弧形面積
\[ \displaystyle \begin{aligned} &= \left( \text{外圓面積} - \text{內圓面積} \right) \times \frac{\text{角度}}{360} \\[10pt] &= \text{圓周率} (\cong 3.1415926) × \left( \text{外半徑}^2 - \text{內半徑}^2 \right) \times \frac{\text{角度}}{360} \\[10pt] &= \pi \left[ \text{外半徑}^2 - \left( \text{外半徑} - \text{厚度} \right) ^2 \right] \times \frac{\text{角度}}{360} \\[10pt] &= \pi \left( 2 \times \text{外半徑} - \text{厚度} \right) \times \text{厚度} \times \frac{\text{角度}}{360} \\[10pt] &= \frac{\pi}{360} \left( 2 \times OR - T \right) T \, \theta \\[10pt] &\cong 0.0087 \, \left( 2 \times OR - T \right) T \, \theta \end{aligned} \]