Semicircle Perimeter

Semicircle Perimeter

A semicircle’s perimeter is the distance around its curved edge plus the straight side across the bottom. That straight side is the diameter, so you need both parts.

1) Identify the radius or diameter

• If the radius is given, use it directly.
• If the diameter is given, remember that the radius is half of it.

2) Find the curved part

The curved edge of a semicircle is half the circumference of the full circle. Using the radius, this curved length is:

• half of (2\pi r), which is (\pi r)

3) Add the straight side

The full perimeter of a semicircle is:

• (\pi r + 2r)

If the diameter (d) is given, you can also write it as:

• (\frac{\pi d}{2} + d)

4) Simplify carefully

Combine terms if possible and leave the answer in exact form unless decimals are requested.

Check your answer

• The perimeter should be a little more than the diameter, because the curved part adds extra length.
• Make sure you included the straight side, not just the arc.
• If you used a radius, the answer should be based on (r); if you used a diameter, the final form should match (d).