Circle Calculator: Radius, Diameter, Circumference & Area
Free circle calculator: find radius, diameter, circumference, or area from one value. Area of a circle formula, how to find circumference, diameter of a circle. Circle geometry solver, units: mm, cm, m, in, ft. Trusted by students and educators. No sign-up—all calculations run locally.
What This Calculator Does & Who It's For
Calculator Purpose & Ideal Users
- What You'll Get:Radius, diameter, circumference, area: All four from one input. Unit support: mm, cm, m, in, ft (linear and area in that unit squared). Precision: 2–10 decimal places. Step-by-step: “How it was calculated” shows the formulas used. Diagram: Parts of a circle (radius, diameter, circumference) for reference.
- Ideal Users:Students & geometry: “Find the area of a circle from circumference,” “diameter of a circle from radius,” homework and exams. Teachers & curriculum: Quick checks and circle formula practice. DIY & construction: Round areas, edging, or circular layouts in your chosen unit. Anyone needing one circle measure from another: No need to memorize formulas—enter one value, get the rest.
- Scope & Limits:Solves only r, d, C, and A from one another. Does not compute chord length, sector area, or arc length (those require angles); the article below explains radius, diameter, chord, tangent, and sector for context. Uses high-precision π (Math.PI) for accuracy. All calculations run in your browser; no data is sent to servers.
This circle calculator solves for r, d, C, and A from any one known value.
What Is a Circle Calculator? Circle Geometry Solver
A circle calculator (or circle geometry solver) finds radius (r), diameter (d), circumference (C), and area (A) from any single known value—enter one, get the other three. Use it to calculate area of a circle from circumference, find diameter of a circle from radius, or apply A = πr². See above for what you'll get and who it's for. Free online, no sign-up.
The Constant π (Pi)
- Circumference:
- Area:
- From C:
- From A:
Pi (π) is the ratio of a circle's circumference to its diameter:
≈ 3.14159. It is the same for every circle, so
and
The calculator uses high-precision π so results are accurate to the decimal places you choose (2–10)
Parts of a Circle: Radius, Diameter, Chord, Tangent, Sector
What This Calculator Solves
This tool solves only for r, d, C, and A from one another. The terms below (radius, diameter, chord, tangent, sector) are for context; the calculator does not compute chord length or sector area.
Radius (r)
- Definition:Half the diameter; the "length from center to edge"
Distance from the center of the circle to any point on the edge. The calculator uses radius as the base for all formulas (d = 2r, C = 2πr, A = πr²).
Diameter (d)
- Definition:Twice the radius; the longest chord
Distance across the circle through the center. d = 2r. Often easier to measure on physical objects than radius.
Chord
- Note:This calculator does not solve for chord length; it solves for r, d, C, and A from one known value.
A line segment whose endpoints lie on the circle. The diameter is a chord that passes through the center.
Tangent
- Note:Used in geometry and trigonometry; not required for the radius–diameter–circumference–area solver.
A line that touches the circle at exactly one point, perpendicular to the radius at that point.
Sector
- Note:For full-circle area use A = πr²; for a sector use an area calculator that supports angles.
A "pie slice" of the circle bounded by two radii and an arc. Area of sector = (θ/360°) × πr², where θ is the angle in degrees.
How to Calculate the Area of a Circle: Area of a Circle Formula
- From radius:
- From diameter:
- From circumference:
The area of a circle formula is
How to calculate the area of a circle: if you know the radius, use A = πr². If you know the diameter, use
If you know the circumference, use
How to find circumference from area: use
This circle calculator does all of that from one value—enter radius, diameter, circumference, or area and get the other three; choose unit (m, ft, etc.) so area shows in that unit squared (m², ft²).
Circle Calculator FAQ
? How do you find the circumference of a circle?
Circumference is the distance around the circle. Use
or
For example, if the radius is 5 m, circumference ≈ 31.42 m. The calculator finds circumference from radius, diameter, or area automatically
? What is the relationship between radius and diameter?
The diameter is exactly twice the radius:
So if you know the radius, double it for the diameter; if you know the diameter, halve it for the radius. The calculator uses this relationship to solve for all circle properties from any one known value
? How many degrees are in a circle?
A full circle is 360 degrees (360°). In radians—another common unit for angles—a full circle is 2π radians (about 6.28). The circle calculator works with linear measures (radius, diameter, circumference) and area; for sector or arc calculations you would use the angle in degrees or radians.
? How do I calculate the area of a circle from circumference?
From circumference C, first find the radius:
Then area
Or use
In this calculator, select "Circumference" as your known variable, enter C, and the tool will show area (and radius, diameter) with your chosen precision and units
? What is pi (π) and why is it used for circles?
Pi (π) is the ratio of a circle's circumference to its diameter: π = C/d ≈ 3.14159. It is the same for every circle. That’s why circumference = πd = 2πr and area = πr². The calculator uses high-precision π (Math.PI) so results are accurate to many decimal places.
? Can I use different units (mm, cm, m, in, ft)?
Yes. Choose your unit from the dropdown; the calculator converts everything internally. Linear results (radius, diameter, circumference) are shown in that unit; area is shown in that unit squared (e.g. m², ft², in²).