Diameter of a Circle Formula
Geometry Formulas: Area, Volume, Perimeter
What is the formula for the diameter of a circle?
Answer
Diameter of a circle
d = 2 · r
Here r is the radius. The result comes out in units.
Where the formula comes from
d = 2 · r
The diameter is any straight line through the centre from edge to edge, and it is made of two radii laid end to end. If what you know is the circumference instead, d = C/π; if it is the area, d = 2·√(A/π).
Worked examples
Let r = 5. Then the diameter of a circle = 10 units.
Let r = 3.2. Then the diameter of a circle = 6.4 units.
The common mistake
Treating the widest measurement of an oval as THE diameter. Both a circle and an ellipse have endlessly many chords through the centre; the circle's are all the same length, which is why it has a single number, while the ellipse's run from the short axis to the long one.
Other quantities: Circle
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- Square Meters to Square Feet (Sq M to Sq Ft) - Convert Square Meters to Square Feet
- Gravel Calculator - Area to cubic yards, tons and coverage
- Topsoil Calculator - Cubic yards, weight and coverage for any area
- Multiplication Facts - Any two numbers from 1 to 9, with the working shown
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Values from the Formula
| If r = | Diameter |
|---|---|
| 1 | 2 units |
| 2 | 4 units |
| 3 | 6 units |
| 4 | 8 units |
| 5 | 10 units |
| 10 | 20 units |
Geometry Formulas: Area, Volume, Perimeter
These are the formulas for the area, perimeter, circumference, volume and surface area of the standard shapes — 31 of them. Each page shows not only the formula but where it comes from, what every letter stands for, two worked examples and the mistake that this particular shape invites. For example, it can help you find out what is the formula for the diameter of a circle? (The answer is: d = 2 · r). Pick the pair — "Area of a circle", for instance — and hit the "Find out" button.
Geometry Formulas: Area, Volume, Perimeter