Circumference to Diameter Calculator
Created by: Emma Collins
Last updated:
To convert circumference to diameter, divide by pi (3.14159). A circle 26 inches around is 8.28 inches across, and one 16 cm around is 5.09 cm across. The answer comes out in the same unit you measured. Enter a circumference below to get the diameter, radius and area.
Circumference to Diameter Calculator
MathDivide the circumference by π (3.14159) to get the diameter.
Circumference to diameter chart
Diameter = circumference ÷ π. Works in any unit: inches in, inches out.
| Circumference | Diameter | Radius |
|---|---|---|
| 1 | 0.318 | 0.159 |
| 2 | 0.637 | 0.318 |
| 3 | 0.955 | 0.477 |
| 4 | 1.273 | 0.637 |
| 4.5 | 1.432 | 0.716 |
| 5 | 1.592 | 0.796 |
| 6 | 1.910 | 0.955 |
| 7 | 2.228 | 1.114 |
| 8 | 2.546 | 1.273 |
| 9 | 2.865 | 1.432 |
| 10 | 3.183 | 1.592 |
| 12 | 3.820 | 1.910 |
| 14 | 4.456 | 2.228 |
| 15 | 4.775 | 2.387 |
| 16 | 5.093 | 2.546 |
| 18 | 5.730 | 2.865 |
| 20 | 6.366 | 3.183 |
| 22 | 7.003 | 3.501 |
| 24 | 7.639 | 3.820 |
| 26 | 8.276 | 4.138 |
| 28 | 8.913 | 4.456 |
| 30 | 9.549 | 4.775 |
| 36 | 11.459 | 5.730 |
| 40 | 12.732 | 6.366 |
| 48 | 15.279 | 7.639 |
| 50 | 15.915 | 7.958 |
| 60 | 19.099 | 9.549 |
| 72 | 22.918 | 11.459 |
| 100 | 31.831 | 15.915 |
How to find diameter from circumference
d = C ÷ π
r = C ÷ 2π
Pi is the ratio of any circle's circumference to its diameter, about 3.14159. So the diameter is always a little under a third of the circumference. For a quick estimate, divide by 3 and take off about 5%.
Worked examples
- A bicycle wheel 84 inches around: 84 ÷ 3.14159 = 26.7 inches across.
- A tree trunk 150 cm around: 150 ÷ 3.14159 = 47.7 cm across.
- A ring size with a 54 mm inside circumference: 54 ÷ 3.14159 = 17.2 mm inside diameter.
Frequently Asked Questions
How do you convert circumference to diameter?
Divide the circumference by pi (3.14159). A circle 26 inches around is 26 ÷ 3.14159 = 8.28 inches across. The answer is in the same unit as the circumference.
What is the formula for diameter from circumference?
d = C ÷ π. It comes from the circumference formula C = π × d. The radius is half of that: r = C ÷ (2π).
What is the diameter of a 16 cm circumference?
16 cm ÷ 3.14159 = 5.09 cm.
What is the diameter of an 8 inch circumference?
8 inches ÷ 3.14159 = 2.55 inches.
How do I measure the diameter of a tree or post?
Wrap a tape measure around it to get the circumference, then divide by 3.14159. Foresters measure trees this way at 4.5 feet above the ground, called diameter at breast height.
Is perimeter the same as circumference?
For a circle, yes. Circumference is the name for a circle’s perimeter, so "perimeter to diameter" uses the same formula.
Sources and References
- NIST Special Publication 811. Unit conventions for lengths and areas.