Circumference of a Circle

A circle has radius $r = 5$ cm.

$C = 2 \times \pi \times 5 = 10\pi \approx 31.4 \text{ cm}$

A wheel has diameter $d = 0.6$ m.

$C = \pi \times 0.6 \approx 1.88 \text{ m}$

In one full rotation, the wheel travels about 1.88 m.

A circle has circumference 50 cm.

$r = \frac{50}{2\pi} = \frac{50}{6.283} \approx 7.96 \text{ cm}$

A bicycle wheel has diameter 70 cm ($r = 35$ cm).

$C = \pi \times 70 \approx 220 \text{ cm} = 2.2 \text{ m}$

Every full turn of the wheel the bike travels 2.2 m. Cycle computers count rotations and use this circumference to calculate speed and distance accurately — the same principle as a car speedometer.

An athletics track has two semicircular bends with inner radius 36.5 m. Lane 1 circumference of each bend:

$C_{\text{bend}} = \pi \times 36.5 \approx 114.7 \text{ m}$

Each outer lane adds about $2\pi \times 1.22 \approx 7.7$ m per bend. This is why runners in outer lanes start further forward — a staggered start ensures everyone runs the same total distance.

A gardener wants to plant flowers around the edge of a circular pond with diameter 4.5 m.

$C = \pi \times 4.5 \approx 14.1 \text{ m}$

They need roughly 14.1 m of border edging. Knowing the circumference prevents buying too little or too much material.