Complete guide
Circle calculator from radius, diameter, circumference or area
Choose any known circle measurement and enter its value. Calculator24 solves radius, diameter, circumference and area without requiring a second input.
Circle measurements
Radius runs from center to edge. Diameter crosses the center and equals two radii. Circumference is boundary length; area is enclosed surface.
Circle formulas
Every measure can be derived from radius.
d=2rC=2πr=πdA=πr²Solve backward
From circumference, divide by 2π for radius. From area, divide by π and take the positive square root.
Units and precision
Radius, diameter and circumference share a linear unit. Area uses the corresponding square unit. π remains unrounded internally.
Worked example
For radius 5, diameter is 10. Multiplying 10 by π gives circumference; multiplying 25 by π gives area.
Convert a known measure to radius
| Known | Radius |
|---|---|
| Radius r | r |
| Diameter d | d/2 |
| Circumference C | C/(2π) |
| Area A | √(A/π) |
Frequently asked questions
What is circle circumference?
It is the distance around the circle: 2πr or πd.
How do I find radius from area?
Divide area by π and take the positive square root.
Is diameter twice radius?
Yes.
Does area use square units?
Yes, such as square inches or square meters.
Is π rounded?
The engine uses JavaScript's full numeric π value and rounds only the display.
What to keep in mind
Results use the same linear unit as the input and its square for area.
Results display up to 8 decimal places; exported numbers preserve calculation precision. This is a calculation summary, not an official certificate.