Circle Calculator
Use this free Circle Calculator to calculate your Circle measurements instantly and accurately.
Circle
Result
What is a Circle Calculator?
A circle calculator computes the radius, diameter, circumference, and area of a circle from any single known value. Enter one measurement and the calculator derives all the others using pi (π) relationships.
How to calculate circle measurements manually (the formula)
Radius r = d/2. Diameter d = 2r. Circumference C = 2πr = πd. Area A = πr2. Given any one value, the others can be derived.
Example calculation
A circle has radius 5. Diameter = 10. Circumference = 2π × 5 = 10π ≈ 31.42. Area = π × 25 ≈ 78.54 square units.
Common mistakes
- Confusing radius and diameter — Radius is half the diameter. Using diameter in a formula that expects radius (or vice versa) changes the result.
- Forgetting to square the radius in the area formula — Area uses r2, not r. Forgetting to square gives a much smaller result.
- Using the wrong value of pi — Using 3.14 instead of the calculator's full π constant may cause rounding differences in precise work.
Frequently asked questions
What is pi (\u03c0)?
Pi (\u03c0) is a mathematical constant approximately equal to 3.14159. It is the ratio of a circle's circumference to its diameter.
How do you find the circumference of a circle?
Use C = 2\u03c0r or C = \u03c0d, where r is radius and d is diameter. Multiply 2 by pi by the radius.
How do you calculate the area of a circle?
Use A = \u03c0r\u00b2. Square the radius and multiply by pi.
What is the relationship between radius and diameter?
The diameter is twice the radius: d = 2r. The radius is half the diameter: r = d/2.
How do you find the radius from the circumference?
Divide the circumference by 2\u03c0: r = C/(2\u03c0). For circumference 31.42, r \u2248 5.
What is the difference between circumference and area?
Circumference is the one-dimensional distance around a circle (like perimeter). Area is the two-dimensional space inside the circle.