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Circle Circumference Calculator

Calculate the boundary circumference, surface area, diameter, or radius of any circle. Input any parameter to solve for all others instantly.

Circle Calculator
Fill in one property and choose units to calculate the rest.
Line segment from center to edge
cm
Straight line width through center
cm
Total perimeter length around edge
cm
Total two-dimensional flat space inside
cm²
Interactive VisualizationHover or tap dimensions to highlight
r: 10 cmd: 20 cmC: 62.83 cmA: 314.16 cm²
Result: Radius of 10 cm results in a Circumference of 62.831853 cm
Table of Contents
Circle Circumference Formulas & Calculations
How boundary circumference is mathematically derived from other circular dimensions.

A circle represents all points in a two-dimensional plane that are at a fixed distance (the radius) from a central point. The constant ratio of any circle's circumference to its diameter is represented by the Greek letter π (Pi), approximately equal to 3.14159.

1. Circumference from Radius
C = 2 × π × r

Multiply the radius by 2, then multiply by Pi. This is the standard definition equation.

2. Circumference from Diameter
C = π × d

Multiply the diameter directly by Pi. Useful because diameter is easier to measure directly in real-world scenarios.

3. Circumference from Area
C = 2 × √(π × A)

Multiply the area by Pi, take the square root of that result, and then multiply by 2.

Real-World Circle Benchmarks
Compare the dimensions and circumference values of familiar circular items.

US Quarter Coin

Diameter: 0.95 in (2.43 cm)

Circumference ≈ 2.98 in (7.63 cm)

Standard Compact Disc (CD)

Diameter: 4.72 in (12.0 cm)

Circumference ≈ 14.83 in (37.70 cm)

NBA Basketball Hoop

Diameter: 18.0 in (45.7 cm)

Circumference ≈ 56.55 in (143.6 cm)

Large Pizza (14-inch)

Diameter: 14.0 in (35.6 cm)

Circumference ≈ 43.98 in (111.8 cm)

💡 Did you know? Because the circumference is directly proportional to the radius, a 14-inch pizza has exactly1.4 times the boundary circumference of a 10-inch pizza, even though its surface area is nearly double (1.96x)!
Quick Radius Presets
Choose common radii values in the selected unit to pre-fill the calculator.
Radius to Circle Circumference Reference Table
Reference mapping of integers to resulting diameters, circumferences, and areas.
Radius (cm)Diameter (cm)Circumference (cm)Area (cm²)
126.2833.142
2412.56612.566
51031.41678.540
102062.832314.159
153094.248706.858
2040125.6641256.637
50100314.1597853.982
100200628.31931415.927
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