圓面積計算器
計算圓的面積、周長和直徑。
Calculating the Area and Circumference of a Circle
Overview
The area of a circle measures the space inside it, and the circumference measures the distance around it — both derived from the radius. This calculator finds them instantly from the radius, diameter, or circumference in your browser.
How to Use (Step by Step)
- 1
Enter a known measurement
Provide the radius, diameter, or circumference.
- 2
Calculate
The tool derives the radius and computes area and circumference.
- 3
Read the results
Both area and circumference are shown together.
How It Works
Area = π × r², and circumference = 2 × π × r, where r is the radius and π is about 3.14159. If you know the diameter instead, the radius is half of it; if you know the circumference, the radius is circumference ÷ (2π). The tool accepts whichever measurement you have and computes the rest.
When to Use This
Working out material needed for a circular area like a garden bed or table. Solving a geometry problem. Finding the circumference for fencing or trim around a round object.
Frequently Asked Questions
Area = π × radius². For a radius of 5, that is π × 25 ≈ 78.54 square units.