What this calculator does
The diameter of a circle is the straight-line distance across it through the centre, from one edge to the opposite edge. It is twice the radius, and it is usually the easiest measurement to take directly with a tape measure or calipers, since finding the exact centre point to measure a radius is often harder in practice.
This calculator leads with the diameter, worked out from whichever measurement you already have: the radius, the area, or the circumference. Enter one of the three and it converts straight to the diameter, with the other measurements shown underneath for reference.
The formula
The diameter is simply twice the radius: d = 2r. If you know the area instead, the radius is recovered first with r = √(area/π), and if you know the circumference, r = circumference/2π; either way the diameter follows from doubling that radius.
| Term | Meaning |
|---|---|
| d | Diameter: the distance across the circle through the centre, d = 2r. |
| r | Radius: the distance from the centre to the edge, half the diameter. |
| π | Pi, approximately 3.14159, the constant ratio linking a circle’s diameter to its circumference. |
The inputs explained
| Field | What to enter |
|---|---|
| Value | Enter the radius, area or circumference, whichever measurement you already have. |
| This value is the | Select which measurement the value above represents. |
When to use it
Specifying a pipe, pot or drum
Plumbing, planters and storage drums are almost always sized by diameter, so converting a measured radius or circumference into a diameter matches the units the product is actually sold in.
Working from a measured circumference
Wrapping a tape measure around a tree trunk, pipe or tank gives the circumference directly; converting that to a diameter is often the number actually needed for a fit or a cut.
Checking a stated area against a diameter
If a circular surface is specified only by its area, such as a tabletop or a hole, working back to the diameter tells you the size to actually order or cut.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does diameter change with the radius?
The diameter, circumference and area for a range of radius values.
| Radius | Diameter | Circumference | Area |
|---|---|---|---|
| 1 | 2.000 | 6.283 | 3.142 |
| 2 | 4.000 | 12.566 | 12.566 |
| 5 | 10.000 | 31.416 | 78.540 |
| 10 | 20.000 | 62.832 | 314.159 |
| 20 | 40.000 | 125.664 | 1,256.64 |
| 50 | 100.000 | 314.159 | 7,853.98 |
Working back to diameter from area or circumference
A radius-5 circle’s area and circumference, each entered back in to check the diameter recovered matches.
| The value entered is the | Diameter | Radius |
|---|---|---|
| Area | 2.523 | 1.262 |
| Circumference | 1.592 | 0.7958 |
Questions
What is a diameter?
The diameter is the distance straight across a circle, passing through its centre, from one side to the other. It is the widest measurement a circle has.
What is the diameter formula?
Diameter equals twice the radius: d = 2r. If you know the area or circumference instead, work out the radius first, then double it.
What is the diameter of a circle if you only know the area?
Divide the area by π and take the square root to get the radius, then double that figure for the diameter: d = 2√(area/π).
Is diameter the same as radius?
No. The radius runs from the centre to the edge; the diameter runs all the way across through the centre and is exactly twice the radius.
For the circumference or the area as the headline figure instead, see the circumference calculator or the area of a circle calculator. For every circle measurement from a single input at once, see the full circle calculator.