A while ago I uploaded a set of resources on calculating the areas of shapes on flags, see here:
A number of people messaged me asking if I had one for circles - and here is is now! I had to get creative with finding flags I could use, and I'd recommend that you have already had a lesson on Area and Circumference before this, as the questions get very challenging!
If you like this resource, then please check out the rest of my stuff! https://www.tes.com/teaching-resources/search/?f=authorId%5B2095097%5D