The paper introduces the concept of quantum weight as a fundamental property of quantum many-body systems, characterized by the ground-state static structure factor and describing long-wavelength density fluctuations. The quantum weight is shown to be related to the inverse dielectric function, which describes electron energy loss, through a sum rule. This relationship allows for the derivation of upper and lower bounds on the quantum weight of real materials based on their electron density, static dielectric constant, and plasmon energy. For systems with short-range interactions or Coulomb systems in reduced dimensions, a sum rule relating the quantum weight to the optical conductivity is derived, establishing a connection with the quantum geometry of many-body ground states. The work highlights the quantum weight as a key material parameter that can be experimentally determined, providing a powerful tool for studying electronic structure and dynamical response in solids. The paper also discusses the connection between quantum weight and the many-body quantum metric, showing that they are equal in systems with short-range interactions or Coulomb systems in reduced dimensions, but unequal in 3D Coulomb systems due to the influence of Coulomb screening.The paper introduces the concept of quantum weight as a fundamental property of quantum many-body systems, characterized by the ground-state static structure factor and describing long-wavelength density fluctuations. The quantum weight is shown to be related to the inverse dielectric function, which describes electron energy loss, through a sum rule. This relationship allows for the derivation of upper and lower bounds on the quantum weight of real materials based on their electron density, static dielectric constant, and plasmon energy. For systems with short-range interactions or Coulomb systems in reduced dimensions, a sum rule relating the quantum weight to the optical conductivity is derived, establishing a connection with the quantum geometry of many-body ground states. The work highlights the quantum weight as a key material parameter that can be experimentally determined, providing a powerful tool for studying electronic structure and dynamical response in solids. The paper also discusses the connection between quantum weight and the many-body quantum metric, showing that they are equal in systems with short-range interactions or Coulomb systems in reduced dimensions, but unequal in 3D Coulomb systems due to the influence of Coulomb screening.