Right Triangle Calculator
Right triangle sides and angles by Pythagoras.
Geometry Calculators
Use tool
Triangle Calculator
Solve a triangle from whatever you know about it.
Triangle Calculator tool
The angle between the two sides, or between the two angles.
For two angles and a side.
—
Area
—
Perimeter
—
Angles
—
Type
—
Height on side a
| Measure | Value |
|---|
About the Triangle Calculator
Triangle Calculator solves a triangle from whatever you know: three sides, two sides and the angle between them, two angles and the side between them, or just a base and a height for the area alone. Each route uses the appropriate rule -- the cosine rule, the sine rule or Heron's formula -- and the working shows which one it applied.
Impossible triangles are refused rather than producing a number. Three lengths only form a triangle if any two of them add up to more than the third, so 1, 2 and 10 is rejected with that explanation. Two angles summing to 180 or more is refused for the same reason.
Base and height give the area and nothing else, and the page says so. Infinitely many triangles share a base and a height, with completely different side lengths and angles, so reporting them would be inventing information you did not supply.
The page shows its working, not just its answer. Where there is a formula it is printed with your own numbers substituted in, so you can check the result rather than take it on trust -- which is the difference between a calculator you can use for something that matters and one you cannot.
Everything runs inside your browser. What you type is never uploaded, never logged and never stored, and the calculator keeps working if your connection drops after the page has loaded.
Looking for something this does not cover? FreeToolsHub.org is another hub of free online tools that need no account, and it is worth a look if triangle calculator is only part of what you are working on.
How to use the Triangle Calculator
- Choose what do you know and measurements inThe fields below change to match what you pick, so only the figures that matter to your case are asked for.
- Enter side a, side b and side cDecimals are fine, and thousands separators are accepted and ignored.
- Read the area, perimeter and anglesThe figures update as you type. The Calculate button is there if you would rather press something, but nothing waits for it.
- Copy or check the resultCopy the summary with the copy button, or open the table to check it row by row and read the formula with your own numbers in it.
Features
- Shows area, perimeter, angles, type and height on side a together, so you are not running it twice
- A results table you can read across
- Prints the formula with your own figures substituted in
- Updates as you type, with no button to press and no page reload
- Copy the result in one click
- Free, no sign-up, and it runs entirely in your browser
Who uses this tool
The triangle calculator is used by writers, developers, students, marketers and anyone else who needs the job done once without installing software. Common cases include:
- Finding the area from three measured sides
- Solving for a missing side or angle
- Checking whether three lengths can form a triangle
- Classifying a triangle as acute, right or obtuse
Frequently asked questions
What is Heron's formula?
A way to get the area from the three sides alone, without needing a height. It uses the semi-perimeter, and the working shows it applied to your figures.
Why were my three lengths rejected?
Because the two shorter ones do not add up to more than the longest. That is the triangle inequality, and lengths that break it cannot close into a triangle.
Can I use mixed units?
No. Put every measurement in the same unit and choose it from the list; the result is then converted for you.
Related tools
If the triangle calculator is not quite what you need, these other free tools solve closely related problems, and there are 16 more in geometry calculators.
Area Calculator
Area and perimeter for ten shapes, with the formula and the working.
Geometry Calculators
Use tool
Perimeter Calculator
Perimeter of rectangles, circles, triangles and polygons.
Geometry Calculators
Use tool
Trigonometry Calculator
Sin, cos, tan and the inverse functions.
Geometry Calculators
Use tool
Regular Polygon Calculator
Regular polygon area, angles, apothem and diagonals.
Geometry Calculators
Use tool
Checked and rebuilt on 2026-10-14. The method this calculator uses is described in the section above, and corrections go through the contact page.