What is the 3-4-5 method?

A way to lay out a right angle with a triangle whose sides are in the ratio 3:4:5. Measure 3 along one line and 4 along the other from the corner. When the distance between those two points is exactly 5, the corner is 90°, because 3² + 4² = 5².

Does the 3-4-5 rule work in metric?

Yes. Any unit works as long as the ratio holds: 0.6-0.8-1.0 m, 1.2-1.6-2.0 m, or 3-4-5 m. Use the biggest one that fits the layout.

What are batter boards?

Two stakes with a level crosspiece, set a few feet outside each corner of a layout. Strings tie to nails on the crosspieces, so the lines cross over the real corners and stay put while you dig or build forms underneath.

How close should the diagonals be?

On a shed or small building, within about 1/8 in (3 mm) is a good target. If the plans for a larger building or slab give a tolerance, follow that instead.

Why use 6-8-10 or 12-16-20 instead of 3-4-5?

A bigger triangle shrinks the angle error from a small measuring miss. A 1/8 in miss on a 3-4-5 ft triangle leaves a 20 ft wall about 1 in out. The same miss on a 12-16-20 ft triangle leaves it about 1/4 in out.

MeasuringLayout & Site WorkSquare a layout with 3-4-5

How do you square a building layout with string and batter boards?

The stakes are in, the strings are up, and the corner looks square. It always looks square. Then the slab goes in an inch out and every sheet of sheathing after it gets a tapered rip. BAS Renovations walks through the fix on a whiteboard model: the 3-4-5 method, run on string lines tied to batter boards, then a corner-to-corner check before anybody picks up a shovel.

⚡ Rule of thumb

Run string lines between batter boards, mark 3 ft along one line and 4 ft along the other from where they cross, and slide a string until the distance between the marks is exactly 5 ft. Use the biggest multiple that fits (6-8-10, 12-16-20), then confirm the two corner-to-corner diagonals are equal.

Starting point

3-4-5, 6-8-10 or 12-16-20 ft (0.6-0.8-1.0, 1.2-1.6-2.0 or 3-4-5 m) — the biggest that fits

Measured

3 and 4 along the strings from their crossing point · 5 straight across between the two marks

Confidence

Widely accepted

▶ Source video — Squaring and layout explained — BAS Renovationswatch on Facebook ↗

Applies to

Process: Site layout — sheds, decks, footings, slabs, paver pads, small buildings
Materials: Batter boards (two stakes + a crosspiece), mason's string, nails
Setup: Tape measure, hammer, string line, tape flags or a marker
Orientation: Batter boards a few feet outside each corner, so the strings cross over the real corner
Best for: Corners too big for any carpenter's square

Why it matters

A triangle with sides 3, 4 and 5 is always a right triangle, because 3² + 4² = 5² (9 + 16 = 25). That's the Pythagorean theorem run backwards, and it squares a corner far bigger than any square you own can reach.

Batter boards are the other half. They hold the strings a few feet outside the real corners, so you can dig, form and pour underneath without knocking the layout. And when the 5 reads long, you nudge the string along the crosspiece. You don't pull a stake.

Size matters more than people think.

Miss the 5 ft diagonal of a 3-4-5 by 1/8 in (3 mm) and the corner is off about a quarter of a degree. Carry that down a 20 ft (6.1 m) wall and the far end is out roughly 1 in (26 mm). The same 1/8 in miss on a 12-16-20 leaves that wall out about 1/4 in (7 mm).

What good looks like

The 3-4-5 (or the multiple) reads exactly, measured from the string crossing
Opposite sides match before you bother with the diagonals
Both corner-to-corner diagonals of the rectangle match
Strings are tight, at the same height, and tied to marked nails on the crosspieces

How to apply it

Set batter boards (two stakes and a level crosspiece) a few feet outside each corner, so the strings will cross over the real corner with room to work underneath.
Stretch string between opposite batter boards for the two sides that meet at your first corner. Tie off to nails on the crosspieces.
From the point where the strings cross, measure 3 units along one line and 4 along the other. Flag both with tape.
Measure straight across between the two flags. Slide one string along its crosspiece until that reads exactly 5.
Mark the crosspiece where the string landed, so you can re-string it to the same spot after you dig.
Set the other two sides parallel at your planned dimensions, and check that opposite sides are equal.
Double-check: measure both corner-to-corner diagonals. Equal diagonals on a layout with equal opposite sides means the whole rectangle is square.

Test it on your setup

Diagonals: on a shed-size layout, aim for the two diagonals within about 1/8 in (3 mm) of each other. Tighten that if the plans give you a number.
Swap corners: run 3-4-5 again from the opposite corner. It should read true there too.
Plumb down: drop a plumb bob from each string crossing and check the stakes or forms sit right under it.

Exceptions & tradeoffs

Small triangles on big layouts amplify error. Go as big as the site lets you: 12-16-20 on a garage, not 3-4-5.

Equal diagonals alone don't prove a rectangle. A lopsided four-sider (an isosceles trapezoid) has equal diagonals too. Get opposite sides equal first, then the diagonals tell the truth.

Mind you, a long tape sags. Pull it tight and keep it level between the marks, or the 5 reads long when the corner is fine.

The strings set the lines, not the grade. Squaring and leveling are two different jobs.

Common mistakes

Measuring 3 and 4 from the edge of a stake instead of from the string crossingMeasure from where the strings actually intersect; the stakes just hold them
Squaring one corner, getting excited, and skipping the diagonal checkAlways finish with both corner-to-corner diagonals
Strings sagging or tied at different heightsPull them tight and set the crosspieces level with each other
Using 3-4-5 ft on a 24 ft garage because those are the numbers everybody remembersScale it up: 12-16-20 ft or bigger

Frequently asked questions

What is the 3-4-5 method?A way to lay out a right angle with a triangle whose sides are in the ratio 3:4:5. Measure 3 along one line and 4 along the other from the corner. When the distance between those two points is exactly 5, the corner is 90°, because 3² + 4² = 5².
Does the 3-4-5 rule work in metric?Yes. Any unit works as long as the ratio holds: 0.6-0.8-1.0 m, 1.2-1.6-2.0 m, or 3-4-5 m. Use the biggest one that fits the layout.
What are batter boards?Two stakes with a level crosspiece, set a few feet outside each corner of a layout. Strings tie to nails on the crosspieces, so the lines cross over the real corners and stay put while you dig or build forms underneath.
How close should the diagonals be?On a shed or small building, within about 1/8 in (3 mm) is a good target. If the plans for a larger building or slab give a tolerance, follow that instead.
Why use 6-8-10 or 12-16-20 instead of 3-4-5?A bigger triangle shrinks the angle error from a small measuring miss. A 1/8 in miss on a 3-4-5 ft triangle leaves a 20 ft wall about 1 in out. The same miss on a 12-16-20 ft triangle leaves it about 1/4 in out.

BAS RenovationsSource & visual reference

Squaring a string layout on batter boards with 3-4-5 and its 6-8-10 / 12-16-20 multiples, then double-checking corner to corner — Squaring and layout explained ↗ at 0:08–1:28.

Summarized and independently edited by Start Here from BAS Renovations' Facebook clip, read off the frames and caption with no audio. The 3-4-5 multiples, the batter boards and the corner-to-corner double check are written right on his model. The metric triangles, the 1/8 in tolerance, the error math and the plumb-bob check are mine. The frames don't show exactly how he slides and re-ties the string on the crosspiece, so that step is the standard way to do it, not necessarily his.