Circle Calculator
Solve a circle from radius, diameter, circumference or area — any one gives the rest.
About Circle Calculator
A circle is completely defined by a single measurement: its radius. Every other property follows directly from r. The diameter is twice the radius. The circumference (perimeter) is 2πr ≈ 6.28318·r. The area enclosed is πr² ≈ 3.14159·r². π (pi) is an irrational constant approximately equal to 3.14159265.
Formulas
Diameter
d = 2r
- r
- Radius
- d
- Diameter
Circumference
C = 2πr = πd
- C
- Circumference
- r
- Radius
Area
A = πr²
- A
- Area
- r
- Radius
Worked example
With the default values below, Circle Calculator returns Radius (r) of 5.000000. Change any input above and every figure here is recalculated in your browser.
Inputs used
- I know theone of Radius (r), Diameter (d), Circumference (C), Area (A)
- Radius (r)
- Radius (r)minimum 0
- 5
- Diameter (d)minimum 0
- 10
- Circumference (C)minimum 0
- 31.4159
- Area (A)minimum 0
- 78.5398
Results
- Radius (r)
- 5.000000
- Diameter (d)
- 10.000000
- Circumference (C)
- 31.415927
- Area (A)
- 78.539816
Step by step
- 1Radius= 5
- 2Diameter= 10
- 3Circumference= 31.41592654
- 4Area= 78.53981634
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 6 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
What is π (pi) and why does it appear in circle formulas?
π is the ratio of a circle's circumference to its diameter — it's approximately 3.14159265. It appears in every circle formula because it's the fundamental constant connecting a circle's linear dimensions (radius) to its rotational measures (arc length, area).
How do you find the circumference from the area?
First find the radius from the area: r = √(A/π). Then compute C = 2πr. For example, if A = 100, then r = √(100/π) ≈ 5.642, and C = 2π × 5.642 ≈ 35.45.
What is the difference between circumference and perimeter?
They mean the same thing: the total length of the boundary. 'Perimeter' is the general term for polygons; 'circumference' is specifically used for circles and curved shapes.