Area of a Semicircle Calculator
Half a circle: half of πr². The perimeter is the curved half-circumference plus the straight diameter.
A = πr² / 2
Perimeter: P = πr + 2r
Area of the semicircle
25.1327ft²
- Perimeter
- 20.5664 ft
- A = π × 4² / 2 = 25.1327 ft²
Area in other units
| m² | 2.3349 |
| cm² | 23,349.1 |
| ft² | 25.1327 |
| in² | 3,619.11 |
| yd² | 2.7925 |
| acres | 0.000577 |
| hectares | 0.000233 |
How to find the area of a semicircle
Formula: A = πr² / 2. Perimeter: P = πr + 2r.
- Measure every length in the same unit. Mixing feet and inches is the most common mistake; convert first (6 in = 0.5 ft).
- Put the numbers into the formula above. The calculator shows each substitution with your numbers.
- Area comes out in square units (ft², m²). If you need a different unit, open "Area in other units".
Why the area of a circle is πr²
Cut a circle into wedges like a pizza and lay them tip-to-tail. The more slices, the closer the strip gets to a rectangle: its height is the radius r, and its length is half the circumference, πr. So the area is πr × r = πr².
Strip ≈ 3.142r long × r tall
Questions people ask
Where does a semicircle show up?
Arched windows, bay windows and the rounded ends of running tracks are measured as semicircles.
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