How do I check if a wall or framing is square?

Use diagonal measurement to prove a 90° corner: on a right-angled frame the diagonal is fixed by the two sides (Pythagoras, a² + b² = c²). Measure the two wall legs, find the target diagonal, then measure corner to corner — when your tape matches, the frame is square.

To square a whole rectangle — a wall, floor, or deck — measure both diagonals corner to corner. When the two diagonals are equal, the rectangle is square. If one is longer, push the frame toward the shorter diagonal until they match.

On a single corner, mark 3 ft along one leg and 4 ft along the other; the straight line between those marks is exactly 5 ft when the corner is 90°. Slide one leg until it reads 5 ft.

For small layouts and individual corners, a framing square (the big L) or a speed square gives an instant 90° reference without measuring diagonals. Seat it into the corner to mark or check square.

For large rooms, additions, or whole-house layouts, a self-leveling laser square projects a 90° reference across the floor, and a pro framer can lay out and brace square far faster. Worth it when the area is too big to tape diagonals accurately or the stakes are high.