圓環面積
\[ \displaystyle \begin{aligned} &= \text{外圓面積} - \text{內圓面積} \\[10pt] &= \frac{1}{4} \times \text{圓周率} (\cong 3.1415926) × \left( \text{外徑}^2 - \text{內徑}^2 \right) \\[10pt] &= \frac{\pi}{4} \times \left[ \text{外徑}^2 - \left( \text{外徑} - 2 \times \text{厚度} \right) ^2 \right] \\[10pt] &= \pi \left( \text{外徑} - \text{厚度} \right) \times \text{厚度} \\[10pt] &= \pi \left( OD - T \right) T \end{aligned} \]