How to use the Circle Area Calculator
Select the measurement you already know. The value field follows that choice, and Unit sets the length unit for the three linear results and the squared unit for Area. After a calculation, the result panel shows all four measurements and the formulas used to derive them.
- Choose Radius, Diameter, Area, or Circumference.
- Enter the positive value in the field and choose a Unit.
- Click Calculate.
- Read the four measurements and the steps under Formula & steps.
- Click Copy result or Download result (.txt) if you need the output elsewhere.
Changing the value, Unit, or measurement option hides the current result until you click Calculate again. Reset clears the value, returns Unit to Meters (m), and selects Radius.
Circle area calculator example
With Radius selected, enter 5 and choose Meters (m) in Unit. The calculator derives the other three measurements and formats the values shown in the result panel.
Known value: Radius = 5 m
Radius: 5 m
Diameter: 10 m
Circumference: 31.415927 m
Area: 78.539816 m^2The result panel lists the intermediate formulas. The text area contains the known value, all four results, and numbered steps for copying or downloading.
Calculate circle area from radius, diameter, or circumference
The calculator first derives a radius when the known value is something else. It uses r = d / 2 for Diameter, r = sqrt(A / pi) for Area, and r = C / (2 * pi) for Circumference. It then applies d = 2 * r, C = 2 * pi * r, and A = pi * r^2.
For a non-meter unit, the steps convert the entered value to meters, perform the calculation, and convert the results back to the selected unit. The calculation uses JavaScript's built-in value of pi. Displayed values are formatted, so they can differ from an unrounded intermediate value.
Circle calculator units and input rules
Unit contains Millimeters (mm), Centimeters (cm), Meters (m), Kilometers (km), Inches (in), Feet (ft), Yards (yd), and Miles (mi). Radius, diameter, and circumference use the selected length unit. Area uses its squared version.
The input parser accepts digits with an optional decimal point and at least one digit after it. Blank input, zero, negative values, commas, signs, scientific notation, invalid numbers, and values above 1e12 produce an error. The result stays hidden when validation fails.
Choose the measurement you have
Use Radius for the distance from the center to the edge, Diameter for the distance across the circle through the center, Area for the enclosed surface, or Circumference for the distance around the circle. Each option changes the value label and its hint.
When Area is selected, enter a value in the squared version of Unit. For example, Meters (m) means square meters for the input and output area, while the returned radius, diameter, and circumference use meters.
Choose another calculator for other shapes
Browse Math Calculators for equations, fractions, sequences, geometry, matrices, vectors, number theory, or scientific calculations. The Geometry Calculator covers slope, distance, midpoint, triangle area, polygon area, and common solid measurements. Use the Cylinder Volume Calculator for cylinder volume or surface areas, or the Cone Volume Calculator for a right circular cone's volume, surface areas, or slant height.
Frequently asked questions
How do I calculate the area of a circle?
Choose Radius, enter the value, select Unit, and click Calculate. The calculator applies A = pi * r^2 and shows the area with the other circle measurements.
Can I find a circle's radius from its area?
Choose Area and enter the positive value in the squared version of the selected Unit. The calculator uses r = sqrt(A / pi) to derive the radius.
What is the difference between diameter and circumference?
Diameter is the distance across the circle through its center, while circumference is the distance around it. Choose either one as the known measurement to calculate the other values.
Why did the circle area calculator reject my value?
The value must be greater than zero, no greater than 1e12, and formatted as digits with an optional decimal point. Commas, signs, scientific notation, blank input, and invalid numbers are rejected.
Does the Circle Area Calculator send my measurement anywhere?
The calculation and the measurement you enter are processed locally in your browser. The calculator code does not make a network request with that value.