From comparing Pizza Sizes to find the best value, to computing a Circular Sector's arc length and chord, solving an Annulus (ring) area, finding a circle's Area and Circumference, or locating the circumcircle of a triangle — this collection handles the full range of circle geometry problems.
Browse the Most Popular list to see which tools others open most, or check the Trending tab for newly popular additions. Every calculator is free, needs no account, and works on any device.
- Volume Calculator
- Area Calculator
- Perimeter Calculator
- Surface Area Calculator
- Circle Calculator
- Rectangle Calculator
- Triangle Calculator
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Arc Length from Degrees CalculatorCalculate the arc length of a circle from its radius and central angle in degrees. Also shows radians and full circumference instantly.
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Annulus (Ring) Area CalculatorCalculate the area of an annulus (ring) from its outer and inner radii using A = π(R² − r²). Also shows ring width and circumferences.
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Perimeter of a Circle CalculatorCalculate the perimeter (circumference) of a circle from its radius using P = 2πr. Also shows diameter and area. Fast, free and accurate.
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Complete the Square to Find Circle Center & RadiusConvert the general circle equation x²+y²+Dx+Ey+F=0 to standard form. Instantly find the center (−D/2, −E/2) and radius by completing the square.
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Arc Length Calculator (s = rθ)Calculate the arc length of a circle from its radius and central angle using s = rθ. Supports angles in radians or degrees, with instant results.
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Area of a Circle CalculatorCalculate the area of a circle from its radius using A = πr². Also returns the diameter and circumference instantly. Free online geometry tool.
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Circle Perimeter from Diameter CalculatorCalculate the perimeter (circumference) of a circle from its diameter using C = πd. Instantly get circumference, radius, and area for any diameter.
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Semicircle Perimeter CalculatorCalculate the perimeter of a semicircle from its radius using P = πr + 2r = r(π + 2). Free, instant, and shows the arc and diameter parts.
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Circle Area from Diameter CalculatorFind the area of a circle from its diameter instantly. Uses A = πd²/4. Also shows radius and circumference. Fast, free, and accurate.
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Circle Area CalculatorCalculate the area of a circle instantly with A = πr². Enter the radius to get area, diameter, and circumference in seconds.
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Circle Area and Circumference CalculatorCalculate a circle's area, circumference, radius, and diameter from any one known value using A = πr² and C = 2πr. Fast and accurate.
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Radius from Diameter CalculatorFind the radius of a circle from its diameter instantly. Enter any diameter and this calculator returns the radius using r = d/2.
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Radius from Area CalculatorCalculate the radius of a circle from its area using r = √(A/π). Also returns diameter and circumference instantly. Free online geometry tool.
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Diameter from Circumference CalculatorFind a circle's diameter from its circumference using d = C / π. Instantly get diameter, radius, and area for any circumference value.
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Circle Area from Circumference CalculatorFind a circle's area directly from its circumference using A = C² / (4π). Instant, accurate results plus the radius and diameter.
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Circle Circumference from Area CalculatorFind the circumference of a circle from its area instantly. Enter the area and get the circumference, radius, and diameter using C = 2√(πA).
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Diameter from Circumference CalculatorFind the diameter of a circle from its circumference using d = C / π. Instantly calculate diameter and radius from any circumference value.
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Radius from Circumference CalculatorFind the radius of a circle from its circumference using r = C / (2π). Also returns diameter and area instantly in the same unit.
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Diameter from Area CalculatorFind a circle's diameter from its area using d = 2√(A/π). Instantly get diameter, radius, and circumference from any area value.
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Area from Circumference CalculatorFind the area of a circle from its circumference using A = C² / (4π). Also returns radius and diameter instantly. Free online geometry calculator.
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Circle Moment of Inertia (Area) CalculatorCalculate the area moment of inertia of a solid circle from its radius or diameter using I = πr⁴/4 = πd⁴/64. Free engineering tool.
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Partially Filled Circle Area CalculatorCalculate the filled cross-section area and fill percentage of a partially filled circle or cylindrical tank from radius and fill depth.
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Radius from Area of a Circle CalculatorFind the radius of a circle from its area using r = √(A/π). Instantly get radius, diameter, and circumference for any circle area.
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Circumference of a Circle CalculatorCalculate the circumference of a circle from radius or diameter using C = 2πr = πd. Get circumference, diameter, and area instantly with worked examples.
Frequently asked questions
What circle problems can I solve here?
You can calculate a circle's area, circumference, and perimeter; find sector area, arc length, and chord length; solve annulus (ring) dimensions; determine a circular segment's area from a central angle; find the circumcircle of a triangle; and compare pizza sizes to see which gives the most value.
How accurate are the results?
All calculators apply standard Euclidean geometry formulas — A = πr², C = 2πr, and related identities — using high-precision arithmetic. For safety- or construction-critical work, verify results against the governing standard and have a qualified engineer review the figures before proceeding.
Are these calculators free?
Yes — every circle calculator here is completely free to use. There is no subscription, no account needed, and no feature locked behind a paywall — open any tool and get your answer instantly.
Do you save the measurements I enter?
No. All calculations run entirely in your browser. Your measurements are never transmitted to our servers, logged, or stored in any form — nothing you enter leaves your device.
Which circle calculator is most popular?
The Most Popular list is sorted by real usage. The Circle Area and Circumference Calculator and the Pizza Size Calculator are perennially popular, but the Trending tab shows which tools are rising fastest in usage right now.
