円計算機

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はじめに

Circle Calculator — Area, circumference and diameter from any radius. Enter Radius to get an instant, accurate result.

計算式

Area = πr². Circumference = 2πr. Diameter = 2r.

ステップごとの説明

1. Enter the Radius.
2. Click Calculate to see your result instantly.

実例

Example: With Radius = 5, the Circle Calculator gives Area: 78.5398, Circumference: 31.4159, Diameter: 10.

よくある質問

Can I calculate from the diameter instead of the radius?

Enter half the diameter as the radius — all formulas here are radius-based.

What value of π is used?

PHP's built-in M_PI constant, accurate to about 15 significant digits.

What inputs does the Circle Calculator need?

You'll need to provide: Radius. All fields use sensible defaults, so you can see a working example immediately and then adjust the values to match your own numbers.

How accurate is the Circle Calculator?

The Circle Calculator applies the formula exactly as calculated — Area = πr². Circumference = 2πr. Diameter = 2r. — so results are precise for the inputs you provide. Accuracy depends on entering correct, realistic input values.

Is the Circle Calculator free to use?

Yes, the Circle Calculator is completely free, requires no signup, and runs instantly in your browser.

円計算機について

The Circle Calculator uses a real, verifiable formula — Area = πr². Circumference = 2πr. Diameter = 2r. — so results are accurate every time, not an approximation.