How do I set out a square base or a right angle on the ground?

Establishing a baseline, turning a right angle off it with a 3-4-5 triangle, proving the rectangle with its diagonals, and putting the whole setting-out onto profile boards so it survives the digging.

Difficulty: intermediate
Time: 1 hr
Read: 5 min

Short answer

Set a baseline first and peg it. From one corner, measure 3 units along the baseline and 4 units out on the line you want square, then adjust until the diagonal between those two points reads exactly 5 units. Prove the finished rectangle by checking both diagonals are equal, and transfer everything onto profile boards before you dig, because the corner pegs come out.

A shed base, a patio, a greenhouse, a fence line, a set of foundations — all of them start with a rectangle on the ground, and all of them look wrong forever if that rectangle is not square. There is no square big enough, so the right angle is made rather than measured, using the only triangle whose sides guarantee it.

What you'll need

Long tape, ideally 30m open reel
Timber pegs and a lump hammer
Builder's line
Timber for profile boards — pegs and battens
Spray marker paint (optional)
A second person

Setting out a right angle on the ground with a 3-4-5 triangle and profiles

Step by step

Establish the baseline first.One line that everything else references — usually parallel to a house wall, a boundary or a path, at a fixed distance from it. Measure that offset at both ends so the baseline is genuinely parallel rather than approximately so, and peg both ends.
Check the reference itself is straight.House walls bow and fences wander. Pull a line along the reference and look: if it is out by 30mm over its length, decide now whether to follow it or ignore it, because the decision is invisible later and obvious in the finished work.
Measure 3 units along the baseline from the corner peg.Any unit, but use metres rather than millimetres — 3, 4 and 5 metres is far more accurate than 300, 400 and 500mm, because the same few millimetres of measuring error represents a much smaller angle over the longer side. Mark the point precisely.
Swing the second line out and set 4 units.Run a line roughly at right angles and measure 4 units along it from the same corner peg. Do not fix it yet — it needs to swing.
Adjust until the diagonal is exactly 5 units.Measure from the 3 mark to the 4 mark and move the second line until that distance reads exactly 5. When it does, the second line is at 90 degrees to the baseline. This is the whole method and it is exact, not approximate.
Extend the line rather than re-measuring it.Peg the square line well beyond the 4 unit mark by sighting along it or running a taut line through both points. Extending a proved line preserves the accuracy; measuring a new angle at the far end does not.
Set the remaining corners and prove with the diagonals.Complete the rectangle by measuring the two known side lengths, then measure both diagonals. In a true rectangle they are equal. If they differ, the shape is a parallelogram — move two opposite corners along the longer diagonal by half the difference each and re-check.
Build profile boards outside the working area.A pair of stakes with a horizontal batten nailed across, set at least a metre clear of where you will dig, at each corner. Bring each string line out to the profile and cut a small saw kerf or drive a nail where it crosses.
Take the lines off, dig, and put them back.The corner pegs are inside the excavation and will be destroyed. The profiles are not, so the setting-out can be restored exactly at any point during the job — which is what they are for.
Set the levels while you are at it.Mark a datum height on each profile, all at the same level, before digging starts. Depths, falls and finished levels are then all measured down from a line you can see, rather than from ground that is about to disappear.

Tips

Scale the triangle up as far as the site allows. 6, 8 and 10 metres is better than 3, 4 and 5, and any multiple works.
Two people and a taut tape is much more accurate than one person and a sagging one. This is a job where a helper genuinely changes the result.
For a small base, an accurately cut plywood triangle or a large framing square gets you started, but always prove the finished rectangle with the diagonals rather than trusting the square.
Spray-mark the line positions on the ground as well. Lines get knocked, cut and walked through, and a paint line lets you get back to roughly the right place before restoring exactly from the profiles.

Common mistakes

Using a small 3-4-5 triangle and extending it a long way — A 5mm error over a 300mm side becomes a very large error at the far end of a 6 metre run. Set the triangle out as large as the site permits.
Trusting a house wall or a fence to be straight and square — Neither usually is. Check the reference before building everything on it, and decide deliberately whether to follow it or to be truly square.
Not checking the diagonals at the end — It is possible for each corner to look right and the whole rectangle still to be out. The diagonal check is the only proof and it takes two minutes.
Setting out with pegs only — Every corner peg is in the hole you are about to dig. Without profiles, the setting-out has to be redone from scratch after excavation, usually less accurately.

Questions people ask

Why 3, 4 and 5?

Because 3 squared plus 4 squared equals 5 squared, so a triangle with those sides must contain a right angle. Any multiple works — 6, 8, 10 or 1.5, 2, 2.5 — and larger is better because the angular effect of a measuring error shrinks as the triangle grows.

What if the site is sloping?

Measure horizontally, not along the slope, or the triangle is distorted. Hold the tape level and use a plumb bob to drop the point down to the ground, or set the whole thing out on level string lines at a datum height above the ground.

Do I need profiles for a small shed base?

For a small paved base you can often work from pegs and a couple of reference points on a wall. For anything you excavate, and for anything with foundations, profiles pay for themselves the first time a peg gets knocked out.