The Fixin’ Post — How to Check if a Wall Is Square: 3-4-5 Method Carpentry Guide

If you’re framing out a wall and wondering whether your corner is square—but you don’t have a framing square or laser level on hand—don’t sweat it. There’s a time-tested trick that’s been used on job sites and in back gardens for generations: the 3-4-5 method carpentry technique.

It’s simple, accurate, and all you need is a tape measure. Whether you’re building a shed, laying out a loft extension, or just trying to keep things tidy in your workshop, this method will keep your corners true.

What Is the 3-4-5 Method?

The 3-4-5 method is a practical way to check if a corner forms a perfect 90-degree angle. It’s based on the Pythagorean Theorem—the same one you learned in school:

If you measure 3 units along one wall, 4 units along the other, and the diagonal between them is exactly 5 units, your corner is square.

Pro tip: You can scale this up to 6-8-10 or 9-12-15 for larger builds. Just keep the ratio.

Easy Analogy: The Pizza Slice Test

Think of it like this: imagine a slice of pizza. One edge is 3 inches, the other is 4 inches, and the diagonal from crust to tip is 5 inches. That triangle has a perfect right angle. That’s what you’re recreating with your tape measure.

Step-by-Step: How to Check if a Wall Is Square

Here’s how to use the 3-4-5 method on-site:

1. Pick your corner — internal or external.

2. Measure 3 feet (or meters) along one wall and mark it.

3. Measure 4 feet along the adjacent wall and mark that.

4. Measure the diagonal between the two marks. If it’s exactly 5 feet, your wall is square.

If it’s off, adjust your framing until the diagonal hits the mark.

Why It Works in Real-World Carpentry

This method is a lifesaver when:

• You’re working solo and need a quick check

• You’re on a remote site without your full kit

• You’re building something custom like a shed, cabin, or extension

It’s not just theory—it’s field-tested geometry that works when it counts.

Out in the wild west of home carpentry, not every jobsite has a laser level. But a tape measure and the 3-4-5 method? That’s all you need to square things up. It’s one of those timeless tricks that earns its place in your mental toolbox.