The geometry of spreading of plumes beneath mid-oceanic ridges is investigated by three-dimensional numerical modeling, with the goal of characterizing the width of the plume along the ridge, or the ''waist width''. Chemically buoyant plumes are modeled, in order to compare to previously reported laboratory tank experiments. The plume is generated near the bottom of the box and rises and forms a mushroomhead with some entrainment of surrounding fluid. The head flattens at a considerable depth beneath the ridge before rising to the surface. Once the plume reaches the surface the head is quickly divided by the diverging plates, and the waist width is found to have reached a steady-state value, W. The results show that W is proportional to the square root of the volumetric flux of the plume divided by the diverging plate speed, which is consistent with previously reported experimental data. Our scaling law gives an independent method for estimating the volumetric flux of mantle plumes. The calculated plume fluxes are two to four times larger than previous estimates. If plume buoyancy is purely of thermal origin, then excess temperatures can be estimated from the fluxes. For Iceland, Azores and the Gal¿pagos, the calculated excess temperatures are 140, 57, and 51 ¿C respectively, in agreement with recent, independent estimates from modeling of gravity and bathymetry. ¿ American Geophysical Union 1995