Triangle and Geometry Calculator
Choose a shape and calculation
This free Triangle and Geometry Calculator covers the shape calculations that come up again and again in homework, DIY and everyday measuring. Triangles can be worked out two ways: from a base and height when you want a quick area, or from all three sides, which gives the perimeter, the area using Heron's formula, and the triangle's type — equilateral, isosceles or scalene — plus a note if it happens to be right-angled. Circle, rectangle and square modes round out the set with areas, perimeters, circumferences and diagonals.
The three-sides mode also protects you from an impossible triangle. If the two shorter sides together are not longer than the longest side, no triangle can exist with those measurements, and the calculator says so plainly instead of printing a nonsense area from a formula pushed beyond its limits. Units are whatever you choose to work in — centimetres, metres, inches or feet — as long as you use the same unit for every measurement in one calculation; areas then come out in the matching square units. Everything runs locally in your browser, so your numbers are never uploaded or stored.
How to use
- Choose the shape and method from the drop-down list.
- Enter the measurements that appear for that method.
- Use the same unit, such as centimetres, for every value.
- Click Calculate to see area, perimeter and other results.
- Switch methods any time; only the fields you need are shown.
Frequently asked questions
What is Heron's formula?
It finds a triangle's area from its three side lengths alone, without needing the height. You halve the perimeter to get s, then take the square root of s times (s minus each side). It is ideal for triangular plots of land measured side by side.
How can I tell if a triangle is right-angled?
Square the two shorter sides and add them. If the total equals the square of the longest side, the triangle is right-angled — that is Pythagoras' theorem in reverse. The three-sides mode checks this for you automatically.
Why did my three sides get rejected?
A real triangle must satisfy the triangle inequality: any two sides added together must beat the third side. A 2, 3 and 6 set fails because 2 plus 3 is less than 6, so the sides could never meet to close the shape.
Does the unit of measurement matter?
Only for consistency. The maths is unit-free, so metres give square metres and inches give square inches. Mixing units inside one calculation, such as a base in centimetres and a height in inches, would give a wrong answer.