any size: Just multiply all three numbers by the exact same multiplier ($3 \times k$, $4 \times k$, $5 \times k$).
✔
Use tape measure & mark clearly: Use a sharp carpenter pencil or crow's foot mark on the layout line.
✔
Check both diagonals: On any rectangular frame, box, or deck floor, cross-diagonals MUST be equal.
✔
Universal application: Essential for framing walls, deck joists, concrete slab forms, roofs, and subfloor layout.

THE MASTER 3-4-5 SQUARING CHART

Use these quick-reference job site tables. Match your wall run dimensions to get the exact required diagonal instantly.

STARTING AT 9 INCHES INCHES
3 SIDE 4 SIDE 5 (DIAGONAL)
9"12"15"
12"16"20"
15"20"25"
18"24"30"
21"28"35"
24"32"40"
30"40"50"
36"48"60"
48"64"80"
60"80"100"
72"96"120"
LARGER MEASUREMENTS (FEET) FEET
3 SIDE 4 SIDE 5 (DIAGONAL)
6'8'10'
9'12'15'
12'16'20'
15'20'25'
18'24'30'
21'28'35'
24'32'40'
27'36'45'
30'40'50'

WHY IT WORKS: THE MATH BEHIND 3-4-5

The 3-4-5 rule is based on the Pythagorean Theorem formula for right triangles (a² + b² = c²).

3² + 4² = 5²
9 + 16 = 25

Because 9 + 16 equals 25, any triangle with side lengths proportional to 3, 4, and 5 is guaranteed to have an exact 90-degree right angle opposite to the diagonal!

TEST YOUR KNOWLEDGE: 3-4-5 QUIZ

Question 1 of 1: You are framing a 12-foot long wall. What should your 4-side and diagonal measurements be to guarantee a square corner?