Calculate area and perimeter for rectangles, circles, triangles, and other shapes with visual diagrams.
This calculator finds area and perimeter for rectangles, circles, and triangles from the dimensions you type in, then draws a simple diagram scaled to those numbers so the shape looks the way it should before you trust the result. Pick a shape, enter its required measurements - length and width for a rectangle, radius for a circle, and for a triangle either base and height (area only) or three side lengths (for perimeter) - and it runs the matching formula automatically. The most common mix-up is with triangles: perimeter needs all three side lengths, but area only needs base and height, so entering just the two 'area' fields and expecting a correct perimeter back will not work.
A concrete input and expected output from the current implementation.
Input
Rectangle, length = 12, width = 5
Expected output
Area = 60 square units, Perimeter = 34 units
Area is length times width (12 x 5 = 60); perimeter is twice the sum of the two sides (2 x (12 + 5) = 34).
Each shape uses its own closed-form formula rather than a generic estimate: rectangles use A = l*w and P = 2(l+w); circles use A = pi*r^2 and circumference = 2*pi*r; triangles use A = 0.5*base*height for area, but perimeter requires all three side lengths rather than base and height alone.
After computing the numbers, the tool draws an outline of the shape proportioned to the values entered, with dimensions labeled on the sides. A rectangle that is much longer than it is wide will actually render elongated, which makes it easier to spot a transposed length and width or a diameter value typed into the radius field.
Yes. The area and circumference formulas both expect radius (A = pi*r^2, C = 2*pi*r). If the field is labeled radius and you type a diameter instead, the area comes out about four times too large and the circumference about twice too large, so check the field label before entering the number.
No. The tool only goes forward from a shape's dimensions to area and perimeter; it does not invert the formulas. If you already have an area figure and need the perimeter, you need the shape's actual side lengths or radius, not just the area number.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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