Master the 345 Rule for Spot‑On Right Angles
Ever tried to line up a wall or a floor joist and ended up with a wonky corner? It’s frustrating, wastes time, and can cost you money. The 345 rule (also called the 3‑4‑5 method) is a tiny trick that fixes that problem fast. All you need is a tape measure and a bit of math – no fancy tools, no laser levels.
How the 3‑4‑5 Rule Works
Think of a right‑angled triangle. The sides follow a simple ratio: 3 units, 4 units, and 5 units. If you measure 3 meters on one side, 4 meters on the other, and the diagonal comes out to exactly 5 meters, you have a perfect 90‑degree corner. The magic is that the numbers work for any scale – use centimeters, feet, or inches – as long as the proportion stays the same.
Here’s a quick step‑by‑step:
- Mark the start point of your line (Point A).
- Measure 3 units along the first line and put a marker (Point B).
- From Point A, measure 4 units at the angle you think should be 90 degrees and mark (Point C).
- Measure the distance between Points B and C. If it reads 5 units, you’ve nailed a right angle.
Using the Rule on Site
Most builders use the 345 rule when laying foundations, fitting walls, or setting up temporary braces. Start with a short test triangle – say 300 mm, 400 mm, and 500 mm – so you don’t waste material. Once the test is spot‑on, scale up using the same ratio. For longer runs, you can multiply the numbers (6‑8‑10, 9‑12‑15, etc.).
Common pitfalls: forgetting to account for board thickness, using a stretched tape measure, or setting markers on uneven ground. Keep your tape taut, write on a flat surface, and double‑check the diagonal before committing to concrete pours.
Another handy tip is to use a carpenter’s square as a quick visual check. If the square fits snugly after you’ve confirmed the 3‑4‑5, you’ve got an extra layer of confidence.
In practice, the 345 rule saves you from re‑working walls, reduces material waste, and keeps your project on schedule. It’s a small step that makes a huge difference, especially on DIY builds where precision tools might be out of reach.
Give it a try on your next layout. Measure, adjust, and watch the corners fall into place. You’ll wonder how you ever built without it.
