How to Solve Any Triangle Online Step by Step

To solve a triangle online, choose the case that matches what you already know — SSS, SAS, ASA/AAS or SSA — enter your three values, and the solver applies the law of sines or cosines to return every remaining side and angle plus the area and perimeter. You need exactly three pieces of information, at least one of which is a side. This guide shows how to pick the right case and read the results, all running privately in your browser.

Step 1: identify what you know

Every triangle is fully determined by three parts as long as one is a side. Match your known values to a case:

Step 2: enter your three values

Select your case, then type the values into the labelled fields — sides in your chosen unit and angles in degrees. The solver picks the correct law automatically, so you never have to remember which formula applies to which case.

Step 3: read all the results

The output gives all three sides, all three angles in degrees, and below them the area and perimeter. A quick sanity check: the three angles should sum to 180 degrees, and the longest side should sit opposite the largest angle. If either looks off, re-check your inputs.

Step 4: watch for special cases

Two situations get flagged for you. In the ambiguous SSA case, two, one, or zero valid triangles can exist for the same inputs; when a second triangle is possible the solver adds a note showing the alternative angle. And if your values break the triangle inequality — one side longer than the sum of the other two — or force the angles past 180 degrees, it explains that the triangle is impossible instead of returning a misleading number.

Private, browser-based trigonometry

Every calculation runs locally in JavaScript in your browser. Nothing you type is uploaded or stored, so you can work through homework, a surveying figure or an engineering check quickly and privately — and it keeps working offline once the page has loaded.

Try the Triangle Solver — free and 100% in your browser.

FAQ

How many values do I need to solve a triangle?

Exactly three, and at least one must be a side. Three angles alone (AAA) fix the shape but not the size, so they are not enough to find actual side lengths.

Which case should I pick if I have a right triangle?

Use whichever case matches your known parts. If you know two sides, SSS or SAS applies; if you know a side and an angle beyond the right angle, ASA/AAS or SSA fits. The right angle is just one of your known angles.

What do I do when the solver flags the SSA case?

Read both solutions it offers. Depending on the real-world situation you may need the acute or the obtuse triangle; the note shows the alternative angle so you can choose the one that fits your problem.

Why would a triangle be impossible?

Because the numbers cannot form a closed shape — typically one side exceeds the sum of the other two, or the given angles already add to 180 degrees or more. The solver says so plainly rather than inventing a result.

Related free tools

• Pythagorean Theorem Calculator — solve right-triangle sides fast.
• Area Calculator — find areas of many shapes.
• Slope Calculator — compute slope and line values.
• Scientific Calculator — trig functions and more in the browser.

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