To find the circumference of a circle, you use one of these formulas:

• C=πdC=\pi dC=πd, where ddd is the diameter of the circle.
• C=2πrC=2\pi rC=2πr, where rrr is the radius of the circle.

Since the diameter is twice the radius (d=2rd=2rd=2r), both formulas are equivalent

Steps to calculate circumference:

1. If you have the radius, multiply it by 2 to get the diameter.
2. Multiply the diameter by π\pi π (approximately 3.14) to get the circumference.

Or directly:

• Multiply the radius by 2, then multiply by π\pi π.

Example:

• For a circle with radius 9 cm: C=2×π×9=18π≈56.52C=2\times \pi \times 9=18\pi \approx 56.52C=2×π×9=18π≈56.52 cm

• For a circle with diameter 12 feet: C=π×12=12π≈37.68C=\pi \times 12=12\pi \approx 37.68C=π×12=12π≈37.68 feet

Alternative method:

• You can physically measure the circumference by wrapping a string around the circle and then measuring the string's length

In summary, the circumference is the distance around the circle, calculated by multiplying the diameter or twice the radius by π\pi π