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# Circumference review

Review the basics of circumference and try some practice problems.

### What is circumference?

Circumference is the distance around the outside of a circle (its perimeter!).
Want a review of circle vocabulary terms (like pi, radius, and diameter)? Check out this video.

### Finding circumference of a circle

To find the circumference of a circle, we can use either of the following formulas:
$\text{Circumference}=\pi ×\text{diameter}$
$\text{Circumference}=2×\pi ×\text{radius}$

### Example 1: Finding circumference when given diameter

Find the circumference of a circle with a diameter of $3$.
The formula for the circumference of a circle that uses diameter is:
$C=\pi d$
$C=\pi 3$
$C=3\pi$
We can stop here and write our answer as $3\pi$. Or we can plug in $3.14$ for $\pi$ and multiply.
$C=3.14\cdot 3$
$C=9.42$ units
The circumference of the circle is $3\pi$ units or $9.42$ units.

### Example 2: Finding circumference when given radius

Find the circumference of a circle with a radius of $8$.
The formula for the circumference of a circle that uses radius is:
$C=2\pi r$
$C=2\pi 8$
$C=16\pi$
We can stop here and write our answer as $16\pi$. Or we can plug in $3.14$ for $\pi$ and multiply.
$C=3.14\cdot 16$
$C=50.24$ units
The circumference of the circle is $16\pi$ units or $50.24$ units.

### Practice

Problem 1
Suppose the radius of a circle is $5$. What is its circumference?
Either enter an exact answer in terms of $\pi$ or use $3.14$ for $\pi$ and enter your answer as a decimal.

Want to try some more circumference of circle problems? Check out this exercise.

## Want to join the conversation?

• To find area with circumference, you divide the circumference by pi, to get your diameter. Then, you find the radius by dividing the diameter by 2. You then multiply your radius by itself and multiply that product by pi.
Hope this Helps! :)
• why do we need to measure circles
• You don't really, but if you need to find your tire size, that's the way to do it. Also, a lot of math you won't really need for practicality, but you'll need it for high school which eventually leads up to college and your future job.
• Is the answer in the second practice question wrong?
My calculator said 9π is 28.27, not 28.26.

Ps: if it is wrong I don't see a way to report it.
• If you use π then you will get 28.27.
If you use 3.14 then you will get 28.26.
• What should I do ,I need help
• You should watch all of the videos again; they will help!

• yall why are the questions from 4 years ago-
• The questions with the most votes are 4 years old, but if you go to the Sort by: and choose Recent, you will see questions that are 9, 10, and a month ago. Recent questions generally do not get many votes.
• Who made pie because whoever did must of been really smart
• No one really made it, Archimedes of Syracuse, was the first to calculate it.
• hey i need help i don't understand can someone help me this is on my finals
• and for area mutiply the radius by itself then multiply it by pie (3.14)
• why era there so much khan academy assinments
• im sorry it was my fault
• Hello, since the weather is terrible today, and my wifi is oging in and out, can someone explain this to me thanks!
• Do you guys know the solutions to calculate the volume of a ball? I don't know, but I know that it can be solved.