Circle Calculator
Enter any one of a circle's radius, diameter, circumference, or area and instantly get the other three — runs 100% in your browser with no upload.
Radius: 1 Diameter: 2 Circumference: 6.283185 Area: 3.141593
🔒 Calculated in your browser — nothing is uploaded. Uses Math.PI; plain numbers, no units.
What this circle calculator does
Pick which measurement you already know — the radius, diameter, circumference, or area — type its value, and the calculator instantly returns the other three. It works in every direction, so you can go from a known area back to the radius just as easily as from a radius forward to the area. Everything recalculates live as you type, with no button to press and nothing to submit.
The formulas it uses
All four quantities depend on a single radius r, so the tool first derives r from whatever value you supply, then computes the rest. It uses diameter = 2r, circumference = 2πr, and area = πr². Going backwards, radius = diameter ÷ 2, radius = circumference ÷ (2π), and radius = √(area ÷ π). The constant π comes from JavaScript's built-in Math.PI for full double precision, and results are rounded for display while the underlying math stays exact.
Frequently asked questions
Which value should I enter?
Choose the measurement you know from the dropdown — radius, diameter, circumference, or area — then type its number. The tool derives the radius internally and fills in the other three values automatically.
Is anything I type uploaded to a server?
No. Every calculation runs locally in your browser with JavaScript. The number you enter is never sent, stored, or logged anywhere.
What value of pi does it use?
It uses JavaScript's Math.PI, which is the closest double-precision value of π (about 3.141592653589793), so results are accurate to many decimal places.