Known radius
r = 5 ftd = 10 ft, C = 31.4159 ft, A = 78.5398 ft^2
Circle Calculator
Use this free circle calculator to start from radius, diameter, circumference, or area and find the other circle measurements with formula steps, units, copy, and history.
Unit-aware inputsFormula noteExample numbersTab-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.
Recent answers
Recent circle answers will appear here.
Privacy note
Geometry answers and history stay in this browser tab while you work.
Copy answer only writes the current result to your clipboard when you press it.
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 a known radius into diameter, circumference, and area.
Work backward from diameter, circumference, or area to find radius first.
Check circle formulas for geometry class, quick diagrams, or simple layout planning.
Keep length units and square units straight before copying the result.
Save circle measurements and formula steps into notes or homework checks.
Quick examples
Known diameter
d = 10 inr = 5 in, C = 31.4159 in, A = 78.5398 in^2
Known circumference
C = 31.4159 cmr is about 5 cm and d is about 10 cm
Known area
A = 78.5398 m^2r is about 5 m and C is about 31.4159 m
Patio diameter
d = 24 ftr = 12 ft and A = 452.3893 ft^2
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 CalculatorSee slower examples, input meanings, formula notes, result limits, and mistakes to avoid.
- CalculatorsBrowse related calculators and utilities in the same category.
- Calculator and utility guidesFind more guides that use real tool inputs and example numbers.
- All free toolsSearch 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 radius first, then uses that radius to calculate the remaining circle measurements.
What formulas does the Circle Calculator use?
It uses d = 2r, C = 2pi r, and A = pi r^2. If you start from diameter, circumference, or area, it rearranges the matching formula to solve for radius before calculating the rest.
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 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.
How does the calculator work backward from circumference?
It uses r = C / (2pi). For a circumference of about 31.4159, the radius is about 5 because 31.4159 divided by 2pi is close to 5.
Can I enter area to find radius?
Yes. Area mode uses r = sqrt(A / pi) to work backward from a known area. After it finds radius, it calculates diameter and circumference from that radius.
Why does the area result use square units?
Radius, diameter, and circumference are lengths, so they use units such as ft or cm. Area covers a flat surface, so it uses square units such as ft^2 or cm^2.
What should I double-check before trusting a circle result?
Check whether your known value is radius or diameter, keep the unit label consistent, and remember that measured real objects may not be perfectly round.
Does this calculator handle arcs, sectors, or partial circles?
No. It calculates full-circle radius, diameter, circumference, and area. Arc length, sector area, rings, and partial circles need separate inputs and formulas.
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 or you are comparing circles with rectangles, triangles, trapezoids, and parallelograms.
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.