What this calculator does
The diameter of a circle is often the one measurement already known, printed on a pipe, a wheel or a lid, or taken directly with calipers across the widest point. From that single figure, every other property of the circle follows.
This calculator starts from the diameter of a circle you already have and works outward: the radius, the circumference and the area, all returned from that one input, rather than asking you to convert to a radius first.
The formula
The radius is simply half the diameter: r = d/2. Circumference follows directly from the diameter as C = πd, without needing the radius as an intermediate step. Area uses the radius once it has been halved, A = πr², or equivalently A = π(d/2)².
| Term | Meaning |
|---|---|
| d | Diameter: the distance straight across the circle through its centre. |
| r | Radius: half the diameter, the distance from the centre to the edge. |
| π | Pi, approximately 3.14159, linking the diameter to both circumference and area. |
The inputs explained
| Field | What to enter |
|---|---|
| Diameter | The diameter of the circle: the full distance across it through the centre. |
When to use it
Working from a labelled pipe or drum diameter
Plumbing fittings, pots and drums are almost always specified by diameter, so starting from that figure and getting the radius, circumference and area in one step saves a manual conversion.
Checking area of circle with diameter for a costing sheet
Turf, glass or paving priced by area needs the area of circle with diameter converted through the radius first; this calculator does that conversion automatically from the diameter alone.
Answering a circle diameter homework question
A geometry problem that states the diameter and asks for the radius, circumference or area is solved directly here without a separate halving step written out by hand.
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.
Diameter of circle formula across a range of sizes
The radius, circumference and area worked out from a range of diameter values.
| Diameter | Radius | Circumference | Area |
|---|---|---|---|
| 1 | 0.5000 | 3.142 | 0.7854 |
| 2 | 1.000 | 6.283 | 3.142 |
| 5 | 2.500 | 15.708 | 19.635 |
| 10 | 5.000 | 31.416 | 78.540 |
| 20 | 10.000 | 62.832 | 314.159 |
| 50 | 25.000 | 157.080 | 1,963.50 |
Questions
What is the diameter of a circle?
The diameter is the straight-line distance across a circle, passing through its centre, from one edge to the opposite edge. It is the widest measurement a circle has, and exactly twice the radius.
How do you find the radius from the diameter of a circle?
Divide the diameter by 2. A diameter of 10 gives a radius of 5, since the radius is always exactly half the diameter.
What is the area of a circle diameter formula?
A = π(d/2)², squaring half the diameter and multiplying by π. It gives the same result as the more familiar A = πr² once the radius has been found from the diameter.
How do you find circle diameter from a measured circumference instead?
Divide the circumference by π: d = C/π. This calculator instead assumes the diameter is already known and works outward to the other measurements from there.
For a calculator that starts from a radius, area or circumference and works toward the diameter instead, see the diameter calculator. For circumference as the headline figure from a radius or diameter, see the circumference calculator.