A square corner is the quiet hero of every build. Get the foundation square at the start and every wall, every door frame, every cabinet falls into place naturally. Get it wrong and you spend the rest of the build cutting tapered packers to make doors close. The Chippy Tools check square calculator is the simplest possible Pythagoras app — enter a length and width, get the expected diagonal, and compare it to the diagonal you measure on site. If the two match, the work is square. If they don’t, fix it before pouring concrete or nailing off boards.
What the check square calculator does
The Chippy Tools check square calculator takes two inputs:
- Length — one side of the rectangle being checked
- Width — the perpendicular side
And returns one output:
- Diagonal — the corner-to-corner distance the rectangle would have if it’s truly square
Compare the calculated diagonal to the actual diagonal you measure with a tape across the work. If the two match, the rectangle is square. If the measured diagonal is longer than calculated, the rectangle is racked one way; shorter, racked the other way. Adjust until they match.
The app runs locally on iOS and Android with no internet required. The same underlying right-triangle math drives the Stair Calculator, the Roof Pitch Calculator, and the Triangle Calculator — the check square calculator just frames it for the specific “is this rectangle square?” workflow.
Squaring a rectangle: the diagonal check
A rectangle is square (i.e. has four 90° corners) if and only if its diagonals are equal. So instead of measuring four corner angles with a square or protractor, you measure two diagonals and check they match — that’s the squaring shortcut every framer uses.
In the diagram: (1) length, (2) width, (3) diagonal, (4) opposite diagonal (should equal 3).
For a 4m × 3m rectangle, both diagonals should measure exactly 5m. If diagonal A measures 5.05m and diagonal B measures 4.95m, the rectangle is racked 50mm out of square — push the longer-diagonal corner toward the centre until both diagonals are 5.00m and brace it before nailing off.
The Chippy Tools check square calculator returns the calculated diagonal once. You measure the two actual diagonals and compare both to the calculated value (and to each other). All three should match within your tolerance.
How to calculate the diagonal of a rectangle
The diagonal of a rectangle is the hypotenuse of the right triangle formed by its length and width:
diagonal = √(length² + width²)
For a worked example, a 4m × 3m foundation has a diagonal of √(16 + 9) = √25 = exactly 5m. A 6.0m × 4.5m deck has a diagonal of √(36 + 20.25) = √56.25 = 7.5m. The Chippy Tools calculator handles the square-root arithmetic — enter the two measurements and read the diagonal directly.
For an imperial example, a 16-foot × 12-foot deck has a diagonal of √(256 + 144) = √400 = exactly 20 feet. The 3-4-5 Pythagorean triple multiplied by 4 gives the 12-16-20 ratio, which is another perfect on-site shortcut: a 16-foot length, 12-foot width, and 20-foot diagonal mean the rectangle is exactly square.
How to calculate the diagonal of a square
A square is a rectangle where length = width, so the diagonal simplifies:
diagonal = side × √2 ≈ 1.414 × side
For a 4m × 4m square, the diagonal is 4 × 1.414 ≈ 5.66m. For a 12-foot × 12-foot square, the diagonal is 12 × 1.414 ≈ 16.97 feet (16 feet 11¾ inches). The Chippy Tools calculator handles both the general rectangle and the square-special-case — just enter the same value for both length and width.
For checking a building or foundation that’s meant to be a true square, always measure both length and width independently rather than assuming they’re equal — a “square” foundation that’s actually 4.00m × 3.97m is more common than not, and assuming the side length means you’ll square it to the wrong target diagonal.
The 3-4-5 method (and its multiples)
The 3-4-5 method is the oldest squaring trick on a job site. It uses the smallest Pythagorean triple — three integer side lengths that satisfy a² + b² = c². The 3-4-5 says: mark 3 units from a corner along one side, mark 4 units from the same corner along the other side, and the diagonal between those marks should measure exactly 5 units when the corner is square.
