How to use the circle calculator
- In I know the, pick Radius, Diameter, Circumference or Area.
- Type the Value. It must be greater than zero.
- Read the four results: Radius, Diameter, Circumference and Area.
- Check the Formulas box. Its first line shows how the radius came from your value.
What it does and when to use it
In real life you rarely measure a circle’s radius directly. You can wrap a tape around a tree trunk or a pipe (circumference), measure across a table top (diameter), or read an area off a plan. This tool turns whichever value you have into all the others. Typical jobs: sizing a round tablecloth, finding the diameter of a pipe from its girth, working out how big a circular patio of a given area must be, or checking textbook circle questions.
For part of a circle, use the arc length calculator. For oval shapes, the ellipse calculator covers area and perimeter. The sphere calculator handles the 3D version.
How it works
The tool first finds the radius r from what you entered:
- From the diameter: r = d/2
- From the circumference: r = C/(2π)
- From the area: r = √(A/π)
Then it applies the standard formulas: d = 2r, C = 2πr and A = πr².
Worked examples
From a tape measure, step by step. A tape around a post reads 50 cm.
- Find the radius: r = C ÷ (2π) = 50 ÷ 6.283185307 = 7.957747155.
- Double it for the diameter: 15.91549431 cm.
- Area: π × 7.957747155² = 198.9436789 cm².
The Formulas box opens with r = C/(2π) = 50/(2π) = 7.957747155. So the post is about 15.9 cm across.
From the diameter. A round table is 30 inches across. The radius is 15, the circumference is 94.24777961 in, and the top has Area: 706.8583471 square inches.
From the area. A circular flower bed must cover 50 m². The tool gives Radius: 3.989422804, so a rope of about 3.99 m pegged at the centre marks out the edge. The edge will be 25.06628275 m long.
Limits and tips
- Pick the right option in I know the. A value of 30 treated as a radius instead of a diameter doubles every length and quadruples the area.
- Results show up to 10 significant figures. Round at the end.
- The calculation runs in your browser.
Frequently asked questions
How do I find the diameter from the circumference?
What value of π does the tool use?
Why does doubling the radius make the area four times bigger?
Does it handle units?
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