Area of trapezoids, triangles, rectangles, circles, ellipses and more — with diagrams, unit conversion and formula breakdowns.
All dimensions must be greater than 0. For 3-side triangles, the three sides must satisfy the triangle inequality or no triangle exists. Results update live; switching the unit converts your numbers.
How to use this tool
Choose a trapezoid, triangle, rectangle, square, circle, ellipse or parallelogram. A small outline diagram of the shape appears beside the results.
Type each measurement as a number greater than 0 and pick your length unit — millimetres, centimetres, metres, inches or feet. Changing the unit converts the numbers you already entered.
Use base × height when you know the base and the perpendicular height, or switch to 3 sides (Heron) when you only know the three side lengths. Heron mode checks that the sides can actually form a triangle.
The area updates as you type, shown in square centimetres, square metres, square feet and acres, with the substituted formula underneath so you can check the working.
Formula & worked example
Trapezoid with parallel sides 10 and 6 and height 4: A = ((10 + 6) / 2) × 4 = 32. A circle of radius 7 gives A = π × 7² = 153.94. A triangle with base 8 and height 5 gives A = (1/2) × 8 × 5 = 20. A 3-4-5 triangle by Heron's formula gives s = 6 and A = √(6 × 3 × 2 × 1) = 6.
Limitations — read this before you rely on it
This calculator assumes flat, perfect shapes measured in a single plane. It does not handle irregular plots, curved boundaries that are not circles or ellipses, or land measured over slopes — for an irregular plot, split it into rectangles and triangles and add the areas. Results are only as accurate as the measurements entered.
Frequently asked questions
Choose a shape, enter its dimensions and pick a unit. The calculator applies the shape's area formula instantly, converts the answer into square centimetres, square metres, square feet and acres, and shows the substituted formula so you can verify it.
The rectangle area formula is length × width, with both measurements in the same unit. A rectangle 5 cm long and 4 cm wide has area 20 cm². In the calculator, choose Rectangle and enter the two sides.
The most common triangle area formula is (1/2) × base × height, where the height is measured perpendicular to the base. That is the formula for the area of a triangle with base 8 and height 5: (1/2) × 8 × 5 = 20. If you only know the three sides, use Heron's formula instead.
Use Heron's formula: first find s = (a + b + c) / 2, half the perimeter, then area = √(s(s − a)(s − b)(s − c)). For sides 3, 4 and 5, s = 6 and the area is 6. The calculator's 3 sides (Heron) mode does this and warns you if the sides cannot form a triangle.
For an equilateral triangle — all three sides equal — the area is (√3/4) × side². A side of 6 gives (√3/4) × 36 = 15.59. You can also use Heron's formula with all three sides set to the same value; the calculator flags equilateral triangles in its breakdown.