Area Calculator
This free area calculator finds the area, and for most shapes the perimeter, of a circle, square, rectangle, triangle, or trapezoid. Nothing you enter is sent anywhere or stored.
How to Use It
- Choose a shape: Circle, Square, Rectangle, Triangle, or Trapezoid.
- Enter its dimensions.
- Area and perimeter update instantly, no submit button.
How It Works
Each shape has its own standard formula for area, built from whichever dimensions define that shape. A circle only needs a radius, a square only needs a side length, while a rectangle needs both a length and a width, and a trapezoid needs both of its parallel bases plus a height. Perimeter, where shown, simply adds up the distance around the shape's edge.
Formulas
circle area = π × r², circumference = 2 × π × r
square area = s², perimeter = 4 × s
rectangle area = l × w, perimeter = 2 × (l + w)
triangle area = 0.5 × base × height
trapezoid area = 0.5 × (base1 + base2) × height
A worked example: a circle with a radius of 5 has an area of about 78.54 square units (π times 5 squared) and a circumference of about 31.42 units (2 times π times 5). A rectangle 6 units long and 3 units wide has an area of 18 square units and a perimeter of 18 units, a coincidence of these particular numbers, not a general rule.
Everything runs client-side in your browser. Nothing you enter is sent anywhere or stored.
Frequently Asked Questions
What is the difference between area and perimeter?
Area measures the amount of 2D space a shape covers, like how much flooring or paint a surface needs. Perimeter (or circumference for a circle) measures the distance around a shape's edge, like how much fencing or trim a border needs. The two answer different questions and are not interchangeable, even though it is common to mix them up.
Why do triangle and trapezoid not show a perimeter here?
Perimeter needs every side length added together. This calculator collects base and height for a triangle, and both bases plus height for a trapezoid, which is enough to find area but not enough to know every side length, so no perimeter figure is shown for those two shapes.
How do I find the area of a shape that is not listed?
Most irregular shapes can be broken down into rectangles and triangles, whose areas can each be calculated separately and then added together. This is the standard approach for L shaped rooms, oddly shaped plots of land, and similar real world layouts.
Can you give a real-world example?
Buying flooring, tile, or paint are common ones. A rectangular room 6 meters by 3 meters needs 18 square meters of flooring, its area, while the trim or baseboard running around its edge needs 18 meters, its perimeter, the same two numbers this calculator produces for a rectangle.