A circle calculator helps you find measurements such as radius, diameter, circumference, and area. These values are useful in school geometry, design, and everyday measurement problems. The Circle Calculator provides a convenient way to check your calculations.
Circle measurements explained
The radius (r) is the distance from the centre to the edge. The diameter (d) passes through the centre and is twice the radius. The circumference (C) is the distance around the circle. The area (A) measures the space enclosed by it.
Essential circle formulas
| Measurement | Formula |
|---|---|
| Diameter | d = 2r |
| Radius | r = d/2 |
| Circumference | C = 2πr = πd |
| Area | A = πr² |
Pi (π) is approximately 3.14159. Using a more precise value of π improves numerical accuracy.
How to use the circle calculator
- Open the Circle Calculator.
- Identify the measurement you already know, such as radius or diameter.
- Enter the value in the appropriate field supported by the tool.
- Calculate and review the available measurements.
- Check that your units are consistent.
Worked example: radius of 5 cm
For a circle with radius 5 cm, the diameter is 10 cm. Circumference is 2π(5) = 10π ≈ 31.42 cm. Area is π(5²) = 25π ≈ 78.54 cm².
Find radius from circumference
Rearrange C = 2πr to get r = C/(2π). If a circle’s circumference is 20π metres, then its radius is 10 metres.
Find radius from area
From A = πr², rearrange to r = √(A/π). If the area is 49π square units, the radius is 7 units.
Common mistakes
- Confusing diameter with radius.
- Using πr instead of πr² for area.
- Reporting area in linear units rather than square units.
- Rounding π too early in a multi-step calculation.
Frequently asked questions
What is the diameter of a circle with radius 8?
The diameter is 16 in the same length unit.
What is the difference between circumference and area?
Circumference measures distance around a circle; area measures the surface inside it.
Can circumference and diameter have the same units?
Yes. Both are lengths, while area uses squared units.
Does the circle formula work for an oval?
No. An ellipse requires different formulas.
Use the online circle calculator to check your radius, area, diameter, or circumference calculations.
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