Circle Calculator — Area, Circumference & Diameter
Quick answer: Enter radius, diameter, area, or circumference to find all other circle measurements.
Calculate any circle measurement from any starting value: enter the radius, diameter, area, or circumference and instantly get all other properties. Also calculates sector area and arc length for any central angle.
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Last reviewed: April 2026Report an error
Area
78.539816
Area = πr² = π × 5² = 78.539816. Circumference = 2πr = 31.415927.
Radius
5
Diameter
10
Area
78.539816
Circumference
31.415927
Sector / Arc (optional)
Sector Area
19.634954
Arc Length
7.853982
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How to Use This Circle Calculator Calculator
- 1Select what you know: radius, diameter, area, or circumference.
- 2Enter the value.
- 3Read area, circumference, diameter, and radius.
- 4Optionally enter a sector angle to get sector area and arc length.
Frequently Asked Questions
- A = πr², where r is the radius. π ≈ 3.14159. Example: radius 5 → A = π × 25 ≈ 78.54.
- C = 2πr = πd. It is the perimeter (total boundary length) of the circle. For r=5: C = 2π×5 ≈ 31.42.
- r = √(A/π). If area = 50: r = √(50/π) ≈ 3.99.
- A sector is a "pie slice" defined by two radii and an arc. Sector area = (θ/360) × πr². Arc length = (θ/360) × 2πr, where θ is the central angle in degrees.
- Diameter = 2 × radius. The radius goes from center to edge; the diameter goes all the way across through the center.
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