Sometimes you can measure the distance around a circular object with a tape measure but need the diameter — this solves that reverse-lookup question.
How it works
Dividing the circumference by pi gives the diameter; halving the diameter gives the radius.
What this does not include
This does not include measurement error correction — a tape measure wrapped around an imperfectly circular object introduces some inaccuracy that this calculator can’t detect or adjust for.
How to use this calculator
- Enter the measured circumference.
A worked example
A circumference of 31.4159: diameter = circumference ÷ π = 10.
What the variables mean
| Variable | Meaning |
|---|---|
| Circumference | Distance around the circle |
| Diameter | Distance across the circle through its center |
Edge cases worth knowing
This is the direct reverse of the standard circumference formula (C = πd) — dividing by π instead of multiplying by it.
A circumference of zero makes the diameter zero too, a degenerate but valid case the calculator declines to show since there’s no real circle to describe.
Frequently asked questions
Why measure circumference instead of diameter directly?
For many round objects (a tree trunk, a pipe, a round table), wrapping a tape measure around the outside is much easier and more accurate than trying to measure straight across the middle.
How is this different from the site’s circumference calculator?
That one goes the other direction — starting from radius or diameter to find circumference; this one starts from circumference to find diameter and radius instead.
Does the unit of measurement matter?
No — the result comes out in the same unit as whatever circumference you entered, whether inches, centimeters, or feet.
