Fluids and melts in planetary interiors significantly influence geodynamic processes from volcanism to global‐scale differentiation. The roles of these geofluids depend on their viscosities (η). Constraining geofluid η at relevant pressures and temperatures relies on laboratory‐based measurements and is most widely done using Stokes' Law viscometry with falling spheres. Yet small sample chambers required by high‐pressure experiments introduce significant drag on the spheres. Several correction schemes are available for Stokes' Law but there is no consensus on the best scheme(s) for high‐pressure experiments. We completed high‐pressure experiments to test the effects of (a) the relative size of the sphere diameter to the chamber diameter and (b) the top and bottom of the chamber, that is, the ends, on the sphere velocities. We examined the influence of current correction schemes on the estimated viscosity using Monte Carlo simulations. We also compared previous viscometry work on various geofluids in different experimental setups/geometries. We find the common schemes for Stokes' Law produce statistically distinct values of η. When inertia of the sphere is negligible, the most appropriate scheme may be the Faxén correction for the chamber walls. Correction for drag due to the chamber ends depends on the precision in the sinking distance and may be ineffective with decreasing sphere size. Combining the wall and end corrections may overcorrect η. We also suggest the uncertainty in η is best captured by the correction rather than propagated errors from experimental parameters. We develop an overlying view of Stokes' Law viscometry at high pressures. Plain Language Summary Liquids and vapors, collectively known as fluids, occur throughout the Earth and other planets. Compared to solid rock, fluids can move rapidly due to their lower viscosities and hence influence and promote important geologic processes. The large span of pressures and temperatures inside planets influences the viscosities of fluids. Measuring a fluid viscosity at relevant pressures is most often done by tracking a sphere that sinks in the fluid. The sinking speed of the sphere is converted to the fluid viscosity by balancing forces which cause and oppose the sinking, known as Stokes' Law. To create the right pressures, unique devices are used which require small chambers to house the fluid and sphere. The small chambers affect the sinking speeds and hence Stokes' Law becomes inaccurate. There are several corrections for the chamber effects on the sphere. However, there is no consensus on which correction should be used for high‐pressure measurements. We examined the chamber effects on the sinking speeds in high‐pressure experiments. We calculated the fluid viscosity using each correction and considered the uncertainties. We find that the corrections produce unique values of viscosity and are not equal. Future work should carefully consider the choice in correction. Key Points We performed high‐pressure experiments to evaluate drag acting on falling spheres in Stokes' Law viscometry We evaluated different correction schemes for Stokes' Law and used Monte Carlo simulations to evaluate the uncertainties in each scheme The best correction may be for drag due to the chamber walls, while other corrections may be ineffective or over‐correct the viscosity