Circumference of a Circle Calculator
The circumference of a circle is C = 2 π r, the distance once around the edge, and this circumference calculator works it out the moment you enter a radius. You can also start from the diameter or the area, switch to the perimeter of a semicircle, and read the answer in inches, feet, centimeters, and meters. Circumference is just another word for the perimeter of a circle, and the calculator shows the formula with every result.
- From radius, diameter, or area
- C = 2πr or πd
- Semicircle perimeter
- Many length units
- Formula shown
Last updated June 15, 2026 Method: C = 2πr Reviewed by the Calcowa math team
Enter a positive value to see the circumference.
Show all length units
C = 2π × 5 = 31.42 in
What is the circumference of a circle?
The circumference of a circle is the distance once around its edge, found with C = 2 × π × r, where r is the radius. It's the circle's perimeter. For a radius of 5 inches, the circumference is about 31.4 inches.
Pi is the link: every circle's circumference is exactly pi times its diameter, which is why C = πd, and since the diameter is twice the radius, that's the same as 2πr. If you've got the area instead, you can work back to the radius first, which the calculator does for you.
How do you find the circumference of a circle?
To find the circumference, multiply 2 times pi by the radius, or pi by the diameter. Here's the full sequence:
- 1
Find the radius or diameterMeasure the radius, or the diameter across the middle.
- 2
Multiply by piFor the diameter, multiply by pi: C = πd.
- 3
Or use 2πrFor the radius, multiply by 2 and by pi: C = 2πr.
- 4
Convert the unitsConvert to feet, meters, or other units if that's what you need.
Circumference of a circle with the diameter
If you measured straight across the middle, you have the diameter, and the circumference is simply C = π × d. There's no halving to do, because the formula already uses the full width. For a diameter of 10 inches, the circumference is π × 10, about 31.4 inches. Set the known value to Diameter and the calculator uses πd for you.
Circumference of a circle with the radius (2πr)
When you know the radius, the distance from the center to the edge, the circumference is C = 2 × π × r. That 2πr is the same as πd, since the diameter is two radii. Double the radius, multiply by pi, and you've got the way around. To go the other way and find the radius from the circumference, use r = C ÷ (2π).
Perimeter of a semicircle
A semicircle's perimeter isn't just half the circumference, because the straight edge across the middle counts too. It's the curved half, πr, plus the diameter, 2r, so the perimeter of a semicircle is πr + 2r, or πr + d. For a radius of 5, that's about 15.7 + 10 = 25.7. Switch the Portion to Semicircle and the calculator adds the flat edge for you.
Circumference and area of a circle
These two get mixed up because both use pi and the radius. The circumference, 2πr, is the distance around the edge, measured in plain length units. The area, πr², is the space inside, measured in square units. One is a length, the other a length squared. This calculator shows the area beside the circumference, and the area of a circle calculator covers the space inside in full.
A circumference example
Say you've got a round pool with a diameter of 8 meters and you want the trim around the edge. Multiply the diameter by pi to get the circumference straight away.
C = π × 8 = 8π ≈ 25.13 m
that's the edging you'd need to go all the way around
Set the known value to Diameter, type 8 in meters above, and you'll get the matching feet and inches at once.
Units and accuracy
Calcowa shows the circumference in millimeters, centimeters, meters, inches, feet, and yards all at once, plus the exact answer in terms of pi. The results use the full value of pi, not a rounded 3.14, so they're accurate for fencing, sewing, engineering, and school work alike.
| Unit | Best for | Good to know |
|---|---|---|
| Inches (in) | Small circles, pipes, lids | Default when you enter inches |
| Feet (ft) | Pools, tables, garden beds | 1 ft = 12 in |
| Centimeters (cm) | Lab and school work | 1 in = 2.54 cm |
| Meters (m) | Large circles and tracks | 1 m = 3.281 ft |
| In terms of pi (π) | Exact math answers | Leaves the result as a multiple of pi |
Frequently asked questions
What does circumference mean?
Circumference is the distance all the way around a circle, its perimeter. If you could unroll the edge into a straight line, the circumference is how long that line would be, and it always equals pi times the diameter.
The circumference of a circle is C = 2 × π × r, or C = π × d using the diameter. For a radius of 5 inches, that's 2 × π × 5, about 31.4 inches. It's the distance once around the edge.
Multiply the diameter by pi: C = π × d. The diameter already spans the full width, so you don't halve anything. Set the known value to Diameter in the calculator and it uses πd straight away.
Rearrange the formula to r = C ÷ (2π). Divide the circumference by 2 times pi and you've got the radius. The diameter is then twice that, or just C ÷ π.
Yes. The perimeter of a circle is its circumference, the distance all the way around. We use the word circumference for circles and perimeter for straight-sided shapes, but for a circle they mean the same length, 2πr.
A semicircle is half a circle plus the straight edge across it, so its perimeter is half the circumference plus the diameter: πr + 2r, or πr + d. It's not just half the circumference, because the flat side counts too. Switch the Portion to Semicircle to see it.
A sphere's circumference is the distance around its widest circle, the great circle, which is C = 2πr just like a flat circle. So the same formula gives the circumference of a ball, a planet, or any sphere.
2πr is the formula for the circumference of a circle: 2 times pi times the radius. It comes from the fact that the circumference is always pi times the diameter, and the diameter is twice the radius, so πd becomes 2πr.
Related calculators
Working with circles and round solids? These geometry tools pair well with the circumference.
The space inside the same circle.
Cylinder volumeA circle base pushed up a height.
Sphere surface areaThe skin of a ball, also from r.
Need a circumference fast?
Try the calculator above, or browse every shape in the geometry hub.