Every measurement of a circle follows from a single one. Pick which value you know — radius, diameter, circumference or area — type it in, and the calculator fills in the other three. Handy for homework, DIY layouts and design work; everything is worked out in your browser.
Enter a value to solve the circle.
How it works
The whole circle hinges on the radius. From it, the diameter is 2r, the circumference is 2πr, and the area is πr². When you enter a different value, the tool reverses the matching formula to recover the radius first — for example radius = √(area ÷ π) — then rebuilds the rest.
π (pi) is an irrational number, so unless you start from the radius or diameter the answers are rounded — here to four decimal places. Round-tripping (area → radius → area) therefore lands back within a tiny rounding margin rather than exactly.
The tool is unit-agnostic: enter centimetres, metres or inches and the outputs share that unit, with area in the squared version. It expects a positive number, since a circle cannot have a zero or negative size.
Practical examples
Radius 5 → the rest
Enter a radius of 5. The diameter is 10, the circumference is about 31.4159, and the area is about 78.5398 — the textbook circle in one step.
Sizing a round rug from area
You have room for about 3 m² of floor. Choose “area”, enter 3, and the radius comes out near 0.977 m — so a rug up to roughly 1.95 m across fits.
From a measured circumference
You wrap a tape around a pipe and read 31.4 cm. Choose “circumference”, enter 31.4, and the diameter is about 9.995 cm — the size to match a fitting.
Frequently asked questions
What is the difference between radius and diameter?
The radius is the distance from the centre to the edge; the diameter is all the way across through the centre, so the diameter is exactly twice the radius.
What is the formula for the area of a circle?
Area = π × r², where r is the radius. If you only know the diameter, halve it first; if you know the circumference, the tool finds the radius as circumference ÷ (2π).
How do I find the circumference?
Circumference = 2 × π × r, or equivalently π × diameter. It is the distance around the circle — what a tape measure reads when wrapped around it.
How can I get the radius from the area?
Rearrange the area formula: radius = √(area ÷ π). Choose “area” as your known value and the tool applies this automatically.
What value of pi does it use?
It uses the full-precision π built into the browser (about 3.14159265), not a rounded 3.14, so results are as accurate as the display precision allows.
Why are some results not perfectly round numbers?
Because π is irrational. Only when you start from the radius or diameter can the other whole-number relationships stay exact; area and circumference bring π in and are rounded to four decimals.
What units should I use?
Any single length unit. Whatever you enter — cm, m, inches — the radius, diameter and circumference come back in that unit, and the area in its square (cm², m², and so on).
Does this work for a sphere or cylinder?
No — it covers a flat (2D) circle only. A sphere’s surface area and volume, or a cylinder’s volume, use the circle’s area or circumference as a building block but need extra formulas.
Is anything I enter stored?
No. The calculation runs entirely in your browser and nothing is uploaded or saved.
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