Common multiples of 3-4-5 cover most domestic spans:
| 3-unit side | 4-unit side | 5-unit diagonal |
|---|---|---|
| 3 ft | 4 ft | 5 ft |
| 6 ft | 8 ft | 10 ft |
| 9 ft | 12 ft | 15 ft |
| 12 ft | 16 ft | 20 ft |
| 3 m | 4 m | 5 m |
| 6 m | 8 m | 10 m |
| 0.9 m | 1.2 m | 1.5 m |
The 6-8-10 is the version most carpenters use because the numbers are big enough to expose small framing errors but small enough to mark out with a single tape pull. The Chippy Tools check square calculator handles any rectangle, not just Pythagorean triples — useful when the foundation, room, or deck isn’t a clean ratio.
Square Up On the Job Site
Download Chippy Tools and use the check square calculator on iOS or Android. Pythagoras at the tap of a button — works offline, in metric or imperial.
Common applications
The check square calculator covers any “is this rectangle square?” workflow:
- Foundations and slabs — before pouring concrete, confirm the formwork is square. A 50mm error at slab stage is a 50mm error in every wall above.
- Decks and verandahs — square the bearer-and-joist frame before nailing off the deck boards. Once the boards are on, you can no longer adjust.
- Rooms and walls — confirm a stud-wall frame is square before plasterboard. A racked wall hides under plaster but shows up the day a door is hung.
- Tile and paver layouts — square the layout before setting the first tile or paver. Out-of-square layouts mean tapered cuts at every wall.
- Sheds, garages, and outbuildings — square the slab and the framing independently. Sheds in particular are easy to get out of square because they don’t tie into the house’s geometry.
- Fences — square fence corners using the 3-4-5 method before setting any posts.
For each of these, the workflow is the same: measure length and width, calculate the expected diagonal, then measure the actual diagonal and compare.
Tolerances and what’s “square enough”
How accurate does the measured diagonal need to be? It depends on what’s coming next:
- Joinery and cabinet work — within 2mm (1/16") over a typical opening. Tight tolerance because cabinet doors and drawer fronts show every error.
- Domestic framing — within 5mm (1/4") over a typical room. Doors and architraves can hide small errors but more than 5mm and the door reveal opens up unevenly.
- Concrete formwork — within 10mm over a typical slab. Once concrete is poured, the error is permanent — better to be 10mm tight before pouring than 10mm loose after.
- Rough framing and decking — within 10–15mm. Decking boards forgive small errors at the edge cuts; rough framing is hidden behind cladding.
- Site setting-out — within 25mm over a building footprint. Larger tolerances at the property-pegs stage; tighter as the work climbs the structural hierarchy.
Don’t aim for zero — that’s impossible with a tape measure on a windy site. Aim for “consistent and within tolerance” — the same diagonal measured three times by three people should give the same answer to within tolerance.
Imperial and metric units
Chippy Tools accepts millimetres, centimetres, metres, feet, inches, or feet and inches in any combination. The calculator handles unit conversion internally so you can mix metric inputs (in mm) with imperial outputs (in feet and inches) — useful when the architectural drawings are in mm but the timber is sold in linear feet.
The diagonal calculation is unitless — Pythagoras holds for any consistent units. The calculator just makes sure your inputs are converted to a single unit before computing the diagonal and then displays the result in whatever unit you picked.
Why use a check square calculator on your phone
The Chippy Tools app is built for tradespeople who need calculations on-site without internet. The check square calculator pairs in the same app with the Stair Calculator, Decking Calculator, Roof Pitch Calculator, and Triangle Calculator — square the foundation, calculate the deck, the stair stringer, and the roof pitch in one workflow without re-entering measurements.
Web check-square calculators break in the back corner of a job site with no signal. The Chippy Tools calculator runs locally — the answer is on screen in under a second. Pull up the calculator straight from your home screen widget when a customer asks “is this room square?” during the walk-through.
Related calculators
- Triangle Calculator — the underlying right-triangle math, useful for any non-rectangular geometry
- Decking Calculator — once the deck frame is square, calculate the deck boards and joists
- Stair Calculator — same Pythagoras applied to stair stringer length
- Roof Pitch Calculator — same rise-over-run math applied to rafters and pitch