# How to Find the Area of a Circle Using Radius

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To find the area of a circle, you take pi times the radius squared, or A = pi r^2. Using this formula, you can find the area of a circle if you know the radius -- or the diameter -- by plugging in your values and solving for A. Pi is approximated as 3.14.

## Diameter

Even if you're given the diameter of a circle, you can still find the area of a circle using the radius. The radius is one half the diameter, so to get the radius, divide the diameter by 2, plug in the result and solve. If you have a diameter of 16, your radius is 8. Square 8 to get 64, then multiply by pi: A = pi x 64. This gives you an area of 201.06.

## Fractions

No matter what your diameter looks like, you will still divide the diameter in half to find the radius. If your diameter is a fraction, such as 5/9, use properties of fractions to write it with a numerator divisible by two, and then divide. The fraction 5/9 becomes 10/18, and the radius is 5/18. Square 5/18 to get 25/324 by squaring the top and bottom of the fraction. Your formula becomes A = pi (25/324). Simplified, your answer is A = 0.24, rounded.