3-4-5 rule calculator computes the exact diagonal hypotenuse required for a perfect 90-degree corner. Input your Wall A and Wall B lengths to generate the target diagonal and tape pull variance.
Squaring a Corner Without Trusting Your Eye
A wall that “looks square” can be off by an inch over six feet — and that inch turns into a visible gap once flooring, cabinets, or fence panels go in. The 3-4-5 rule sidesteps that problem entirely: if two sides of a right angle measure 3 and 4 units, the diagonal connecting them must measure exactly 5 units, or the corner isn’t 90 degrees. This calculator scales that ratio to whatever measurement you’ve actually got — a 9 ft leg, a 4.5 m leg, whatever — and tells you the other two numbers you need to check against.
It also reports the area, perimeter, and angle breakdown of the resulting triangle, plus a side-by-side conversion into the opposite measurement system, which is handy if your tape measure and your plans don’t agree on units.
How the Scaling Works
Everything here is built on one ratio: 3:4:5. Whatever single measurement you enter — and whichever of the three sides it represents — the tool divides that number by its position in the ratio (3, 4, or 5) to find a scale multiplier, then multiplies that factor across all three sides.
Picking Which Side You Know
The “Target Known Dimension” dropdown matters more than it looks. If you measured the shorter wall and entered 9 ft as the “Short Leg,” the tool divides 9 by 3 to get a multiplier of 3, then reports the long leg as 12 ft and the diagonal as 15 ft. Enter that same 9 ft as the “Long Leg” instead, and the multiplier becomes 2.25 — giving a short leg of 6.75 ft and a diagonal of 11.25 ft. Same number, completely different triangle, depending on what you tell it that number represents.
Angles, Area, and the Opposite-Unit Conversion
The corner angles (90°, 53.13°, 36.87°) never change — that’s baked into the geometry of any 3-4-5 triangle regardless of scale, so this card is essentially a constant reference rather than a calculation. The area and perimeter cards do scale with your input, and the fourth card mirrors the long leg, short leg, and diagonal into a second unit system (feet convert to meters, inches to centimeters, and so on) so you can cross-check against a tape measure marked in the other system.
The Part People Get Wrong: Switching Units Mid-Job
Change the “Measurement Unit” dropdown from feet to centimeters, and the length field doesn’t just relabel — it resets to a fresh default (3 becomes 30, since the tool assumes you’re now thinking in centimeters rather than converting your old feet value). Same thing happens switching the “Target Known Dimension” dropdown: the input snaps back to 3, 4, or 5 depending on which leg you selected. If you’re mid-calculation and just want to see the result in a different unit, don’t touch these dropdowns — re-enter your actual measured value after switching, or you’ll end up squaring a corner based on the tool’s placeholder number instead of your tape measure reading.
Worked Example: Squaring a 9×12 Deck Frame
Laying out a deck frame, one side measures 9 ft and the adjacent side measures 12 ft — a perfect 3-4-5 ratio at a multiplier of 3. Set the unit to feet, the known dimension to “Short Leg,” and enter 9.
- Multiplier: 9 ÷ 3 = 3.000x
- Long leg: 4 × 3 = 12.000 ft (matches the deck’s other side)
- Required diagonal: 5 × 3 = 15.000 ft
- Leg difference: 12 − 9 = 3.000 ft
- Area: (9 × 12) ÷ 2 = 54.00 sq ft
On site, that means: measure 9 ft along one joist, 12 ft along the perpendicular joist, mark both points, then stretch a tape between them. If it reads anything other than exactly 15 ft, the frame is racked — pull it square until that diagonal hits 15 ft, then brace it before it shifts back.
3-4-5 Rule Calculator FAQs
What happens if I enter 0 or a negative number for the length?
The calculator clears all results and shows a “Data Required” warning instead of returning zero or negative dimensions — the math requires a positive multiplier, so anything at or below zero is rejected outright.
Why did my input value change by itself when I switched the unit?
Each unit has its own default starting value tied to the 3-4-5 ratio — feet and meters default to 3/4/5, centimeters to 30/40/50, millimeters to 300/400/500. Switching units resets the field to that unit’s default rather than converting your previous number, so you’ll need to re-enter your actual measurement after changing units.
If I know the diagonal instead of a leg, does the calculator still work?
Yes — selecting “Diagonal” as the known dimension flips the hero result to show the required short leg instead of the diagonal, since the diagonal is now your input rather than your target. The long leg and all other figures scale from the same multiplier either way.
Do the corner angles ever change based on my measurements?
No. A 3-4-5 triangle is always similar to every other 3-4-5 triangle — scaling the sides up or down never changes the angles (90°, 53.13°, 36.87°). That card exists for reference, not as a calculated output specific to your numbers.
What’s the “Converted” card actually showing me?
It takes your three calculated side lengths and re-expresses them in a paired unit system — feet pair with meters, inches pair with centimeters, centimeters pair with inches, and millimeters pair with inches. It’s there so you can check your result against a metric tape even if you measured in imperial, or vice versa.