Circle Calculator

A sector is a 'pie slice' of a circle — the region bounded by two radii and the arc between them. A 90° sector is a quarter circle, 180° is a semicircle, 360° is the whole circle. Sector area = (angle/360°) × full circle area.

The arc is the curved edge of the sector along the circle. The chord is the straight line connecting the two endpoints of the arc. The segment is the area between the chord and the arc — calculated as sector area minus the triangle area formed by the two radii and the chord.

An annulus is a ring — the area between two concentric circles (circles with the same centre). Ring area = π × (R² − r²), where R is the outer radius and r is the inner. Common applications: washers, pipes, donuts, race tracks, anywhere a hollow circular shape appears in engineering or everyday objects.

Pick from millimetres, centimetres, metres, kilometres, inches, feet, or yards. Areas display in square units (cm², m², ft²...). Switch units any time without re-typing — the unit selector only changes labels, not the underlying numbers.

π (pi) is the constant ratio of a circle's circumference to its diameter — approximately 3.14159. The area of any circle equals π × r². This relationship was known and approximated by ancient civilisations (Babylonians, Egyptians, Indians) long before its irrationality was proven.

JavaScript uses IEEE 754 double-precision floating-point for π and √, giving roughly 15-17 significant digits. For everyday geometry, surveying, engineering drawings — far more than precise enough. Customize mode lets you pick how many decimal places to display (2 / 3 / 4 / 6).

No. Radius is a distance, so it must be non-negative. Negative inputs are rejected.