An annulus is the flat ring shape left when a smaller circle is cut from the centre of a larger one.
How to use the Annulus Area Calculator
- Outer radius
- Inner radius
Results update as you type, and every field accepts the units shown in its menu.
How it works
Subtract the inner circle’s area from the outer circle’s area.
Inputs explained
| Field | What to enter |
|---|---|
| Outer radius | Units: m, cm, mm, km, in, ft … |
| Inner radius | Units: m, cm, mm, km, in, ft … |
What each result means
- Ring area = pi×([Outer radius]^2 – [Inner radius]^2)
- Ring width = [Outer radius] – [Inner radius]
Example values
How the main result changes when Outer radius changes and everything else stays at the example values above:
| Outer radius (m) | Ring area (m²) |
|---|---|
| 5 | -75.4 |
| 7.5 | 22.78 |
| 10 | 160.22 |
| 12.5 | 336.94 |
| 15 | 552.92 |
| 20 | 1,102.7 |
Formula
Area = π(R² − r²)Worked example
Inputs — Outer radius: 10 m; Inner radius: 7 m.
Results — Ring area = 160.22 m²; Ring width = 3 m.
Frequently asked questions
Can I use diameters?
Halve them first to get radii.
What if the inner radius is larger?
Swap them; the outer radius must be bigger.
What formula does the Annulus Area Calculator use?
Area = π(R² − r²).