Circle Calculator · Practical guide
Circumference to Diameter Calculator
Convert a measured circumference back to a diameter with one division.
At a glance
The Short Answer
Diameter equals circumference divided by pi: d = C ÷ π. A circle with a circumference of 31.4159 inches has a diameter of almost exactly 10 inches.
Interactive tool
Circle Calculator
Enter your own values, then compare the result with the examples below.
Results
Why It Works
Circumference is defined as C = πd — the distance around the circle is always about 3.14159 times the distance across it. Rearranging the formula gives the diameter directly. If you also need the radius, divide the circumference by 2π instead: r = C ÷ (2π).
Worked Examples
| Circumference | Diameter | Radius |
|---|---|---|
| 31.4159 in | 10 in | 5 in |
| 62.8319 ft | 20 ft | 10 ft |
| 100 cm | 31.831 cm | 15.9155 cm |
Using the Calculator Above
The tool accepts radius or diameter, so do the single division first — C ÷ π — and enter the result as the diameter. It then reports the matching radius, area, and circumference (which should match your original measurement, confirming the division). This is handy when you have wrapped a tape measure around a tree trunk, pipe, or circular rug and need the straight-across measurement.
A Practical Note on Measuring
Real-world circumference measurements are approximate — a tape that sags or rides over bumps inflates the reading. Measure snugly, and treat the resulting diameter as an estimate rounded to a sensible precision rather than an exact figure.
Frequently Asked Questions
What is the diameter of a circle with a 100 cm circumference?
Divide by pi: 100 ÷ 3.14159 ≈ 31.831 cm.
How do I get the radius from the circumference?
Divide the circumference by 2π (about 6.28318). A circumference of 31.4159 gives a radius of 5.
Is the ratio really the same for every circle?
Yes. That constancy is the definition of pi — every circle's circumference is exactly π times its diameter, regardless of size.
Can this calculator take circumference directly?
It takes radius or diameter, so divide your circumference by π once, then enter the result as the diameter to see every other property of the circle.