Diameter
4u
Twice the radius.
Mathematics · Geometry
Circle calculator — radius, diameter, circumference and area
Start from radius, diameter, circumference or area and calculate the other circle quantities on one page.
Created v1.0
By Thibaut Grzelak, Mechanical Analysis Engineer
03
Results
Circumference
12.57u
Length of the circular boundary.
Area
12.57u²
Area enclosed by the shape.
Radius
2u
Results show up to four significant digits for readability; internal calculations retain full precision.
See calculation details
01Radius
Formula
r = vSubstitution
2Result
2
02Diameter
Formula
d = 2rSubstitution
2 × 2Result
4
03Circumference
Formula
C = 2πrSubstitution
2π × 2Result
12.57
04Area
Formula
S = πr²Substitution
π × 2²Result
12.57
01Formulas and symbols
Formulas used
r
d = 2r
C = 2πr
S = πr²
r: radius; d: diameter; C: circumference; S: area.
| Symbol | Meaning |
|---|---|
| v | Known value |
| r | Radius |
| d | Diameter |
| C | Circumference |
| S | Area |
02Validation example
Reference numerical case
- r = 2 u gives d = 4 u, C = 4π u and S = 4π u². Entering any one of these in its corresponding mode returns the same circle.
03How it works
A circle has diameter 2r, circumference 2πr and area πr². An area input is inverted with r = √(S/π); a circumference input gives r = C/(2π).
04Limits and conventions
A positive value and a Euclidean circle. Lengths use one arbitrary unit u; area uses u². Changing the known quantity preserves the last input entered for each mode, not the physical size.
05Common mistakes
Area is in squared units. Do not enter a diameter as a radius: that doubles the circumference and quadruples the area.
06Practical questions
How are radius, diameter, circumference and area related?
The diameter is d = 2r, circumference is C = 2πr and area is A = πr². Knowing any one positive quantity is enough to recover the radius and therefore all the others.
Can I calculate a circle from its area or circumference alone?
Yes. From circumference, r = C/(2π); from area, r = √(A/π). The calculator then derives all the other circle quantities from that radius.