Math
Circle Calculator
Calculate radius, diameter, circumference, and area. Also compute arc length, sector area, chord length, segment area, annulus (ring) area, and circle equations with clear steps.
Calculate Circle Measurements in Multiple Modes
Choose a tab based on what you know. Set units and rounding once, then calculate and copy results.
Area outputs are shown in square units (e.g., cm²). Angles are in degrees.
Radius, diameter, circumference, area — plus a small formula table you can reference.
Core circle formulas: d = 2r, C = 2πr, A = πr².
Radius plus the matching derived values so you can verify the result.
Radius conversions: r = √(A/π), r = C/(2π), r = d/2.
With θ in degrees: arc = (θ/360)·2πr, sector = (θ/360)·πr²,
chord = 2r·sin(θ/2), segment = sector − (1/2)r²·sin(θ) (minor segment).
Annulus (ring) area: A = π(R² − r²), thickness: t = R − r.
Standard form: (x − h)² + (y − k)² = r². General form expands to x² + y² + Dx + Ey + F = 0.