Circle Calculator
Type any single circle measurement — radius, diameter, circumference, or area — and the other three are calculated for you as you type.
Example
With a known radius of 5:
Diameter = 2 × 5 = 10
Circumference = 2 × π × 5 ≈ 31.4159
Area = π × 5² ≈ 78.5398
Switch "Known value is" to Area and enter 78.5398 and the radius comes back as 5.
How it works
Pick which value you know, enter it, and the tool applies the standard circle formulas (d = 2r, C = 2πr, A = πr²) to derive the radius first and then every other measurement.
Good to know
The Circle Calculator converts between the four core measurements of any circle: radius, diameter, circumference, and area. You enter just one value you already know, tell it which measurement that value represents, and it instantly fills in the other three. It is built for students checking geometry homework, makers and DIYers sizing round materials, and anyone who needs a quick, accurate conversion without juggling formulas.
Reach for it whenever you have a partial figure and need the rest. Common cases include knowing a pipe or tabletop diameter and wanting the area, having a tape-measured circumference and needing the radius to draw or cut a circle, or working backward from a target surface area to find how wide the circle must be. Because every relationship runs through the radius, the tool handles each direction equally well, including the trickier reverse step of recovering radius from area using radius = √(area ÷ π).
To read the result, note that the large number at the top is always the radius the calculator derived from your input; the three smaller stats below it are the diameter, circumference, and area computed from that radius. Keep these points in mind:
- All four figures share the same unit family: if you enter centimeters, circumference is in centimeters and area is in square centimeters.
- Displayed numbers are rounded to 6 significant figures, while the math runs at full double precision, so light rounding differences when you re-enter a result are expected.
One practical caveat: enter only positive numbers, since a circle cannot have a negative or zero-derived dimension, and the field will reset to a blank state otherwise. If you plan to feed an answer into another calculation, copy the on-screen value as shown rather than retyping a truncated version, because that rounded figure can drift slightly from the exact internal value.
Frequently asked questions
How does it work if I only know the area?
It reverses the area formula: radius = √(area ÷ π). From that radius it then computes diameter (2r), circumference (2πr), and confirms the area, so one input gives you all four measurements.
What value of π is used?
It uses JavaScript's Math.PI, which is π to full double-precision (about 3.141592653589793). Displayed answers are rounded to 6 significant figures for readability while calculations stay at full precision.
Is my data uploaded anywhere?
No — this calculator runs entirely in your browser; nothing is uploaded.
Is it free?
Yes, completely free with no sign-up and no limits.
People also ask
How do you find the area of a circle from its diameter?
Halve the diameter to get the radius, then apply area = π × radius². For a diameter of 10, the radius is 5 and the area is π × 25 ≈ 78.54. The calculator does this automatically when you set the known value to Diameter.
What is the difference between circumference and diameter?
The diameter is the straight distance across a circle through its center, while the circumference is the distance all the way around its edge. The circumference is always about 3.14159 (π) times the diameter.
How do you calculate the radius from the circumference?
Divide the circumference by 2π. For example, a circumference of 31.4159 divided by 6.2832 gives a radius of about 5. Set the known value to Circumference to have the tool compute it for you.
Why is π used in every circle calculation?
π is the fixed ratio of a circle's circumference to its diameter, so it appears in every formula that links a circle's edge or area to its width. This calculator uses π at full double precision (about 3.141592653589793) for accuracy.
Is the diameter always twice the radius?
Yes. By definition the diameter passes through the center and equals two radii, so diameter = 2 × radius for every circle, regardless of size.
Can this calculator work in inches, centimeters, or any unit?
Yes, it is unit-agnostic. Whatever unit you enter applies to all linear results, and area comes out in that unit squared, so consistency depends only on the unit you choose for your input.
How accurate are the results shown?
Calculations run at full floating-point precision, but the displayed answers are rounded to 6 significant figures for readability. Very large or very small values are shown in exponential notation instead.
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