The angle strain in cyclobutane is

When carbon bonded with four other atoms, then it forms a tetrahedral shape with an angle of ${109}^{\circ }28\text{'}.$ But due to angle strain, the bind angle between two carbon atoms reduced to $90°.$ The change in bond angle is calculated as:

$\begin{array}{ccc}\mathrm{Change} \mathrm{in} \mathrm{bond} \mathrm{angle}& =& \frac{\mathrm{Expected} \mathrm{bond} \mathrm{angle}-\mathrm{Observed} \mathrm{bond} \mathrm{angle}}{2}\\ & =& \frac{109°28\text{'}-90°}{2}\\ & =& \frac{19°28\text{'}}{2}\\ & =& 9°44\text{'}\end{array}$