Sinking particles are important in delivering carbon to the deep ocean where it may be stored out of contact with the atmosphere. Particle sinking velocity strongly influences the amount of carbon reaching the deep ocean, and is thought to be strongly affected by particle size. Here we carried out an extensive literature review (62 datasets) into the size-sinking velocity relationship, and find the relationship is much weaker for studies examining particles in situ (median R² = 0.03) compared with ex situ studies (median R² = 0.35). This may be because particles examined in the laboratory have more uniform properties than those studied in situ, and represent only a subset of particles from the natural environment. Our findings suggest a simple relationship between size and sinking velocity may be insufficient when calculating sinking particulate fluxes in the ocean; considering different particle types individually will enable more accurate calculations of particulate fluxes.