How precisely can Flow Induced Dispersion Analysis (FIDA) measure protein size in solution? In some cases it can go down to the ångström scale: in this study, using a Fidabio Fida 1, FIDA resolved a hydrodynamic-radius change of only 0.64 ± 0.04 Å per molar of the osmolyte betaine, alongside the viscosity measurements the work relied on. This level of sensitivity is system- and condition-dependent, not a fixed figure for every sample, protein or application. The study itself shows that adding betaine (up to 1 M) sharpens a subset of weak signals in protein HSQC spectra, an effect the authors attribute to reduced fluctuations in the surrounding water. It is an example of using FIDA to measure protein hydrodynamic radius and viscosity in solution, in some cases resolving size differences down to the ångström scale.