For elastic scattering by a real short-range potential in three dimensions, assume that the fixed-energy scattering matrix is unitary at wave number $|k|>0$, that $E_k=\hbar^2|k|^2/(2m)$ is not a threshold or embedded eigenvalue, and that the scattering amplitude is normalized by \begin{align*} \psi_k^{(+)}(x)=e^{ik\cdot x}+f(|k|\hat{x},k)\frac{e^{i|k||x|}}{|x|}+o(|x|^{-1}) \end{align*} as $|x|\to\infty$. Then \begin{align*} \sigma_{\mathrm{tot}}(k) = \int_{S^2}|f(|k|\omega,k)|^2\,d\omega = \frac{4\pi}{|k|}\operatorname{Im} f(k,k). \end{align*}