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Quickly calculate the area of a circle by entering the radius, diameter, or circumference. Supports custom units and precision settings.
Overview
Understand what the tool solves, how it works, and the boundaries of its data.
Enter a positive radius to calculate the area of a circle. Radius is the distance from the center to the edge. The calculator also reports circumference, the distance around the circle, and diameter, the distance across it through the center. These outputs use the same length unit as your radius, with area expressed in that unit squared.
Area: A = πr²
Circumference: C = 2πr
Diameter: d = 2r
For a radius of 5 cm, area is π × 5² = 25π ≈ 78.5398 cm²; circumference is about 31.4159 cm and diameter is 10 cm. The tool displays numerical results to four decimal places. It accepts radius only; diameter and circumference are results, not alternate input modes.
Guide
Follow the workflow and verify inputs and outputs with practical examples.
Use the distance from the circle's center to its boundary. If you know only the diameter, halve it before entering the value.
The area and related values appear as the input changes. A blank, nonnumeric, zero, or negative radius does not produce a result.
Area is in square units; circumference and diameter are in linear units. Copy the result you need and keep the unit attached when recording it.
Use cases
See how the tool fits into real work and everyday tasks.
A student enters a known radius to compare the area with a hand calculation, then checks circumference and diameter from the same circle.
A homeowner measuring a circular tabletop can enter its radius and use the area as a geometric estimate, provided the surface is actually circular and the measurement units are consistent.
Q&A
Find concise answers to common questions and confusing cases.
Divide the diameter by two to get the radius, then enter that radius. The calculator does not accept diameter as its input field.
The values are numeric. Interpret area in the square of your input unit—for example, a radius in centimeters gives area in square centimeters—and circumference and diameter in centimeters.
The calculation uses π and shows four decimal places. The displayed value is a rounded decimal approximation; keep an exact expression such as 25π when a problem requests an exact answer.
Notes
Review scope, result limitations, and important precautions before use.
This calculation models a complete, flat circle. It does not calculate a sector, arc, hole, shaded region, or a three-dimensional object's surface. Measurement error in the radius affects the area because the radius is squared. For physical projects, confirm the measurement and units before using the result for material estimates.
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