Circle Calculator
Calculate radius, diameter, circumference, and area of any circle from one known value. Free geometry calculator with formulas, charts, and instant results.
About This Calculator
The Circle Calculator computes all fundamental circle properties — radius, diameter, circumference, and area — from any single known measurement. Whether you know the radius, diameter, circumference, or area, simply select the known value type from the dropdown and enter your number. The calculator instantly derives all other dimensions using standard geometric formulas. This tool is ideal for students learning geometry, teachers preparing lessons, engineers designing circular components, construction workers measuring round structures, and DIY enthusiasts working on craft or home improvement projects.
Four core formulas power this calculator: diameter d = 2r, circumference C = 2πr, circumference C = πd, and area A = πr². The mathematical constant π (pi, approximately 3.14159265) is sourced from JavaScript's built-in Math.PI for maximum precision. The results panel displays all four computed values with up to four decimal places, alongside a pie chart showing the proportional relationship between dimensions and a bar chart for direct value comparison. The calculator uses the modern chart pattern with Chart.js for responsive, interactive visualizations that work on all screen sizes.
How to Use
Using the circle calculator is straightforward. First, select the type of value you know from the dropdown — Radius, Diameter, Circumference, or Area. Then enter the numeric value in the input field. Click the Calculate button to compute all missing circle properties instantly. The results section shows radius, diameter, circumference, and area with their computed values. Toggle between the Pie and Bar chart modes to visualize the relationships. Your inputs are saved in the page URL, allowing you to bookmark or share the calculation with a single link.
Frequently Asked Questions
How do you calculate the circumference of a circle?
The circumference of a circle is calculated using C = 2πr, where r is the radius. Alternatively, use C = πd where d is the diameter. For a circle with radius 10, the circumference is approximately 62.83 units. Our calculator computes circumference instantly from any known circle dimension.
What is the formula for the area of a circle?
The area of a circle is calculated using A = πr², where r is the radius. Multiply pi (approximately 3.14159) by the square of the radius. For a circle with radius 5, the area is approximately 78.54 square units. You can also find area from diameter, circumference, or directly from the area field.
How do you find the diameter of a circle from circumference?
To find the diameter from circumference, divide the circumference by pi: d = C / π. For example, if the circumference is 31.42 units, the diameter is approximately 10 units. Our calculator automatically converts between any circle measurement you enter.
How do you calculate radius from area?
To find radius from area, use the formula r = √(A / π), where A is the area. For a circle with area 78.54 square units, the radius is approximately 5 units. Select Area from the dropdown and enter your value to get the radius and all other circle properties.
What is the relationship between radius and diameter?
The diameter of a circle is exactly twice the radius: d = 2r. Conversely, the radius is half the diameter: r = d/2. This means if you know either the radius or diameter, you can instantly find the other. Our calculator handles all these conversions automatically.
What units does the circle calculator support?
The calculator works with any consistent unit system. If you enter the radius in meters, diameter and circumference are in meters and area in square meters. Use inches, feet, centimeters, or any unit as long as it is used consistently throughout.
Is this circle calculator free to use?
Yes, all calculators on Calculy are completely free with no registration, limits, or hidden fees. You can bookmark or share any calculation via the URL.