Area and Perimeter Calculator
This area and perimeter calculator brings the four most common geometric shapes — circle, square, rectangle, and triangle — into a single tool. Pick a shape, enter its measurements, and both the area and the perimeter are computed instantly, updating live as you change any value. It is handy for homework and geometry classes, but also for everyday tasks such as measuring a room, estimating fencing, working out how much paint or flooring you need, or planning a garden bed. Every result appears as a large figure so you can read it at a glance.
Area
Perimeter
Area is expressed in square units (units²) and perimeter in linear units, based on whatever unit you use for the measurements.
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How it works
Each shape has its own formulas. For a circle, the area is π × r² and the perimeter (circumference) is 2 × π × r, where r is the radius. For a square, the area is side² and the perimeter is 4 × side. For a rectangle, the area is width × height and the perimeter is 2 × (width + height). For a triangle, the area is base × height ÷ 2, where the height is measured perpendicular to the base, and the perimeter is simply the sum of the three sides a + b + c. Because a triangle can take many forms, the area is based on its base and height while the perimeter uses the three side lengths. All results share the same unit you enter: the area comes out in square units and the perimeter in linear units.
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Preguntas frecuentes
- What is the difference between area and perimeter?
- Area measures the amount of surface a shape covers, expressed in square units such as square metres. Perimeter measures the distance around the outside of the shape, expressed in linear units such as metres. For a rectangle of 8 by 5, the area is 40 square units (how much space it fills) while the perimeter is 26 units (how far you would walk around its edge).
- How do I calculate the area and perimeter of a circle?
- For a circle you only need the radius, the distance from the centre to the edge. The area is π × radius², and the circumference (the circle's perimeter) is 2 × π × radius. With a radius of 5, the area is about 78.54 square units and the circumference is about 31.42 units. If you know the diameter instead, the radius is simply half of it.
- Why does the triangle use both a height and three sides?
- A triangle's area depends on its base and the perpendicular height from that base, using area = base × height ÷ 2. But its perimeter depends on the actual lengths of all three sides, which the height alone does not give you. That is why the calculator asks for the base and height for the area, and the three side lengths a, b, and c for the perimeter.
- What units should I use?
- You can use any unit you like — centimetres, metres, inches, feet — as long as you are consistent across all the measurements for a shape. The calculator does not assume a specific unit. Whatever unit you enter, the perimeter comes out in that same linear unit and the area comes out in that unit squared.
- How is the area of a square or rectangle found?
- A square's area is its side multiplied by itself (side²) and its perimeter is four times the side. A rectangle's area is width multiplied by height, and its perimeter is 2 × (width + height). A square is really just a rectangle whose width and height are equal, so the same logic applies to both.