Circle Calculator

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.

  • Complete the Square to Find Circle Center & Radius
    Convert 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.
  • 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.
  • Circumference to Diameter Calculator
    Convert a circle's circumference to its diameter instantly with d = C / π. Free online calculator that also gives the radius. Works with any unit.
  • RPM to Angular Velocity Calculator
    Convert RPM to angular velocity in radians per second instantly. Uses ω = RPM × 2π / 60, with degrees/second and Hz outputs too.
  • Perimeter of a Circle Calculator
    Calculate the perimeter (circumference) of a circle from its radius using P = 2πr. Also shows diameter and area. Fast, free and accurate.
  • Centripetal Acceleration Calculator
    Calculate centripetal acceleration and angular velocity from tangential velocity and radius using a = v²/r and ω = v/r. Free physics tool.
  • Bolt Circle Diameter Calculator
    Calculate the center-to-center chord spacing between bolt holes from the bolt circle diameter (BCD) and hole count, or back-solve the BCD.
  • Circular Motion Speed Calculator
    Convert RPM and radius into linear (tangential) speed and angular velocity. Uses v = 2πr·(RPM/60) = ωr for any rotating object.
  • Wheel Revolutions to Distance Calculator
    Convert wheel revolutions into distance travelled. Enter wheel diameter and number of turns to get distance in meters, km, miles, and feet.
  • Circle to Sphere Surface Area Calculator
    Calculate the surface area of a sphere from its radius using SA = 4πr². Also returns diameter and volume. Fast, free, and accurate geometry tool.
  • Circle to Sphere Volume Calculator
    Calculate the volume of a sphere from its radius using V = (4/3)πr³. Also returns diameter and surface area instantly with full precision.
  • Arc Length from Radius and Radians Calculator
    Calculate arc length from a circle's radius and central angle in radians using s = r × θ. Free, instant, with chord length too.
  • Circle Degrees to Arc Length Calculator
    Convert a circle's central angle in degrees and radius into arc length. Free calculator with the L = (θ/360) × 2πr formula, radians, and circumference.
  • Degrees-Minutes-Seconds to Decimal Degrees Calculator
    Convert latitude/longitude in degrees, minutes and seconds (DMS) to decimal degrees instantly using decimal = D + M/60 + S/3600.
  • Belt Length Around Two Pulleys Calculator
    Calculate the approximate length of an open belt around two pulleys from their diameters and center distance using the standard belt-length formula.
  • Annulus (Ring) Area Calculator
    Calculate the area of an annulus (ring) from its outer and inner radii using A = π(R² − r²). Also shows ring width and circumferences.
  • Arc Length from Degrees Calculator
    Calculate the arc length of a circle from its radius and central angle in degrees. Also shows radians and full circumference instantly.
  • Circumference of a Circle Calculator
    Calculate the circumference of a circle from radius or diameter using C = 2πr = πd. Get circumference, diameter, and area instantly with worked examples.
  • Circumference to Area Calculator
    Convert a circle's circumference to its area instantly with A = C²/(4π). Also shows radius and diameter. Free, accurate geometry calculator.
  • Circle to Cylinder Volume Calculator
    Turn a circle's radius and a height into a cylinder volume instantly with V = πr²h. Free calculator showing circle area and total cubic volume.
  • Radius from Circumference Calculator
    Find a circle's radius from its circumference using r = C / (2π). Also returns diameter and area instantly. Free, accurate geometry calculator.
  • Area from Radius Calculator
    Calculate the area of a circle from its radius using A = πr². Also returns diameter and circumference instantly. Free, accurate circle area tool.
  • Degrees to Radians Calculator
    Convert degrees to radians instantly. Enter any angle in degrees and get the exact radian value plus its multiple of π using radians = degrees × π / 180.
  • Diameter from Radius Calculator
    Find a circle's diameter from its radius instantly using d = 2r. Also returns circumference and area. Fast, free, and accurate geometry tool.

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.

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