Circle Calculator
Find area, circumference, and diameter from a circle’s radius.
Everything from the radius
A circle is defined entirely by its radius, and from that single number you can find its area, circumference, and diameter. This tool computes all three at once.
A circle of radius 5 has an area of about 78.54, a circumference near 31.42, and a diameter of 10. The calculations use full floating-point pi, so the results are precise.
Precise and private
It applies the circle formulas in your browser, so nothing you enter is uploaded.
Frequently Asked Questions
What does it calculate?
From the radius it gives the area, circumference, and diameter, using pi to full floating-point precision.
What are the formulas?
Area is pi times radius squared; circumference is two times pi times radius; diameter is twice the radius.
Can I start from the diameter?
Halve your diameter to get the radius, then enter that. The relationships are all reported.
Browse the full set of free, private, in-browser tools.