Circle Calculator
Please provide any value below to calculate the remaining values of a circle — radius, diameter, circumference, and area.
Formulas
d = 2r
C = 2πr
A = πr²
C = 2πr
A = πr²
What is a circle?
A circle is the set of all points in a plane at the same distance (the radius) from a fixed point called the center. It is one of the most fundamental shapes in geometry, appearing in wheels, gears, planets, and design.
Parts of a circle
- Center: The fixed point equidistant from every point on the circle.
- Radius (r): Distance from center to any point on the circle.
- Diameter (d): Longest distance across the circle through the center; d = 2r.
- Circumference (C): Distance around the circle; C = 2πr.
- Area (A): Space enclosed; A = πr².
- Chord: Line segment connecting two points on the circle.
- Arc: Portion of the circumference between two points.
- Secant: Line that intersects the circle at two points.
- Tangent: Line that touches the circle at exactly one point.
Circle formulas
Diameter: d = 2r
Circumference: C = 2πr = πd
Area: A = πr²
Example: If r = 5, then d = 10, C ≈ 31.416, A ≈ 78.54.
How to use this circle calculator
- Enter any one value — radius, diameter, circumference, or area.
- Leave the other fields blank (or they will be overwritten with consistent values).
- Click Calculate to fill all four values with step-by-step formulas.
Frequently Asked Questions
Pi is the ratio of a circle's circumference to its diameter, approximately 3.14159. It appears in all circle formulas.
Yes. Enter the area and click Calculate. The tool uses r = √(A/π).
Yes, completely free on ScientificCalculators.site.