Perimeter of a Semicircle Formula
Geometry Formulas: Area, Volume, Perimeter
What is the formula for the perimeter of a semicircle?
Answer
Perimeter of a semicircle
P = π · r + 2 · r
Here r is the radius. The result comes out in units.
Where the formula comes from
P = π · r + 2 · r
The outline of a half-disc is two different things joined: the curved half of the circumference, πr, plus the straight edge across it, which is the diameter 2r. Leave the diameter out and you have measured an arc, not a shape.
Worked examples
Let r = 5. Then the perimeter of a semicircle ≈ 25.71 units.
Let r = 3.2. Then the perimeter of a semicircle ≈ 16.45 units.
The common mistake
Halving the full circumference and stopping there. That gives πr and forgets the straight side — the perimeter comes out about 39 % short.
Found an error in the calculation or the information?
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- Square Feet to Square Meters (Sq Ft to Sq M) - Convert Square Feet to Square Meters
- Square Meters to Square Feet (Sq M to Sq Ft) - Convert Square Meters to Square Feet
- Gravel Calculator - Area to cubic yards, tons and coverage
- Topsoil Calculator - Cubic yards, weight and coverage for any area
- Multiplication Facts - Any two numbers from 1 to 9, with the working shown
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Values from the Formula
| If r = | Perimeter |
|---|---|
| 1 | ≈ 5.14 units |
| 2 | ≈ 10.28 units |
| 3 | ≈ 15.42 units |
| 4 | ≈ 20.57 units |
| 5 | ≈ 25.71 units |
| 10 | ≈ 51.42 units |
Geometry Formulas: Area, Volume, Perimeter
These are the formulas for the area, perimeter, circumference, volume and surface area of the standard shapes — 31 of them. Each page shows not only the formula but where it comes from, what every letter stands for, two worked examples and the mistake that this particular shape invites. For example, it can help you find out what is the formula for the perimeter of a semicircle? (The answer is: P = π · r + 2 · r). Pick the pair — "Area of a circle", for instance — and hit the "Find out" button.
Geometry Formulas: Area, Volume, Perimeter