Circle Calculator
Frequently Asked Questions
How do I calculate the area of a circle if I only know the diameter?
Divide the diameter by 2 to get the radius, then apply the formula A = πr². For example, a circle with a diameter of 10 cm has a radius of 5 cm and an area of approximately 78.54 cm². Our calculator does this instantly — just enter the diameter and all values appear automatically.
What is the formula for the circumference of a circle?
The circumference equals 2 × π × radius, or equivalently π × diameter. Using π ≈ 3.14159, a circle with a 7 cm radius has a circumference of approximately 43.98 cm. The circumference represents the total distance around the outer edge of the circle, equivalent to its perimeter.
Can I calculate the radius if I only know the area?
Yes. Rearrange the area formula A = πr² to get r = √(A / π). For example, if a circle has an area of 50 cm², the radius is √(50 / 3.14159) ≈ 3.99 cm. Simply enter the area value in our calculator and the radius is computed instantly without any manual steps.
What units does the circle calculator use?
The calculator works with any unit you choose — centimeters, meters, inches, or feet. Whatever unit you enter for the radius or diameter, all linear results (radius, diameter, circumference) use the same unit. The area result uses square units. For example, entering meters gives area in square meters (m²).
Why is π (pi) used in circle calculations?
Pi (π ≈ 3.14159) is the constant ratio of a circle's circumference to its diameter. It appears in every circle formula because it captures the fundamental geometric relationship of all circles. Pi is irrational, meaning its decimal expansion never ends, so calculators use a high-precision approximation for accurate results.
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