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
Related Calculations
See Also
- Square Footage Calculator - Width × length in feet to square feet
- Square Feet to Square Meters (Sq Ft to Sq M) - Convert Square Feet to Square Meters
- 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
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
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 |
FAQ
What is the formula for the diameter of a circle?
Diameter of a circle d = 2 · r
Where does this formula come from?
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).
What is the most common mistake here?
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.
How do I use this formula?
Put your own numbers in place of the letters. With r = 3.2, for instance, the formula d = 2 · r gives = 6.4 units.
See Also
- Area of a Hexagon Formula
- Area of a Parallelogram Formula
- Area of a Regular Polygon Formula
- Area of a Rectangle Formula
- Area of a Rhombus Formula
- Area of a Circular Sector Formula
- Area of a Square Formula
- Area of a Trapezoid Formula
- Area of a Triangle Formula
- Circumference of a Circle Formula
- Perimeter of a Regular Polygon Formula
- Perimeter of a Rectangle Formula
- Perimeter of a Semicircle Formula
- Perimeter of a Square Formula
- Perimeter of a Triangle Formula
- Surface Area of a Cone Formula
- Surface Area of a Cube Formula
- Surface Area of a Cylinder Formula
- Surface Area of a Pyramid Formula
- Surface Area of a Rectangular Prism Formula
- Диаметр круга
- Diámetro del círculo
- Diâmetro do círculo