Omni Calculator logo

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 calculator.

What is a semicircle?

A semicircle is simply half of a circle. You'll get a semicircle when you cut a circle along a diameter line — or, in other words, through the circle center. The prefix semi- comes from Latin, meaning half or partly (like in words such as semi-permanent, semi-formal, and semifinal).

💡 Once know the 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\pi\cdot r^2:

Asemicircle=Acircle2=πr22A_{\mathrm{semicircle}} = \frac{A_{\mathrm{circle}}}{2} = \frac{\pi \cdot r^2}{2}

The area of a semicircle is just 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 not to 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:

Psemicircle=Pcircle2+2rP_{\mathrm{semicircle}} = \frac{P_{\mathrm{circle}}}{2} + 2\cdot r


Psemicircle=πr+2r=r(π+2)\begin{align*} P_{\mathrm{semicircle}} & = \pi\cdot r + 2\cdot r \\ & = r\cdot (\pi+2) \end{align*}


Psemicircle=πr+dP_{\mathrm{semicircle}} = \pi\cdot r + d

Where dd 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 a 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 that comes in round, cylindrical, or sphere shapes could also be sliced to approximate semicircles. 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 that would fit perfectly in front of your fireplace🔥:

  1. Input the diameter or radius of the rug. Assume that our fireplace is 4 ft4\ \mathrm{ft} wide, so we'd like to have the same rug length. Enter 4 ft4\ \mathrm{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 ft26.28\ \mathrm{ft^2}.
  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 ft10.28\ \mathrm{ft}.

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


How do I find the perimeter of a semicircle given radius?

To determine the perimeter of a semicircle:

  1. Write down the radius r.
  2. Multiply the radius by π+2: r × (π+2) ≈ 5.14159 × r.
  3. The result is the perimeter of your semicircle. You can verify the result with our online semicircle perimeter calculator.

Is the semicircle perimeter half of circle perimeter?

No, the circumference of a semicircle is greater than half of the circle circumference. This is because the "flat" side of the semicircle also contributes to the perimeter. It is equal to the (semi)circle's diameter. Thus, semicircle perimeter = half of circle's perimeter + circle's diameter.

What is the area of a semicircle with radius 10 in?

The area is 157.08 in sq. To arrive at this answer, recall that the area of a semicircle is half the area of the circle. That is, the semicircle area formula reads Area = πr²/2. Plugging in r = 10 [in], we get 100π / 2 [in sq] ≈ 157.08 [in sq]. Don't hesitate use our online semicircle area calculation if you struggle with the calculations.

Hanna Pamuła, PhD
Radius (r)
Diameter (d)
Semicircle area and perimeter image, with radius, diameter and central angle marked. Formula for perimeter and semicircle area.
Semicircle Area and Perimeter
Area (A)
Perimeter (L)
Check out 11 similar circle calculators ⭕
Arc lengthArea of a circleCircle calc: find c, d, a, r… 8 more
People also viewed…

BMR - Harris-Benedict equation

Harris-Benedict calculator uses one of the three most popular BMR formulas. Knowing your BMR (basal metabolic weight) may help you make important decisions about your diet and lifestyle.

Cos 2 theta

This cos 2 theta calculator will teach you all there is about the double angle formula for the cosine function.

Inverse variation

Use this inverse variation calculator to understand and compute the inverse proportionality between two variables.


Use our titration calculator to determine the molarity of your solution.