Area of a Circle Calculator
Solve area acircle problems step-by-step with our free calculator. See formulas, worked examples, and clear explanations.
Formula
A = pi * r^2
The area of a circle equals pi multiplied by the square of the radius. This can also be expressed as A = pi * d^2 / 4 using the diameter, or A = C^2 / (4 * pi) using the circumference.
Worked Examples
Example 1: Circle with Radius 5 cm
Problem: Find the area, circumference, and diameter of a circle with radius 5 cm.
Solution: Radius r = 5 cm\nDiameter d = 2r = 10 cm\nCircumference C = 2 * pi * r = 2 * 3.14159 * 5 = 31.4159 cm\nArea A = pi * r^2 = 3.14159 * 25 = 78.5398 sq cm\n\nThe area equals exactly 25pi square centimeters.
Result: Area = 78.5398 cm^2 | Circumference = 31.4159 cm | Diameter = 10 cm
Example 2: Circle from Circumference of 50 cm
Problem: A circular garden has a circumference of 50 cm. Find its area.
Solution: Circumference C = 50 cm\nRadius r = C / (2pi) = 50 / (2 * 3.14159) = 7.9577 cm\nArea A = pi * r^2 = 3.14159 * 63.325 = 198.944 sq cm\n\nAlternatively: A = C^2 / (4pi) = 2500 / 12.5664 = 198.944 sq cm
Result: Area = 198.944 cm^2 | Radius = 7.958 cm
Frequently Asked Questions
What is the formula for the area of a circle?
The area of a circle is calculated using the formula A = pi times r squared, where r is the radius of the circle and pi is approximately 3.14159265. This formula can also be expressed in terms of the diameter as A = pi times d squared divided by 4, since the radius is half the diameter. The formula was first rigorously proven by Archimedes around 250 BCE using the method of exhaustion, which approximated the circle with inscribed and circumscribed polygons. The area formula tells us that doubling the radius quadruples the area, because the radius is squared. This quadratic relationship between radius and area is fundamental to understanding how circular measurements scale.
How do you find the area of a circle from the circumference?
To find the area from the circumference, first derive the radius using C = 2 times pi times r, which gives r = C divided by (2 times pi). Then substitute this radius into the area formula A = pi times r squared. Combining these steps yields the direct formula A = C squared divided by (4 times pi). For example, if the circumference is 31.4159 units, the radius is 31.4159 / (2 * 3.14159) = 5 units, and the area is pi times 25 = 78.5398 square units. This relationship is useful when you can measure around a circular object (like using a tape measure) but cannot easily measure the radius directly, which is common in practical applications like measuring pipes or round containers.
What is the difference between the area and the circumference of a circle?
The area and circumference measure fundamentally different properties of a circle. The circumference (C = 2 times pi times r) measures the length of the boundary, which is a one-dimensional measurement expressed in linear units like centimeters or inches. The area (A = pi times r squared) measures the space enclosed within the boundary, which is a two-dimensional measurement expressed in square units like square centimeters or square inches. As the radius increases, the circumference grows linearly (double the radius means double the circumference) while the area grows quadratically (double the radius means four times the area). This distinction is crucial in practical applications like fencing (circumference) versus tiling (area) a circular garden.
Why does the area formula use pi?
Pi appears in the area formula because it is the fundamental constant relating a circle to its radius. Specifically, pi is defined as the ratio of a circle's circumference to its diameter, and this same ratio governs the relationship between area and radius. One intuitive way to understand why is to imagine cutting a circle into many thin triangular sectors and rearranging them into a shape approaching a rectangle. The rectangle has a height equal to the radius r and a width equal to half the circumference (pi times r), giving an area of r times pi times r equals pi r squared. This geometric argument shows that pi is not arbitrarily inserted into the formula but emerges naturally from the fundamental geometry of circles and their constant curvature.
How do you calculate the area of a semicircle or quarter circle?
A semicircle is exactly half of a full circle, so its area equals pi times r squared divided by 2. A quarter circle (quadrant) has an area of pi times r squared divided by 4. More generally, a sector with central angle theta (in degrees) has an area of (theta / 360) times pi times r squared, or equivalently (theta / 2) times r squared when theta is in radians. For example, a semicircle with radius 10 has area = pi * 100 / 2 = 157.08 square units. A 60-degree sector of the same circle has area = (60/360) * pi * 100 = 52.36 square units. These partial area calculations are essential in architecture, engineering, and design where circular arcs and segments appear as parts of larger structures.
What is the relationship between a circle and its inscribed square?
An inscribed square has all four vertices touching the circle, and its diagonal equals the diameter of the circle. If the circle has radius r, the inscribed square has a diagonal of 2r, giving a side length of r times the square root of 2, and an area of 2r squared. The ratio of the circle area to the inscribed square area is pi/2, approximately 1.5708, meaning the circle is about 57% larger in area than its inscribed square. Conversely, the inscribed square covers about 63.66% of the circle area. A circumscribed square (with sides tangent to the circle) has side length 2r and area 4r squared. The circle covers pi/4 or about 78.54% of the circumscribed square area. These ratios appear in Monte Carlo methods for estimating pi.