Listen to this song to learn the formulas for circumference and area of circles. The catchy hook helps students remember that the formula for circumference is "2 pi r" and the formula for area is "pi r squared." The song teaches terms like pi, radius and diameter as it walks students through the process of calculating the circumference and area of example circles.

*Flocabulary. Circles. You know how we roll.*

You can see me spinnin' on the car,

Circumference = 2πr

I roll with it, boy I’m not scared,

Area = πr

^{2}

Squares are square, maybe you’ve found,

I’m not that, you know I get around.

I’m a circle, you know the shape,

But let me tell you what makes me so great.

The distance from my center to my outside,

It’s the craziest, we call that radius.

All you need in your life and times,

Is the length of my radius and you need π.

Why? With those two you can find,

The circumference of a circle, and that’s no lie.

You can find the area too,

Just follow me, and do what I do.

The circumference is the length all around my outside,

To figure it out just multiply

The radius by 2 and by π.

That’s the circumference, my oh my.

You can see me spinnin' on the car,

Circumference = 2πr

I roll with it, boy I’m not scared,

Area = πr

^{2}

Hold up, you want to know how much space

You can find inside of me? Well, OK.

Memorize the formula, so it’s not a barrier,

πr

^{2}, that’s my area.

So let’s see, if my radius is 5 ft,

Then what would my area be?

πr

^{2}and r = 5 ft,

So r

^{2}= 25 ft

^{2}.

And π is close to 3.14,

So let’s use that as the value for sure.

3.14 · 25 ft

^{2}= 78.5 ft

^{2}.

Now, all you need in your life and times,

Is the length of my radius and you need π.

My radius, that’s just

^{1}⁄

_{2}my diameter,

Like a boss, 'cause you’re talking to the manager.

You can see me spinnin' on the car,

Circumference = 2πr

I roll with it, boy I’m not scared,

Area = πr

^{2}

What is the
*
radius
*
? Radius is the distance from the center of the circle to the outer edge. This is not to be confused with the
*
diameter
*
, which is the length across a circle. The radius is always one half the diameter.

What is the

*circumference*? The circumference is the perimeter of a circle, or the distance around the outside of it.

To find the circumference of a circle, use the formula 2πr. This can also be written as πD, since D = 2r.

What is pi? Pi is the relationship between a circle's diameter and its circumference. This relationship is always the same, and it's always Pi. Pi is an irrational number, which means its decimals go on forever, but it is approximately 3.14. Want to get a little more specific? Talk to Lu Chau, the student in China who was able to memorize the first 67,000 digits of pi, in order, without making a mistake.

If you have to solve a problem that involves a radius but are only given the diameter, just remember that you can still use the diameter to solve the problem, since the radius is equal to half of the diameter. The same is true if you need the diameter but are only given the radius.

To find the area of a circle, use the formula πr
^{
2
}
. It's important to remember to square the radius because area is always measured in square units.