Do I have to use a 3:4:5 ratio?

No. Any two values that form a right triangle will work — 3:4:5 is just the most well-known example, and this calculator works for any ratio.

What happens if I enter a new value when all three are already filled?

Type into any of the three fields and the field you filled in longest ago gets recalculated and updated automatically.

What's the tolerance for the on-site verification check?

A difference within about 0.5% of the ideal hypotenuse length is treated as "very close to a true right angle" — a margin that accounts for typical tape-measure accuracy on a job site.

Why specifically 3:4:5? Would other ratios work?

3:4:5 is simply the smallest Pythagorean triple made of whole numbers, which makes it easy to calculate and measure — that's why it's so widely used. There are infinitely many combinations that form a right triangle, including other whole-number triples like 5:12:13 or 8:15:17.

My real-world measurement keeps coming up off. What's causing that?

The most common cause is a tape that wasn't pulled taut, or a starting point that isn't precisely at the corner where the two sides meet. Try re-aligning both sides to the exact same reference point and measuring again.

Is this method actually more accurate than using a protractor?

It can be. A small protractor's markings tend to introduce larger relative error, whereas scaling up to longer sides (like 3m, 4m, 5m) for this method shrinks the error's relative impact.

Right Triangle Side Calculator - Solve Any Side or Hypotenuse

What is this right triangle calculator?

This calculator uses the Pythagorean theorem (sideA² + sideB² = hypotenuse²) to instantly solve for the missing side of a right triangle from any two known values. Enter Side A and Side B and it solves for the hypotenuse (diagonal); enter the hypotenuse and one side and it solves for the other side. The classic "3-4-5 rule" used in carpentry, framing, landscaping, and deck building works perfectly here too, but you're not limited to it — plug in whatever real measurements you have. Change any value and the diagram below redraws to match your actual ratio.

How to use it

Enter any two of Side A, Side B, and the Hypotenuse — the remaining field fills in automatically. If all three are already filled and you enter a new value somewhere, the field you filled in longest ago gets recalculated and updated instead. Mark out the two sides on-site using the calculated hypotenuse, measure the actual diagonal, and enter it in the "On-site verification" field to see how far off it is from the ideal and whether the corner is square.

When it comes in handy

Quickly checking whether a wall or floor corner is square without a framing square or protractor
Squaring up baselines for deck, tile, or landscaping projects
Working out a diagonal length in advance for furniture or woodworking frames
Quickly back-calculating a missing side from two measurements you already have

Why is the 3:4:5 rule so common on job sites?

Plugging 3, 4, and 5 into the Pythagorean theorem (a²+b²=c²) gives 3²+4²=9+16=25=5² exactly — no rounding needed. It's the smallest, simplest combination made entirely of whole numbers, so it's been a longstanding practice in carpentry, landscaping, and tile work to square up a corner with nothing more than a tape measure. Without a protractor, you're relying on the fact that getting the three side lengths in exactly that ratio forces the angle between them to be exactly 90 degrees. Any whole-number multiple of the same ratio — 6:8:10, 9:12:15, and so on — squares a corner just as well.

Good to know when squaring a corner on-site

Scaling the sides up (say, 6:8:10 instead of 3:4:5) reduces the relative impact of measurement error, giving a more precise right angle
When measuring with a tape, make sure it's pulled taut with the 0 mark exactly at the starting point — a loose tape reads longer than the true distance
For larger areas (like a deck floor), also measuring both diagonals and checking they match adds an extra layer of accuracy
You're not locked into 3:4:5 — just enter any two real measured side lengths and it calculates the third, whatever the ratio

Frequently Asked Questions

Do I have to use a 3:4:5 ratio?: No. Any two values that form a right triangle will work — 3:4:5 is just the most well-known example, and this calculator works for any ratio.
What happens if I enter a new value when all three are already filled?: Type into any of the three fields and the field you filled in longest ago gets recalculated and updated automatically.
What's the tolerance for the on-site verification check?: A difference within about 0.5% of the ideal hypotenuse length is treated as "very close to a true right angle" — a margin that accounts for typical tape-measure accuracy on a job site.
Why specifically 3:4:5? Would other ratios work?: 3:4:5 is simply the smallest Pythagorean triple made of whole numbers, which makes it easy to calculate and measure — that's why it's so widely used. There are infinitely many combinations that form a right triangle, including other whole-number triples like 5:12:13 or 8:15:17.
My real-world measurement keeps coming up off. What's causing that?: The most common cause is a tape that wasn't pulled taut, or a starting point that isn't precisely at the corner where the two sides meet. Try re-aligning both sides to the exact same reference point and measuring again.
Is this method actually more accurate than using a protractor?: It can be. A small protractor's markings tend to introduce larger relative error, whereas scaling up to longer sides (like 3m, 4m, 5m) for this method shrinks the error's relative impact.

Right Triangle Calculator

On-site verification (optional)