Scale‐up of orthokinetic agglomeration in stirred‐tank reactors was studied numerically. A computer code based on the lattice‐Boltzmann method was used to perform large eddy simulations. To account for the dependence of agglomeration on hydrodynamics, the kinetic relation proposed by Mumtaz et al. was applied locally. The convection‐reaction equation governing the evolution of the particle‐number concentration was solved at the same resolution as the flow field. Three scale‐up rules were studied: scale‐up at constant Re‐number, constant specific power input, and constant impeller tip speed. Vessel sizes varying from 1 to 10,000 L were simulated. The use of two types of impellers (Rushton turbine and Pitched Blade turbine) allowed for investigating the dependence of agglomeration on the macroscopic flow pattern in a reactor. Inhomogeneities in stirred‐tank flow caused unpredictable scale‐up behavior. Scale‐up at constant power input yielded the most constant reactor performance. Impeller geometry had a drastic impact on the observed agglomeration rate, even if the difference in Po number was taken into account.