A method and device for estimating a density value ρm indicative of a true density ρ or for estimating a viscosity value ηm indicative of a true viscosity η of a fluid is disclosed. For this, a first resonance frequency fR of a first mechanical oscillator in a reference volume and a second resonance frequency fF of a second mechanical oscillator in contact with the fluid are measured. The estimated value ρm or ηm is then derived using these resonance frequencies fR and fF. During this derivation, at least one fluid-temperature- or fluid-pressure-dependent parameter of the fluid is used. Additionally or alternatively, the first (i.e. reference) mechanical oscillator is arranged in contact with a reference fluid. Thus, fundamental errors in the derivation of the estimated value ρm or ηm are reduced and the estimated value becomes more reliable.