The weighted binomial sum $f_m(r)=2^{-r}\sum_{i=0}^r\binom{m}{i}$ arises in coding theory and information theory. We prove that,for $m\not \in\{0,3,6,9,12\}$, the maximum value of $f_m(r)$ with $0\leqslant r\leqslant m$ occurs when $r=\lfloor m/3\rfloor+1$. We also show this maximum value is asymptotic to $\frac{3}{\sqrt{πm}}\left(\frac{3}{2}\right)^m$ as $m\to\infty$.