Mesh size refers to the dimensions of the individual resin beads. The finer the mesh size, the smaller the beads. Smaller beads can pack together more tightly so, for a given volume of resin, fine mesh will offer more ion-exchange surface area and will do more softening. However, because fine mesh resin packs more tightly, it will also reduce flow rate through the softener much more significantly than standard mesh resin. We don't love fine mesh for this reason and don't offer it as a standard product on our website. Most customers who ask about fine mesh, have been misinformed that it's more effective for iron reduction. It's not - you can read about that next. One last note here - if you take a softener that has used standard resin, and then add fine mesh to it, you have to make sure you replace the lower basket at the same time. This basket has slots in it that correspond to the resin bead size. Fine mesh resin will move right through a basket designed for standard mesh resin.