Pi is a special number that always stays the same for any circle. It shows the relationship between a circle's circumference and its diameter.

When you divide the circumference of any circle by its diameter, you always get approximately 3.14. This number is called pi and written as the symbol π.

In calculations, we use:

• π ≈ 3.14 (decimal form)
• π = 22/7 (fraction form)

If you know the diameter (d), use:

$C = \pi d$

If you know the radius (r), use:

$C = 2\pi r$

Remember: diameter = 2 × radius, so d = 2r.

Find the circumference of a circle with a radius of 14 cm. Use π = 22/7.

Solution

$C = 2\pi r$

$C = 2 × \frac{22}{7} × 14$

$C = 2 × 22 × 2$

$C = 88$ cm

The circumference is 88 cm.

A bicycle tyre has a diameter of 63 cm. Find its circumference. Use π = 22/7.

Solution

$C = \pi d$

$C = \frac{22}{7} × 63$

$C = 22 × 9$

$C = 198$ cm

The circumference is 198 cm.

The circumference of a circular plate is 110 cm. Find its diameter.

Solution

$C = \pi d$

$110 = \frac{22}{7} × d$

$d = 110 × \frac{7}{22}$

$d = 5 × 7$

$d = 35$ cm

The diameter is 35 cm.

• Circumference = π × diameter
• Circumference = 2 × π × radius
• Always write the unit of measurement (cm, m, mm, etc.)
• Use π = 22/7 or π = 3.14 as given in the question

In Tanzania, knowing how to calculate circumference is useful when measuring circular objects. For example, if a farmer wants to know how much wire is needed to fence a circular well with a diameter of 7 metres, they calculate the circumference: C = πd = 22/7 × 7 = 22 metres of wire required.