NumberTally Circle Calculator
Enter a circle's radius and instantly get its diameter, circumference, and area — with each formula and every step of the working shown, a labeled diagram, and optional arc-length and sector-area tools. Free, no sign-up.
Advanced options
Three formulas, one radius
Everything about a circle flows from the radius r: diameter = 2r, circumference = 2πr (or πd), area = πr². The circumference is the distance around; the area is the space inside. Note the area grows with the square of the radius — doubling r quadruples the area.
Arcs and sectors
A sector is a pie slice: its area is the fraction θ/360 of the whole circle. The arc length is the same fraction of the circumference. Enter a central angle in Advanced options to see both.
Frequently Asked Questions
How do you find the circumference of a circle?
C = 2πr (or πd). A circle of radius 5 has circumference 10π ≈ 31.42.
How do you find the area of a circle?
A = πr². A circle of radius 5 has area 25π ≈ 78.54.
What is the diameter of a circle?
The full width through the center: d = 2r. It's the longest straight line you can draw inside the circle.
What is pi (π)?
The ratio of any circle's circumference to its diameter, ≈ 3.14159. It's irrational — the decimals never repeat — so we round it.
What is the difference between arc length and sector area?
Arc length is a distance along the curve (θ/360 × 2πr); sector area is the pie-slice region (θ/360 × πr²). Same fraction, different wholes.