Geometry Calculator

What this tool does

Pick a shape, type in the measurements you already have, and get the area, perimeter, volume, or surface area instantly — no formula lookup required. It covers 11 flat (2D) shapes and 7 solid (3D) shapes in one place: circle, rectangle, square, triangle (by three sides, using Heron's formula), right triangle, trapezoid, parallelogram, rhombus, regular polygon, ellipse, and circle sector on the 2D side; cube, rectangular prism, sphere, cylinder, cone, square pyramid, and triangular prism on the 3D side.

Who it's for

Students checking geometry homework, DIYers figuring out how much material a room or a build needs, teachers building worksheets, and anyone who just needs "area of a trapezoid" solved without digging through a textbook. Every result comes with the exact formula used underneath it, so you can see the math, not just the answer.

How to use it

• Choose a shape from the dropdown — it's grouped into 2D Shapes and 3D Shapes.
• Fill in the fields that appear (they change based on the shape you picked).
• Results update automatically as you type — area and perimeter for flat shapes, volume and surface area for solids.
• Pick a unit label (mm, cm, m, in, ft) so your results are labeled correctly, and use Copy Results to grab everything as plain text.

Real-world uses

Working out how much paint or turf covers a triangular yard, calculating the volume of a cylindrical tank or a cone-shaped hopper, checking a trapezoid area for a woodworking cut list, or verifying a textbook problem before a test.

Frequently asked questions

How do I find the area of a triangle with just the side lengths?

Use the Triangle (3 sides) option. It applies Heron's formula: calculate the semi-perimeter s = (a+b+c)/2, then area = √(s(s−a)(s−b)(s−c)). You don't need a separate height measurement.

What's the difference between surface area and volume?

Volume measures how much space a 3D shape occupies (in cubic units, like cm³), while surface area measures the total area of all its outer faces (in square units, like cm²). A shipping box's volume tells you how much fits inside; its surface area tells you how much cardboard was used to make it.

Can this calculate the area of an irregular polygon?

Not directly — the Regular Polygon option assumes every side and angle is equal (like a regular hexagon or octagon). For irregular shapes, break them into triangles or rectangles and calculate each piece separately, then add the areas together.

Why does the ellipse perimeter say "approximate"?

Unlike a circle, there's no exact elementary formula for an ellipse's perimeter. This tool uses Ramanujan's second approximation, which is accurate to within a tiny fraction of a percent for almost any ellipse shape.

Does it matter which unit I pick?

The unit selector only labels your results (cm, m, in, etc.) — it doesn't convert between units. Just make sure all your input measurements use the same unit before calculating.