Circle Calculator
Enter any one value — all others will be calculated.
What is a Circle Calculator?
A circle calculator computes all four fundamental circle measurements — radius, diameter, circumference, and area — from any single input value. Enter any one dimension and the other three are derived instantly using the standard formulas: d = 2r, C = 2πr, and A = πr². This tool uses JavaScript's Math.PI constant for maximum precision.
How to Use
- Enter any one known value — radius, diameter, circumference, or area.
- The remaining three values are calculated and displayed automatically in the results panel.
- The input field you used is highlighted in the results for easy reference.
- Clear or change any field to recalculate from a different starting measurement.
Frequently Asked Questions
What is π (pi)?
Pi (π ≈ 3.14159) is the ratio of a circle's circumference to its diameter. It's irrational (non-repeating decimal) and appears throughout mathematics, physics, and engineering. This calculator uses JavaScript's Math.PI for maximum precision.
What is the formula for circle area?
Area = π × r². Alternatively, Area = π × (d/2)² where d is diameter. Doubling the radius quadruples the area since it's a squared relationship.
What is the difference between circumference and diameter?
Diameter is the straight-line distance across the circle through the center (= 2 × radius). Circumference is the distance around the circle's edge (= π × diameter). Circumference is always about 3.14159× the diameter.
What is a unit circle?
A unit circle has radius = 1, making circumference = 2π and area = π. It's fundamental in trigonometry — any angle θ defines a point (cos θ, sin θ) on the unit circle, which is how sine and cosine are defined geometrically.
Formulas
d = 2r
C = 2πr
A = πr²