It has remained unclear how much of the negative buoyancy force of the slab (FB) is used to pull the trailing plate at the surface into the mantle. Here I present three-dimensional laboratory experiments to quantify the net slab pull force (FNSP) with respect to FB during subduction. Results show that FNSP increases with increasing slab length and dip up to ~8--12% of FB, making FNSP up to twice as large as the ridge push force. The remainder of FB is primarily used to drive rollback-induced mantle flow (~70%), to bend the subducting plate at the trench (~15--30%) and to overcome shear resistance between slab and mantle (0--8%).