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Circle Calculator

Math

Find a circle's radius, diameter, circumference, and area from any single known value. Get all four results instantly, with formulas and a worked example shown.

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Area

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Radius
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Diameter
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Circumference
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This calculator computes your Area, Radius, Diameter, Circumference from the values you enter.

Inputs
Known ValueValue
Outputs
AreaRadiusDiameterCircumference

What is a Circle?

A Circle Calculator computes a circle's radius, diameter, circumference, and area from any single known measurement. Circles show up constantly in everyday calculations — from sizing a round table or garden bed to working out the area of a circular plot — but most people only ever know one measurement at a time, like the distance around an object or its width across the middle.

This calculator removes the need to remember and rearrange four different geometry formulas. Pick which value you already know — Radius, Diameter, Circumference, or Area — enter it, and the calculator instantly works out the other three. It complements the broader Area Calculator, which handles area and perimeter for several shapes including circles, by focusing entirely on circles and accepting any one of the four core properties as a starting point.

How to use this Circle calculator

  1. Select the Known Value dropdown and choose which measurement you already have — Radius, Diameter, Circumference, or Area.
  2. Enter that measurement in the Value field.
  3. Read the Area result card for the headline figure.
  4. Check the Radius, Diameter, and Circumference cards below it for the remaining measurements.
  5. Open the step-by-step breakdown to see exactly how each value was derived from your input.
  6. Change the Known Value selection at any time to recalculate from a different starting measurement.

Formula & Methodology

All calculations start by converting the known value into the radius, then deriving the rest from there:

- From radius: r = value
- From diameter: r = value ÷ 2
- From circumference: r = value ÷ (2π)
- From area: r = √(value ÷ π)

Once the radius is known, the remaining properties follow standard circle formulas:

Diameter = 2r
Circumference = 2πr
Area = πr²

Worked example: For a circle with a known circumference of 44 metres:
- Radius = 44 ÷ (2 × 3.14159) ≈ 7 m
- Diameter = 2 × 7 = 14 m
- Area = 3.14159 × 7² ≈ 153.94 m²

Frequently Asked Questions

A circle calculator finds all four key measurements of a circle — radius, diameter, circumference, and area — when you know just one of them. Instead of remembering and rearranging multiple formulas, you enter a single known value and the calculator works out the rest automatically. This is useful for geometry homework, construction layouts, and any task where you need to convert between a circle's measurements quickly.
The area of a circle is calculated as A = πr², where r is the radius and π is approximately 3.14159. If you only know the diameter, divide it by 2 to get the radius before applying the formula, or simply select Diameter as the known value and let the calculator handle the conversion.
Circumference is calculated as C = 2πr, where r is the radius. It can also be written as C = πd, where d is the diameter, since the diameter is twice the radius. Both formulas give the same result — the distance around the edge of the circle.
The radius is the distance from the centre of the circle to its edge, while the diameter is the full distance across the circle through its centre. The diameter is always exactly twice the radius, so if you know one, you automatically know the other.
Rearranging the area formula A = πr² gives r = √(A ÷ π). Select Area as the known value in the calculator, enter the area, and it will compute the radius along with the diameter and circumference automatically.
No — this calculator is specifically for two-dimensional circles (area and circumference). For three-dimensional sphere calculations like volume and surface area, you would need a dedicated sphere or volume calculator, since the formulas are different.
The [Area Calculator](/area-calculator/) computes area and perimeter for multiple shapes — circles, rectangles, triangles, and more — starting from basic dimensions. The Circle Calculator is dedicated entirely to circles and lets you start from any one of the four properties (not just radius), making it more flexible when you know the circumference or area but not the radius.
The calculator uses the full precision value of π (not a rounded 3.14) for all internal calculations, then rounds the final results to three decimal places for display. This keeps results accurate enough for academic, construction, and design purposes.
Yes — select Circumference as the known value and enter the measured distance around the circle. The calculator will derive the radius using r = C ÷ 2π, and then compute the diameter and area from that radius.
Circle calculations come up in tasks like sizing a circular table, cutting a circular piece of material, laying out a round garden bed, or working out the area of a circular plot of land. Knowing just one measurement — like the distance around a space — is often enough to plan the rest.
Also known as
circle area calculatorcircumference calculatorradius calculatordiameter calculatorcircle geometry calculator