3,4,5 rule - square up a building easily
tecmath · YouTube
000 · 345Start at zero. Square it with three, four, five.
Free layout and estimating calculators for builders and DIYers — built around the oldest rule on the job site. Get the corner right, then get the numbers right, then get it built.
18free calculators
ft + mimperial and metric
$0no sign-up, no paywall
Most-used calculators
Every tool works in feet or metres, shows its formula, and explains the result in plain language.
Layout
3-4-5 Squaring Calculator
Find the exact diagonal for a square corner and check a measured one.
ConcreteConcrete Slab Calculator
Cubic yards, cubic metres and bag counts for slabs and pads.
FramingStair Calculator
Riser count, riser height, treads, total run and stringer length.
RoofingRoof Pitch Calculator
Pitch, angle, slope % and roof area from rise and run.
Decks & fencesDeck Board Calculator
Boards, linear feet, joists and screws for a rectangular deck.
LayoutRectangle Diagonal Check Calculator
Compare two diagonals to prove a frame, slab or room is square.
Why 000345?
Every build starts from a zero point — a corner stake, a ledger end, the first tile line. From there, three units one way and four the other, and a diagonal of exactly five, gives you a right angle you can trust. That sequence, 0-0-0-3-4-5, is the whole idea of this site.
We turn that idea into tools: calculators that square corners, size slabs and footings, lay out stairs and rafters, and count boards, tiles, bricks and paint. Each one explains its method so you can check the math yourself.
Guides
- The 3-4-5 Rule: The Complete Guide to Squaring Corners
Why three, four and five make a perfect right angle, and how to use them on slabs, decks, walls and tile.
- How to Lay Out a Foundation with Batter Boards and String
Build batter boards, pull string lines, square with 3-4-5 and verify diagonals before digging.
- How to Square a Deck Frame (Ledger, Rim Joists and Diagonals)
Square the deck from the ledger out, then prove it with diagonals before decking goes on.
- How Square Is Square Enough? Layout Tolerances Explained
What tolerances pros work to, and why small errors at the corner become big ones at the far end.
Ready to build? Get up to 3 free quotes from local pros.
Tell us about the project in about a minute. We pass it to up to three vetted local contractors who cover your area.
- Free for homeowners, no obligation to hire
- Your details go only to the pros matched to your job
- Bring your calculator numbers — pros quote faster with real measurements
Start with your postal code
Watch it done
Hand-picked how-to videos from builders and educators. Videos load only when you press play.
Square Corner Challenge
Share a project you squared with 3-4-5. Monthly prizes for the best builds, chosen by a judging panel and the community.
Enter the challengeContractors: get local leads
Homeowners arrive with measurements already worked out. Pick your trades and service area, and set your own monthly cap.
Join as a proKeep the tools free
No paywalls, no accounts. Support from readers and sponsors pays for hosting, new calculators and contest prizes.
Support 000345