Squaring up with the 3-4-5 method

Learn the 3-4-5 method to check or mark a perfect right angle using just a tape measure.

SU-01Understand why 3-4-5 worksA corner is 'square' when it's exactly 90 degrees. The 3-4-5 method uses simple maths: any triangle with sides in the ratio 3:4:5 (like 300mm, 400mm, 500mm) always has a right angle between the two shorter sides. It works at any size, so you can use 300-400-500mm for a small job or 900-1200-1500mm for a bigger one - the ratio matters, not the units.

SU-02Get your tape measure and pencil readyYou just need a tape measure (metal, retractable) and a pencil. A helper makes this much easier, since one person holds the tape end still while the other reads the measurement. Working alone is possible but fiddly, because the tape wants to slide.

SU-03Mark the first point along one lineFrom your corner point, measure exactly 300mm (or 400mm - pick whichever line you're on) along one edge and make a small, sharp pencil dot. Keep the tape flat and pulled tight, not sagging, or your measurement will be short. The mistake beginners make is measuring from the edge of the tape's hook rather than the actual corner point - always start the zero mark exactly at your corner.

SU-04Mark the second point along the other lineFrom the same corner, measure exactly 400mm along the second edge (the one at roughly 90 degrees to the first) and mark it. Both lines must start from the same corner point - if you measure from slightly different spots, the whole check falls apart. Take a second to double-check you're still reading the tape from zero, not from an odd number.

SU-05Measure the diagonal between your two marksStretch the tape between the two pencil dots you just made - not back to the corner. If the distance is exactly 500mm, your corner is square. If it's more or less than 500mm, the corner is out of square and needs adjusting before you build, fix, or mark anything else off it.

SU-06Adjust if the diagonal is wrongIf the diagonal reads more than 500mm, the angle is too wide - pivot one line slightly inward. If it reads less, the angle is too tight - pivot it outward. Re-measure the diagonal after each small adjustment until it lands on 500mm. The common beginner mistake here is guessing and moving too far in one go - make small changes and re-check each time.

SU-07Confirm you've got it rightYou've done it correctly when all three measurements hold at the same time: 300mm along one line, 400mm along the other, and 500mm on the diagonal, all measured from the same accurate corner point. For extra confidence on a bigger area, double every measurement (600-800-1000mm) and check again - if it still checks out, your square is solid and reliable, not a fluke.

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