Video narration
Is a pizza measured by the space inside, or the distance around its crust? That mix-up matters. Trace your finger all the way around a plate. That distance is the circumference. Now measure straight across the middle, from edge to edge. That is the diameter-not the radius. The radius goes from the centre to one edge, so the diameter is two radii: d equals 2r. Here's the flip: for every circle, the distance around is pi times the distance across. Circumference, C, equals pi times diameter, d: C equals πd. Pi is the same circle constant every time, approximately 3.14. If you know the radius, replace d with 2r. So C equals 2πr. Don't measure the space inside the pizza for circumference. Measure around its outside: multiply the diameter by π, or the radius by 2π. Keep the same units, such as centimetres or metres.