Area Calculator
Calculate the area and perimeter of squares, rectangles, circles, triangles, and more.
How to Use
- Choose a shape from the tabs at the top.
- Pick your unit of measurement (applies to every shape).
- Enter the required dimensions for that shape.
- Area (and perimeter, where available) update instantly as you type.
When to Use This
- Estimating paint or flooring quantity for a room, where the rectangle and trapezoid tabs between them cover most ordinary floor plans.
- Checking a garden bed or lawn before ordering turf, using the ellipse or polygon tab when the outline is not rectangular.
- Solving a geometry exercise from three triangle side lengths, where the triangle-inequality check catches a combination that cannot exist.
- Working out the circumference of a circular table or pipe straight from its diameter, without halving it to a radius first.
Frequently Asked Questions
About This Calculator
Each shape has its own area formula and some of them hide a subtlety. A triangle given three sides needs Heron's formula, plus a check that the three lengths can close at all. A trapezoid averages its parallel sides before multiplying by the height. An ellipse has a clean area, pi times a times b, but no exact perimeter in elementary functions, which is why approximations exist for it.
Eight shape tabs share one unit selector, which labels the output and applies no conversion, so your measurements must already share a unit. Circles accept a radius or a diameter; triangles accept base and height or three sides; trapezoids and parallelograms add a perimeter only once you fill the optional side fields. The ellipse perimeter uses Ramanujan's second approximation. Results carry zero, two or four decimal places depending on their magnitude.
Notes & Limits
- The unit selector only labels the result. It performs no conversion, so mixing a length in meters with another in feet produces a meaningless number.
- The ellipse perimeter is Ramanujan's second approximation, exact only when the two axes are equal; every other ellipse carries a small approximation error.
- Triangle in base-and-height mode reports no perimeter, because base and height do not fix the three side lengths. Switch to three sides for that.
- Only regular polygons are supported, with 3, 5, 6, 7, 8, 9, 10 or 12 sides. Irregular outlines and three-dimensional solids are not covered.