Diameter of a Circle Calculator

Enter a circle's radius, area, or circumference, select which measurement you know, and this calculator finds the diameter, plus the matching radius, circumference, and area, using standard geometry formulas.

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Method verified  Standard Euclidean geometry formulas for a circle.

1. What Do You Know?

units

2. About the Formula

From radiusd = 2r
From aread = 2 x sqrt(A / pi)
From circumferenced = C / pi
Value of pi used3.14159265

Your Circle's Dimensions

Diameter
-
d = 2r
Radius
-
r = d / 2
Circumference
-
C = pi x d
Area
-
A = pi r²

Calculation Steps

You entered-
Formula used-
Diameter-

All Circle Measurements

Radius-
Diameter-
Circumference-
Area-

How to Find the Diameter of a Circle

The diameter of a circle is the straight-line distance across the circle, passing through its centre. It is the longest chord you can draw inside a circle. Depending on what you already know about the circle, there are three common ways to work out its diameter.

From the Radius

This is the simplest method. The radius is the distance from the centre of the circle to any point on its edge, and the diameter is always exactly twice the radius:

d = 2r

For example, if the radius is 5 cm, the diameter is 2 x 5 = 10 cm.

From the Area

The area of a circle is given by A = pi r^2. To find the diameter from the area, first rearrange this formula to solve for the radius, then double the result:

r = sqrt(A / pi) and then d = 2r

For example, if the area is 78.54 square cm, dividing by pi (3.14159) gives 25, and the square root of 25 is 5. Doubling gives a diameter of 10 cm.

From the Circumference

The circumference is the total distance around the outside of the circle, given by C = pi x d. Since the diameter and circumference are directly proportional, you can find the diameter by dividing the circumference by pi:

d = C / pi

For example, if the circumference is 31.42 cm, dividing by pi gives a diameter of 10 cm.

Diameter Formulas at a Glance

You knowFormula for diameterExample
Radius (r)d = 2rr = 5 → d = 10
Area (A)d = 2 x sqrt(A / pi)A = 78.54 → d = 10
Circumference (C)d = C / piC = 31.42 → d = 10

Once you have the diameter, you can easily find the other measurements: the radius is half the diameter, the circumference is pi multiplied by the diameter, and the area is pi multiplied by the radius squared.

Where This Is Used

Finding the diameter of a circle is a common task in maths homework, engineering drawings, construction (such as pipe or duct sizing), and DIY projects (such as cutting a circular piece of material). Knowing the diameter lets you quickly work out how much material fits inside a circular space, or check that a round object will fit through a gap.

Related Calculators

Sources: Standard Euclidean geometry formulas for circles (National Institute of Standards and Technology, NIST Digital Library of Mathematical Functions, dlmf.nist.gov).

This calculator uses pi to 8 decimal places (3.14159265) for accuracy. Results are rounded to the number of decimal places you select. Units are not converted, so the diameter is returned in the same units as the value you enter.

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