  Modular symmetries offer a dynamic approach to understanding the flavour
structure of leptonic mixing. Using the modular $\mathcal{A}_4$ flavour
symmetry integrated in a type-II seesaw, we propose a simple and minimalistic
model that restricts the neutrino oscillation parameter space and, most
importantly, introduces a sum rule in the physical neutrino masses. When
combined with the mass squared differences observed in neutrino oscillations,
this sum rule determines the absolute neutrino mass scale. This has significant
implications for cosmology, neutrinoless double beta decay experiments and
direct neutrino mass measurements. In particular, the model predicts $\sum_i
m_i \approx 0.1$ eV for both normal and inverted ordering, and thus can be
fully probed by the current generation of cosmological probes in the upcoming
years.
