Skip to content
Kordu Tools
Math & Numbers Runs in browser Updated 30 Mar 2026

Circle Calculator

Calculate radius, diameter, circumference, and area from any single value. Arc length and sector area included with an SVG diagram.

Enter a positive number.

Enter a positive value above to see all circle measurements.

Formulae

Circumference C = 2πr = πd

Area A = πr²

Arc Length L = rθ (angle in radians)

Sector Area A = r²θ / 2 (angle in radians)
Loading rating…

How to use Circle Calculator

  1. Enter any circle measurement

    Type a value into the radius, diameter, circumference, or area field. All other values are calculated immediately.

  2. Read exact and decimal results

    Results appear in both exact form (e.g. 6π) and as a decimal approximation.

  3. Calculate arc length and sector area

    Enter a central angle in degrees or radians to calculate arc length and sector area for that portion of the circle.

  4. Check the SVG diagram

    The circle diagram highlights the currently active measurement — radius, diameter, arc, or sector.

  5. Copy results

    Click the copy button next to any result to copy it to your clipboard.

Circle Calculator FAQ

What is the formula for the circumference of a circle?
Circumference = 2 × π × r, or equivalently π × d, where r is the radius and d is the diameter.

What is the formula for circle area?
Area = π × r², where r is the radius. If you know the diameter, r = d/2.

How do I find the radius from the circumference?
Radius = Circumference / (2π). Enter the circumference into the circumference field and the radius appears automatically.

What is arc length and how is it calculated?
Arc length is the distance along a curved portion of a circle. Arc length = r × θ, where θ is the central angle in radians. For degrees: Arc length = (θ/360) × 2πr.

What is sector area?
A sector is the pie-slice region bounded by two radii and an arc. Sector area = ½ × r² × θ (radians), or (θ/360) × π × r² for degrees.

What does exact form mean?
Exact form leaves π as a symbol, such as 6π cm². This avoids rounding errors and is often required in math exams.

Can I enter circumference to find area?
Yes. Enter any one of radius, diameter, circumference, or area and all others are derived automatically.

What is the difference between diameter and radius?
The radius is the distance from the center to the edge. The diameter crosses the full circle through the center. Diameter = 2 × radius.

Can I use this for homework?
Yes. Formulas are shown alongside every result so you can verify your method and understand the working.

Is any data sent to a server?
No. All calculations run entirely in your browser. Nothing is uploaded or stored.

Background

Enter any one circle measurement and get all the others instantly. Type the radius, diameter, circumference, or area and the calculator applies the correct formula in both directions. Results display in exact form (with π) and as a decimal — handy for exams where exact values are required. An arc and sector section calculates arc length and sector area from a central angle entered in degrees or radians. A live SVG diagram highlights the measurement you are working with, making it easy to visualize each quantity. Useful for geometry classes, engineering, construction, and everyday problems like finding how much fencing fits around a circular garden or the area of a pizza. The change-of-base formula is used internally for precision. All processing runs in your browser with no data sent to any server and no account needed.

Related tools

Area & Perimeter Calculator

Calculate area and perimeter for rectangles, triangles, circles, trapezoids, ellipses, and regular polygons with formula diagrams.

Triangle Calculator

Solve any triangle using SSS, SAS, ASA, AAS, or right-triangle mode. Get sides, angles, area, perimeter, and a live SVG diagram with step-by-step solutions.

Volume & Surface Area Calculator

Calculate volume and surface area for spheres, cylinders, cones, cubes, rectangular prisms, pyramids, and more. Capacity conversion included.

Scientific Calculator

A full-featured scientific calculator with trigonometry, logarithms, exponents, roots, memory functions, and calculation history. Full keyboard support.

Percentage Calculator

Calculate X% of Y, find what percentage X is of Y, or compute percentage increase and decrease. Results update as you type.