(It works better in America, where they put the month before the day so it’s 3/14. Like pi. Get it? Uh? UH?!)

Today’s a day for maths geeks around the world to celebrate the most famous number since 2. We’re talking about pi – **the ratio of the circumference of a circle to its diameter** – which goes on forever and ever, amen, without pattern or repetition.

But even if you’ve memorised that bolded description, what does it actually mean? And why is it useful? And can we please have pies for lunch?

**Pi is a constant number**

It doesn’t matter how big a circle is, its length will always be slightly more than three times its width. Knowing this means instead of trying to measure the actual circle with a ruler, and bursting into tears because it’s impossible, you can measure the diameter and times it by pi (or π, as maths people write it).

**Pi is useful for measurement**

From there, you can use it to work out the area of a circle (A=πr²) and the volume of a cylinder (V=πr²h). If equations stress you out, take a deep breath and break them down.

**A=****πr²**

A means “area”. It’s what we’re trying to find.

On the other side of the equals sign, we’re taking the radius of our circle and squaring it – that’s the r². Then we’re multiplying that by π to get our answer. It’s a useful step to figuring out other stuff, like…

**V=****πr²h**

Here, V means “volume”. So we do everything we did with the equation above, then multiply it by the height of our cylinder – the h – to find out how much space is in this pipe, toilet paper roll or pie dish.

**Pi is delicious**

If you already know that cooking is a kind of science, you won’t be surprised that those maths geeks we mentioned go all in on baking pi pies. So feel free to make a perfectly circular treat in that pie dish you just measured…

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