Circle calculator
Diameter, circumference and area of a circle from its radius.
Enter your values and press Calculate. Your inputs stay in the address, so you can switch language or share the link.
Formula
C = 2πr A = πr² d = 2r
How it works
The diameter is twice the radius, the circumference is 2π times the radius and the area is π times the radius squared, with π ≈ 3.14159.
Worked example
The same calculation with the default values:
- Radius 5
- d = 2 × 5 = 10
- C = 2 × π × 5 = 31.415927
- A = π × 5² = 78.539816
Result: Diameter: 10 · Circumference: 31.41592654 · Area: 78.53981634
Uses radius as the primary input for all three formulas - a value entered as a diameter must be halved first, since using it directly as the radius would overstate circumference by 2× and area by 4×.
Common mistakes
- Confusing diameter and radius - this is the single most common error with circle calculations; always check which one a field is asking for before entering a value.
- Using an approximate value of π with too few digits for a high-precision application - 3.14 is fine for a rough estimate, but this calculator uses a much more precise value internally, so hand calculations with 3.14 will drift from the shown result.
- Forgetting that area scales with the square of the radius, not linearly - doubling the radius quadruples the area, it does not simply double it.
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