Known radius
r = 5Area = 78.5398163397
Circle Calculator
Use this free circle calculator to start from radius, diameter, circumference, or area and find the other circle measurements with steps.
Unit-aware inputs Formula note Example numbers Tab-only history
Radius5 cm
- Diameter
- 10 cm
- Circumference
- 31.4159265359 cm
- Area
- 78.5398163397 cm^2
Steps
- Start from the known radius.
- Use d = 2r, C = 2pi r, and A = pi r^2.
- The radius is 5 cm.
- The area is 78.5398163397 cm^2.
How to use the Circle Calculator
- Choose the known circle measurement: radius, diameter, circumference, or area.
- Enter the known value and an optional unit label.
- Press Calculate circle to find radius, diameter, circumference, and area.
- Use examples, recent answers, or copy the result while solving circle problems.
What people use it for
Convert radius into diameter, circumference, and area.
Work backward from diameter, circumference, or area.
Check circle formulas for geometry class or quick planning.
Copy circle measurements and formula steps into notes.
Quick examples
Known diameter
d = 10Radius = 5
Known circumference
C = 31.4159Radius is about 5
Need the guide or a nearby tool?
Need a slower walkthrough, a related calculator, or the full library? These links keep you close to the task you started.
- How to use the Circle Calculator See slower examples, input meanings, formula notes, result limits, and mistakes to avoid.
- Calculators Browse related calculators and utilities in the same category.
- Calculator and utility guides Find more guides that use real tool inputs and example numbers.
- All free tools Search every live tool, converter, calculator, AI helper, and browser utility.
Frequently asked questions
Quick answers about formulas, units, valid measurements, examples, copying, and private in-browser history.
What circle measurements can I start with?
You can start with radius, diameter, circumference, or area. The calculator finds the remaining circle measurements.
What formulas does the Circle Calculator use?
It uses d = 2r, C = 2pi r, and A = pi r^2. It rearranges those formulas when you start from diameter, circumference, or area.
What do the main Circle Calculator inputs mean?
The main inputs are the numbers, operation, mode, or known values the calculator needs. Keep units consistent, enter percentages the way the page label shows, and use the examples as a quick check before trusting the answer.
How should I read the Circle Calculator answer?
Read the headline answer, then check the supporting lines and examples to understand how the calculator got there. If one input changes, rerun the tool and compare the new answer instead of guessing.
What should I double-check before trusting the Circle Calculator?
Check units, signs, rounding, and the selected mode before copying the answer. If the number feels weird, rerun one of the examples first, then put your own values back in slowly.
What is the difference between radius and diameter?
Radius is the distance from the center to the circle edge. Diameter is the full distance across the circle through the center, so diameter is twice the radius.
Can I enter area to find radius?
Yes. Area mode uses r = sqrt(A / pi) to work backward from a known area.
Should I use Area Calculator instead?
Use Circle Calculator when you want all circle measurements. Use Area Calculator when you only need area for one of several common shapes.
Is my circle history private?
Yes. Recent circle answers stay only in the current browser tab while you use the page. They are not sent to a server.
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