To calculate the area of a circle, multiply the number pi, which is approximately 3.14, by the squared radius. Determine the area of a circle, which will always be an approximation, with instructions from a college-level math teacher in this free video on geometry.

## Video transcription

So how does one calculate the area of a circle? Hi, I'm Jimmy Chang, I've been teaching college mathematics for nine years and calculating the area of a circle is relatively straight forward as long as you A, know the formula and B, know what the radius of a circle is because that's what it takes to figure out the area. Now, the area of a circle formula is A equals Pi r squared or r is the radius. So as I mentioned earlier, as long as you know the radius finding the area of a circle should be pretty straight forward. So, here's an example. Suppose you're given a circle, and you're given that the radius is three and you want to find the area. All you need to do is plug in the three where the r is and you'll be all set. So you have A is equal to Pi times three squared. Now, three squared is nine, so your actual answer, your exact answer is going to be nine Pi. So that's the exact value for area. But if you want to find it in approximate decimal form, then all you need to do is replace Pi with either the exact value on your calculator or you can use three point one four as an approximation and you'll find out what the area of the circle that way. Now, suppose they don't give you the radius outright but they give you something else. Let's just say you have another circle or you're given that the diameter or the distance across is ten. Well, I think you'll see that if the distance across the circle is ten then the radius will just be half of that, so the radius here would be five. And then finding the area should be pretty straight forward from there too. So all you need to do is plug in five where the r is. So A is equal to Pi times five squared. Now, five squared is twenty five, so for all intents and purposes the area of this particular circle is twenty five Pi. That is the exact value, but if you want to find the approximation of it, then just again replace Pi with three point one four and then do the multiplication and you have more approximate value for the area of that circle. So, my name is Jimmy Chang and here are a couple of examples as to how to find the area of a circle.