Circle Formula Calculator
Table of contents
Circle formulasHow to use the area and circumference of a circle formula — An exampleOther helpful tools like the circle formula calculatorFAQsThe circle formula calculator allows you to make circle calculations easily. Keep reading to learn things like:
 The formula to calculate the diameter of a circle given its radius, or vice versa.
 What is the formula for calculating the circumference of a circle?
 What is the formula for the area of a circle?
Circle formulas
Although using the circle formula calculator is the fastest way to make circle calculations, it's crucial to know the formulas to understand the process better.
Formula to calculate the diameter of a circle given its radius, or vice versa
The formula of the diameter of a circle is the most straightforward of the equations and says that diameter (d
) equals twice the radius (r
):
d = 2r
r = d/2
The calculation of circumference and area of a circle formulas are more tricky. Look at the equations in the following sections.
Formula to calculate the circumference of a circle
The formula of the circle circumference (c
) can be expressed in terms of radius, diameter, and area (A
):
c = 2πr
c = πd
c = 2√(πA)
Formula to calculate the area of a circle
We can express the formula to calculate the area of a circle in terms of the radius, diameter, and circumference:
A = πr²
A = πd²/4
A = c²/(4π)
How to use the area and circumference of a circle formula — An example
Suppose you have a circle of radius 3 cm. If you want to calculate its area and circumference, you can do it by following these steps:

Input the radius in the circle area formula:
A = π × (3 cm)² = 28.2743 cm²

Also, input the radius in the circumference formula:
c = 2π × (3 cm) = 18.8496 cm

Check your results with the circle formula calculator. The results should also be
28.2743 cm²
and18.8496 cm
for area and circumference.
Other helpful tools like the circle formula calculator
Now that you know how to calculate the formula of circumference and area of a circle, take a look 👀 at other calculators:
 Circle calc: find c, d, a, r;
 Circle measurements calculator;
 Circumference to diameter calculator;
 Radius of a circle calculator;
 Circle length calculator;
 Circumference and area of a circle calculator;
 Diameter of a circle calculator;
 Circle perimeter calculator;
 Square footage of a circle calculator; and
 Square inches of a circle calculator.
What is the formula for the perimeter of a halfcircle?
π (pi) times radius p_{half} = πr. The formula for the perimeter of a halfcircle is the same formula for the circle circumference, divided by two. That results in the product of π (pi) and the radius.
What is the radius of a 3.65inch diameter circle?
1.825 in. The formula for the diameter of a circle says the diameter is twice the radius. To find the radius, simply divide the diameter by two: r = 3.65 in / 2 = 1.825 in
.