We compute the scalar determinants \(\det (\Delta +M^{2})\) on the two-dimensional round disks of constant curvature \(R=0\) , \({\mp } 2\) , for any finite boundary length \(\ell \) and mass M, with Dirichlet boundary conditions, using the \(\zeta \) -function prescription. When \(M^{2}=\pm q(q+1)\) , \(q\in {\mathbb {N}}\) , a simple expression involving only elementary functions and the Euler \(\Gamma \) function is found. Applications to two-dimensional Liouville and Jackiw–Teitelboim quantum gravity are presented in a separate paper.