Unfortunately, we don't have a π symbol in emoji on iOS. So in order to send that symbol you'll either have to copy it from the text of this article and paste it in … or you can do what we did in the first place: add the Greek keyboard to your iPhone and type it from there!
The steps are simple. Head to Settings > General > Keyboards > Keyboards, tap Add New Keyboard… and choose Greek from the list.
Add a Greek keyboard to your iPhone to enable access to the letter Pi.
Now that you've got the Greek keyboard available, simply switch to it by tapping the Globe icon down in the lower-left corner of your iOS keyboard and, when you cycle through to Greek, tap π!
Wish your friends a Happy π Day by choosing the letter from the Greek keyboard
Happy Pi Day! Or, well, Happy π Day!