Quick answer
Diameter = circumference ÷ π. A circumference of 31.4159 gives a diameter of exactly 10 (since 31.4159 = 10π).
Enter the circumference. The calculator returns the diameter.
Related tools: Circumference Calculator: Circumference of a circle. Circle Diameter Calculator: Diameter of a circle from radius, circumference or area. Diameter to Radius Calculator: Convert a circle's diameter to its radius.
How to use the Circumference to Diameter Calculator
- Enter circumference (above 0).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Circumference | A number | > 0 |
Formula and method
Formula
d = C ÷ π
IEEE-754 double precision; shown to at most 6 decimal places.
Direct application of the circle relationships.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Circumference = 31.41592653589793.
| Result | Value |
|---|---|
| Circumference to Diameter | 10 |
Example 2
Inputs: Circumference = 1.
| Result | Value |
|---|---|
| Circumference to Diameter | 0.31831 |
The first line is the headline result (circumference to Diameter). IEEE-754 double precision; shown to at most 6 decimal places.
A common use
Wrap a tape around a pipe, tree or pole and divide by π. A 62.8 cm girth means a diameter of 20 cm. It is easier than measuring across the middle of a round object.
Accuracy
Use π ≈ 3.14159 for accuracy; 3.14 gives an error of about 0.05%. The value must be positive and in the same units as the answer.
Measuring round objects
d = C ÷ π. A tape measure around a tree trunk reading 94.2 cm gives a diameter of about 30 cm, and a finger circumference of 54 mm is about 17.2 mm across.
Measurement error carries through: 1 mm of error in C is about 0.3 mm in d.
How the result changes with circumference
Holding every other input at the first worked example, this table changes only circumference. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Circumference | Circumference to Diameter |
|---|---|
| 15.71 | 5.000648 |
| 31.42 | 10.001297 |
| 62.83 | 19.99941 |
| 125.7 | 40.011553 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Circumference: entering "0" shows "Circumference must be greater than 0." beside that field and no result.
Choosing between the Circumference to Diameter Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Circumference Calculator](/math/circumference/) | i know the and value | Circumference |
| [Circle Diameter Calculator](/math/circle-diameter/) | i know the and value | Diameter |
| [Diameter to Radius Calculator](/math/diameter-to-radius/) | diameter | Diameter to Radius |
| [Circumference to Radius Calculator](/math/circumference-to-radius/) | circumference | Circumference to Radius |
| [Triangle Calculator](/math/triangle-calculator/) | side a, side b and side c | Area |
Background: geometry measurements
Geometry results carry units that depend on the quantity: a perimeter or length is in linear units such as cm, an area or surface area is in square units (cm²) and a volume is in cubic units (cm³). Use one length unit for every input, because the calculator multiplies the numbers as given.
Flat (2D) shapes have an area and a perimeter; solids (3D) have a surface area and a volume. Two familiar checks: a circle of radius r has area πr² and circumference 2πr, and a sphere has volume (4/3)πr³ and surface area 4πr².
Assumptions and limitations
- Value must be positive
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for circumference to confirm this.
Frequently asked questions
What is the diameter if the circumference is 100?
A circumference of 100 gives d = 100 ÷ π ≈ 31.83.
How do I get the radius?
Divide the diameter by 2.
Does it work for ellipses?
No. The formula d = C ÷ π only applies to circles; an ellipse needs its two axes.
How accurate is it?
As accurate as the measurement. An error of 1 mm in the circumference gives about 0.3 mm in the diameter.
What does the Circumference to Diameter Calculator calculate?
Convert a circle's circumference to its diameter. It returns Circumference to Diameter.
What formula does the Circumference to Diameter Calculator use?
d = C ÷ π. Direct application of the circle relationships.
What do I need to enter in the circumference to Diameter Calculator?
You enter Circumference.
What is an example of a circumference to Diameter result?
With circumference 31.41592653589793, the circumference to Diameter is 10.
Related calculators
- Circumference Calculator — Circumference of a circle.
- Circle Diameter Calculator — Diameter of a circle from radius, circumference or area.
- Diameter to Radius Calculator — Convert a circle's diameter to its radius.
- Circumference to Radius Calculator — Convert a circle's circumference to its radius.
- Triangle Calculator — Area, perimeter, angles and type of a triangle from its three sides.
- Circle Calculator — Radius, diameter, circumference and area of a circle from any one of them.
- Midpoint Calculator — Midpoint of a line segment.
- Endpoint Calculator — Other endpoint of a segment from its midpoint and one endpoint.
- Rise Over Run Calculator — Slope, grade and angle from rise and run.
Sources
- Standard Euclidean geometry formulas (e.g. NIST Digital Library of Mathematical Functions §4, CRC Standard Mathematical Tables)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.