Formula: √(Length² + Width²)
Quick Answer: How to Check Square With the 3-4-5 Method
Measure 3 metres along one side, 4 metres along the perpendicular side, and the diagonal between those two points should be exactly 5 metres if the corner is perfectly square (90°). This is the Pythagorean theorem (3² + 4² = 5²) applied to construction setout.
For larger areas, scale the ratio: 6-8-10, 9-12-15, or 12-16-20 metres. The calculator above does the maths instantly for any size area.
Check Square Calculator — 3-4-5 Method for Builders
Verify if your layout is perfectly square using the 3-4-5 triangle method. Essential for slab setout, framing, decks, and formwork. Enter your two side measurements and the calculator instantly tells you if the corner is 90 degrees.
How the 3-4-5 Method Works
The 3-4-5 method is based on Pythagoras’ theorem: in a right-angled triangle, a² + b² = c². Since 3² + 4² = 9 + 16 = 25 = 5², any triangle with sides in a 3:4:5 ratio must contain a 90-degree angle.
Measure 3 metres along one wall, 4 metres along the adjacent wall. If the diagonal between those points is exactly 5 metres, your corner is 90°. Scale up for larger layouts: 6-8-10, 9-12-15, or any multiple of 3-4-5.
The Mathematics Behind Check Square
The check square calculator uses the Pythagorean theorem to verify right angles:
Diagonal = sqrt(Side A² + Side B²)
If your measured diagonal matches the calculated diagonal, the corner is square. The calculator also shows you the exact error in millimetres and degrees if the corner is out of square.
Common 3-4-5 Multiples for Building
| Side A | Side B | Diagonal | Best For |
|---|---|---|---|
| 1,500mm | 2,000mm | 2,500mm | Small rooms, bathroom setout |
| 3,000mm | 4,000mm | 5,000mm | Standard rooms, decks |
| 6,000mm | 8,000mm | 10,000mm | Large slabs, house setout |
| 9,000mm | 12,000mm | 15,000mm | Commercial slabs, site setout |
Pro tip: Always use the largest multiple that fits your layout. Larger triangles give more accurate results because measurement errors become proportionally smaller.
The Diagonal Method (Full Rectangle Check)
For a full rectangle check, measure both diagonals corner to corner. If both diagonals are equal, the rectangle is square. This is the gold standard for checking slabs and large layouts.
Diagonal = sqrt(Length² + Width²)
For a 10m x 6m slab: Diagonal = sqrt(100 + 36) = sqrt(136) = 11,662mm. Both diagonals should measure 11,662mm plus or minus 3mm.
When to Check Square
- Slab setout — before pouring, verify all corners. This is the most critical check on any residential build
- Frame setout — check wall frames before bracing and sheeting
- Decks and pergolas — ensure post positions are correct before setting in concrete
- Formwork — check before pouring footings or retaining walls
- Fence lines — verify corner posts at right angles to boundary
- Tiling — check room squareness before laying out tile grid
- Brickwork — verify corners before starting brick courses
NCC and Building Tolerances for Squareness
The National Construction Code (NCC) and relevant Australian Standards specify tolerances for building setout:
| Element | Tolerance | Reference |
|---|---|---|
| Concrete slab setout | +/-10mm over any dimension | AS 3600 |
| Slab diagonals | +/-5mm difference | AS 3600 |
| Timber frame walls | +/-5mm in 3m | AS 1684 |
| Brickwork | +/-10mm in 5m | AS 3700 |
| Tiling | +/-3mm in 3m | AS 3958 |
How to Fix an Out-of-Square Layout
- Under 5mm error: Can usually be absorbed in construction tolerances. Adjust string lines slightly
- 5-15mm error: Re-measure your baseline and pivot one side. Reset string lines and re-check
- Over 15mm error: Re-establish your baseline from a known straight line (boundary, existing wall). Set one corner as your reference and work from there
- Always recheck: After adjustment, re-measure all four corners and both diagonals
“Check square twice, pour once. The five minutes you spend checking diagonals before a pour will save you days of remedial work. I always use the 6-8-10 method for house slabs because larger triangles catch smaller errors.”
— Licensed concreter, 15+ years experience, NSW
More than just calculators
Built Simple manages your entire project — estimates, schedules, tasks, and team.