Body weight is a key trait in insects, reflecting size, condition, and energetic status. It also influences their flight performance, reproduction, and ecological interactions. Yet most ecological and taxonomic studies rely on preserved rather than living individuals, despite the substantial mass loss caused by drying and other curatorial procedures. We quantified how preservation affects body mass in moths (Lepidoptera) using three complementary datasets representing major workflows: laboratory-reared Spodoptera littoralis (Boisduval, 1833), freshly collected moths from Germany, and frozen geometrid specimens from Peru. Across datasets, body weight declined markedly across developmental stages and preservation states, with only partial recovery after rehydration. In S. littoralis, females were consistently heavier than males, while egg presence did not influence female mass. By integrating results across workflows, we derived practical correction factors for estimating fresh body weight from dry specimens. These order-level approximations substantially improve the ecological value of preserved material by allowing body-mass data from museum collections to reflect the mass of living insects more accurately. Applying such corrections enhances the reliability of trait-based and comparative analyses that depend on preserved Lepidoptera.