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Find circle area, circumference, and diameter from radius.
⭕ Area = π r². Circumference = 2 π r. Diameter = 2 r.
This calculator finds the area, circumference, and diameter of a circle from its radius. Essential for math class, engineering, and any project involving round shapes.
π (pi) is the ratio of a circle's circumference to its diameter, approximately 3.14159. It appears in every circle formula because it's a fundamental property of circles — and of waves, rotation, and many natural phenomena.
The Circle Calculator lets you figure out circle calculatorinstantly, without reaching for a spreadsheet or doing the math by hand. Whether you're planning a budget, checking a loan, or working through homework, the tool applies the correct formula behind the scenes and shows the result the moment you enter your numbers.
Unlike a static chart or table, this calculator adapts to your exact inputs. You can adjust any value and see the outcome update in real time, which makes it easy to compare scenarios — for example, "what if the rate were 1% lower?" or "what if I paid an extra $50 a month?"
Common uses: people reach for this tool when they need to find a circle area from radius, circumference from diameter calculator, circle area and circumference, or diameter from radius calculator.
Browser-based tools like this one have a few real advantages over installed software or manual methods:
The Circle Calculator is based on the following formula:
A = π·r² C = 2π·r
Variables: A = Area (square units) C = Circumference (units) r = Radius (units) π ≈ 3.14159
Circle area A and circumference C, where r is the radius.
Worked example: Step 1: r = 5 cm. Step 2: A = π × 5² = 3.14159 × 25 ≈ 78.54 cm². Step 3: C = 2πr = 2 × 3.14159 × 5 ≈ 31.42 cm. Result: area ≈ 78.54 cm² and circumference ≈ 31.42 cm.
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