•Fast scanning calorimetry is used to measure Tf of melts up to 5000 K s − 1.•Universal shift factor is provided for strong and fragile glass-forming systems.•Viscosity of melts prone to crystallize is derived by DSC over 6 orders of magnitude. A gap in viscosity data spreads between glass transition and liquidus temperature for glass compositions, which are prone to crystallization. Alternatively, differential scanning calorimetry (DSC) is used to calculate viscosity as both the cooling rate in a DSC experiment and the shear viscosity at the fictive temperature are related by a constant shift factor. However, the low cooling rate (≤ 30 K min−1) of conventional calorimeters limits the accessible range of viscosities around 1012 Pa s. This study reports on measurements using a Flash DSC (FDSC) with cooling rates up to 3 × 105 K min−1. Under such extreme conditions, the fictive temperature is shifted considerably to higher temperatures and viscosity is accessible down to the softening point (viscosity = 106.6 Pa s) without the occurrence of crystallization during measurements. We provide a composition-independent shift factor to retrieve the viscosity of silicate and fluorophosphate melts over six orders of magnitude.