With this semicircle area calculator, you can quickly find the area of half of a circle. What is more, the tool also doubles as a semicircle perimeter calculator, so inputting radius or diameter will help you find the basic features of the shape in the blink of an eye. In the article below, we provide the semicircle definition and explain how to find the perimeter and area of a semicircle. So what are you waiting for? Scroll down!

However, if you're searching for a moment of inertia of a semicircle, check out our comprehensive moment of inertia tool.

What is a semicircle? Semicircle definition

A semicircle is simply half of a circle 🌗. When you cut a circle along a diameter line - or, in other words, through the circle centre - you'll get a semicircle. The prefix semi- comes from Latin, and it means half or partly (like in words such as semi-permanent, semi-formal, semifinals). Now, when you know etymology, you won't forget what a semicircle is: just remind yourself how the semifinals are connected with the finals, and you'll know how to link a semicircle to a circle.

How to find the area of a semicircle? Semicircle area formula

Image of a semicircle area

Knowing the semicircle definition - half of a circle - we can easily write the semicircle area formula using the well-known circle area, πr² :

Areasemicircle = Areacircle / 2 = πr² / 2

The area of a semicircle is just a half of the area of a full circle. But can we use this rule for the circumference of a semicircle?

Perimeter of a semicircle formula (aka circumference of a semicircle)

Image explaining how to find semicircle perimeter - half of the circle circumference + diameter

If you want to calculate the outer boundary of a semicircle - the circumference or perimeter of a semicircle - you need to be careful to not fall into any traps. The circumference of a semicircle is not equal to half of the circle circumference - you need to add also the diameter of the semicircle, as another boundary was created:

Perimetersemicircle = (circumferencecircle / 2) + 2 * radius

Perimetersemicircle = π * r + 2 * r = r * (π + 2)


Perimetersemicircle = π * r + d, where d is the semicircle diameter

Find the area of a half circle with semicircle area calculator

Where do we find semicircular objects in everyday life? The shape of half circle occurs in:

  • home furniture and decorations such as mirrors, tables with semicircular counters, sofas, benches, rugs or even windows.
  • everyday objects like fans, protractors, circle skirts.
  • all food which comes in round, cylindrical or sphere shapes could also be sliced to approximate semicircles. Like for example half a pizza, a slice of watermelon 🍉 or half a cookie 🍪

Let's figure out how to find an area of a semicircular object - a rug which would fit perfectly in front of your fireplace🔥:

  1. Input the diameter or radius of the rug. Assume that our fireplace is 4 ft wide, so we'd like to have the same rug length. Enter 4 ft in the diameter box. Change the length units by clicking on the unit name and selecting the one you need from the drop-down list.
  2. The semicircle area calculator displays the area of half circle: for our rug, it's 6.28 ft².
  3. The tool works as semicircle perimeter calculator as well - e.g., if you want to braid the rug, you can calculate how much lace you'll need. In our case, the perimeter equals 10.28 ft.

Now as you know everything about semicircles, maybe you'd like to check our others circle-related calculators 🔴?

Hanna Pamuła, PhD candidate