Radius (r)
Apka Apna Career Saathi
Apka Apna Career Saathi
Circumference Calculator
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Formula: C = 2πr
Circumference
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CFrom r
Multiplier
6.283185
Circumference (C)
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Note
Same units
How this circumference calculator works
Enter the radius of a circle. The calculator multiplies it by 2π and shows the circumference, the distance around the circle. C = 2πr.
The radius is the straight line from the centre to the edge. The diameter is twice that length. Because the diameter is 2r, the same formula can be written C = πd. This page takes the radius, so a diameter of 15 is entered as 7.5.
π comes from JavaScript’s Math.PI, about 3.141592653589793. Twice that is about 6.283185307, and the multiplier card rounds 2π to 6.283185. The circumference is that factor times your radius, then rounded to at most six decimal places.
Worked example: the field’s sample radius is 7.5. 7.5 × 2π = 47.123889…, which the page shows as 47.12389. The result uses the same units as the radius. A radius of 7.5 cm gives a circumference of 47.12389 cm. Zero is allowed and returns 0. A negative radius is rejected. The ring fills as the radius approaches 10 and stays full above that, while the circumference keeps growing. A radius of 14 still returns 87.964594.
Circumference examples
Each value is 2π times the radius, rounded the same way as the calculator.
| Radius | Circumference | What it marks |
|---|---|---|
| 0 | 0 | A point, no distance around |
| 1 | 6.283185 | 2π × 1 |
| 2 | 12.566371 | Diameter 4 |
| 3.5 | 21.991149 | Diameter 7 |
| 7 | 43.982297 | Diameter 14 |
| 7.5 | 47.12389 | Sample input |
| 10 | 62.831853 | Ring is full |
| 14 | 87.964594 | Diameter 28 |
How to read the result
Multiply by 2π
A radius of 1 has circumference 6.283185, which is one full turn of the multiplier.
Same units
Centimetres in give centimetres out. The tool does not change the unit.
Radius, not diameter
If you measured across the circle, halve that number before you enter it.
The circle
The ring is full at a radius of 10. A radius of 14 still converts to 87.964594.
Frequently asked questions
How is the circumference of a circle calculated?
C = 2πr. Multiply the radius by 2 and then by π. This page uses JavaScript’s value of π and shows the result to at most six decimal places. A radius of 1 gives 6.283185.
What is π on this calculator?
π is Math.PI, about 3.141592653589793. The multiplier card shows 2π, which rounds to 6.283185. Circumference is that multiplier times the radius you enter.
What is the circumference when the radius is 7.5?
7.5 × 2π = 47.123889…, shown as 47.12389. The diameter of that circle is 15, because the diameter is twice the radius.
Is circumference the same as the diameter times π?
Yes. The diameter d is 2r, so C = 2πr is the same as C = πd. This page asks for the radius. If you know the diameter, enter half of it. A diameter of 14 means a radius of 7 and a circumference of 43.982297.
Does this page also calculate the area?
It returns the circumference only, the distance around the circle. Area uses a different formula, πr². For a radius of 1 the circumference is 6.283185 and the area would be about 3.141593, which this tool does not show.
What units does the result use?
The same units as the radius. If the radius is in centimetres, the circumference is in centimetres. The card labelled Same units is a reminder of that. The calculator does not convert between units.
Can the radius be zero or negative?
Zero is allowed and returns a circumference of 0. A negative radius is rejected. The field must be 0 or greater.
Why does the circle fill at a radius of 10?
The ring is a visual scale that is full when the radius reaches 10. Larger radii still convert. A radius of 14 gives a circumference of 87.964594.
How many decimal places are shown?
The result is rounded to six decimal places, and trailing zeros are dropped. A radius of 2 gives 12.566371. A radius of 0 gives 0.
How do I find the radius from a circumference?
Divide the circumference by 2π. r = C ÷ 6.283185… A circumference of 62.831853 corresponds to a radius of 10 on this page.