Circle Calculator
Area, circumference and diameter from any radius.
Amortization schedule
| # | Payment | Principal | Interest | Balance |
|---|---|---|---|---|
Showing first 12 of months.
Growth over time
| Year | Invested | Value |
|---|---|---|
Introduction
Circle Calculator — Area, circumference and diameter from any radius. Enter Radius to get an instant, accurate result.
Formula
Area = πr². Circumference = 2πr. Diameter = 2r.
Step-by-step
- Enter the Radius.
- Click Calculate to see your result instantly.
Real-world example
Example: With Radius = 5, the Circle Calculator gives Area: 78.5398, Circumference: 31.4159, Diameter: 10.
Frequently Asked Questions
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.
About the Circle Calculator
The Circle Calculator uses a real, verifiable formula — Area = πr². Circumference = 2πr. Diameter = 2r. — so results are accurate every time, not an approximation.