Let $K$ be a field endowed with a complete discrete valuation with value group $\Gamma\subset\mathbb{Q}$. Let $\|\ \|_{1}$ and $\|\ \|_{2}$ be two model metrics on the line bundle $L$ of the smooth projective curve $X$ over $K$. Then $\vol(L,\|\ \|_{1},\|\ \|_{2})\in\mathbb{Q}$.