Circle Calculator
Finds a circle's diameter, circumference and area from its radius.
Recent
Formula
How the calculation works
The radius is the distance from the centre of a circle to its edge, and every other measurement in this tool is derived from that single number using the constant pi.
Doubling the radius gives the diameter, the full width across the circle through its centre; for a radius of 7 that diameter is 14 units.
Multiplying twice the radius by pi gives the circumference, while pi times the radius squared gives the area, so the same radius of 7 yields about 43.98 around and 153.94 inside.
Common mistakes
- Do not confuse the radius with the diameter: entering 7 as the radius means the diameter is 14, and treating 7 as a diameter would halve every other answer.
- Keep units consistent, because the circumference and area will come out in those same units and their squares, so mixing centimetres and metres spoils the result.
- Do not round pi to 3 in your head before using this tool, since the built-in Math.PI keeps precision and rounding early distorts the area noticeably.
When to use it
- Use it to size anything round, such as finding how much edge trim a circular table needs or how much paint covers a round lid.
- Use it in geometry homework to cross-check the diameter, circumference and area of a circle from a single radius measurement.
Worked example
A circle of radius 7 has a diameter of 14, a circumference of about 43.98 and an area of about 153.94.
Common questions
Is the circumference the same as the area?
No. Circumference is the distance around the circle's edge, measured in plain units, while area is the space inside it, measured in square units.
Can I enter the diameter instead?
No. This tool wants the radius, so divide a known diameter by two before entering it; otherwise every answer will be doubled.
Related tools
-
Discount Calculator
See what you actually pay after a discount, and how much you save.
-
Tip Calculator
Add a tip to the bill and split it between people.
-
Add Tax / VAT Calculator
Add tax or VAT to a price to get the total.
-
Percentage Calculator
Find what percentage one number is of another.
-
Percentage Change
How much a number rose or fell between two points.
-
Monthly Budget Split
Divide monthly income into needs, wants and savings.
-
Profit Margin & Markup
Profit, margin and markup from a cost and a selling price.
-
Original Price from a Sale
Work back from a discounted price and the discount rate to the original.