Diameter Calculator

What Is a Diameter Calculator?

A diameter calculator determines the diameter of a circle using a measurement that is already known. Depending on the information available, the diameter can be calculated from the circle's radius, circumference or area.

The diameter is the straight-line distance across a circle through its centre. It is an important measurement in geometry and is commonly used in mathematics, engineering, construction, manufacturing and other technical applications.

Why Use the Diameter Calculator?

This free online diameter calculator provides a straightforward way to calculate the diameter without performing the formulas manually.

How to Use the Diameter Calculator

Select the measurement you know, enter its value and choose the appropriate unit. The calculator will then determine the corresponding diameter.

1. Select a calculation method — Choose From Radius, From Circumference or From Area.
2. Enter the measurement — Type the known positive measurement into the input field.
3. Select the unit — Choose the unit that matches the value you entered.
4. Calculate the diameter — Select Calculate Diameter to display the result and formula used.

How to Calculate the Diameter of a Circle

The formula used to calculate a circle's diameter depends on which measurement is known. If the radius is known, the diameter is simply twice the radius. If the circumference is known, divide the circumference by π. If the area is known, take the square root of the area divided by π and multiply the result by two.

For example, a circle with a radius of 5 centimetres has a diameter of 10 centimetres. A circle with a circumference of approximately 31.416 centimetres also has a diameter of approximately 10 centimetres.

Diameter From Radius

The simplest diameter calculation uses the radius. The formula is d = 2r, where d represents diameter and r represents radius. If the radius is 8 metres, the diameter is 16 metres.

Diameter From Circumference or Area

If the circumference is known, use d = C ÷ π, where C is the circumference. If the area is known, use d = 2 × √(A ÷ π), where A is the area. The units of the resulting diameter depend on the units used for the original measurement.

Diameter Calculation Examples

The examples below show how the diameter changes according to the known measurement. When calculating from area, the input represents a square measurement, while the resulting diameter is a linear measurement.

Results are calculated using the mathematical constant π. The calculator displays a suitably precise result based on the value entered, while the underlying calculation uses JavaScript's mathematical value of π.

Frequently Asked Questions