Area of a circle from the diameter, step by step
The area formula wants the radius.
The question
cm²
Area of a circle marked 16 cm
- The 16 cm marked here goes right ACROSS the circle — it is the diameter, not the radius. The area formula wants the radius, so that is the first thing to sort out.
- Half of 16 is 8, so the radius is 8 cm. Skipping this step is what makes an area come out four times too big.
- Area is π × r × r. Square the RADIUS first — only the r is squared, not the π with it: 8 × 8 = 64.
- Then multiply by π: 64 × 3.14 = 200.96 square cm.
How it works.
- 01
The 16 cm marked here goes right ACROSS the circle — it is the diameter, not the radius. The area formula wants the radius, so that is the first thing to sort out.
- 02
Half of 16 is 8, so the radius is 8 cm. Skipping this step is what makes an area come out four times too big.
- 03
Area is π × r × r. Square the RADIUS first — only the r is squared, not the π with it: 8 × 8 = 64.
- 04
Then multiply by π: 64 × 3.14 = 200.96 square cm.