Let $ρ$ be compactly supported on $D \subset \mathbb R^2$. Endow $\mathbb R^2$ with the metric $e^ρ(dx_1^2 + dx_2^2)$. As $δ\to 0$ the set of Brownian loops centered in $D$ with length at least $δ$ has measure $$\frac{\text{area}(D)}{2πδ} + \frac{1}{48π}(ρ,ρ)_{\nabla}+ o(1).$$ When $ρ$ is smooth, this follows from the classical Polyakov-Alvarez formula. We show that the above also holds if $ρ$ is not smooth, e.g. if $ρ$ is only Lipschitz. This fact can alternatively be expressed in terms of heat kernel traces, eigenvalue asymptotics, or zeta regularized determinants. Variants of this statement apply to more general non-smooth manifolds on which one considers all loops (not only those centered in a domain $D$). We also show that the $o(1)$ error is uniform for any family of $ρ$ satisfying certain conditions. This implies that if we weight a measure $ν$ on this family by the ($δ$-truncated) Brownian loop soup partition function, and take the vague $δ\to 0$ limit, we obtain a measure whose Radon-Nikodym derivative with respect to $ν$ is $\exp\bigl( \frac{1}{48π}(ρ,ρ)_{\nabla}\bigr)$. When the measure is a certain regularized Liouville quantum gravity measure, a companion work [APPS20] shows that this weighting has the effect of changing the so-called central charge of the surface.