Grinding exactly what you need sounds simple enough to do on any grinder. It isn't, and the reason why comes down to where retained coffee actually hides inside a grinder in the first place.
Single dosing is loading exactly the amount of beans you're about to grind, grinding all of it, and clearing the grinder out before the next dose, rather than keeping a standing supply of beans sitting in a hopper. The goal is that what comes out is entirely what you just put in, nothing left over from a previous bean, nothing held back for next time.
Here's the part that surprises people: loading a single dose of beans into a regular hopper-fed grinder doesn't automatically get you the same result as a grinder actually engineered for single dosing. The amount of beans you load doesn't change the grinder's internal geometry, and that geometry is where retained coffee actually comes from.
A grinder built for single dosing, like the Niche Zero or DF64, is specifically shaped to avoid corners, chutes, and chambers where grounds could get stuck. A grinder built for continuous hopper-fed use often wasn't designed with that specific goal in mind, so loading it with just one dose still runs into the same trapped-grounds problem it would with any amount of beans.
Two things mainly cause it. The first is simple geometry, corners, ledges, and chambers inside the grinder where ground coffee can physically sit rather than falling straight through to the output. The second, and the one people talk about less, is static electricity.
Freshly ground coffee builds up a static charge as it's cut apart by the burrs, especially with lighter roasts, and that charge makes the grounds cling to the inside walls of the grinder instead of dropping cleanly out. This is a real, physical phenomenon, not just a design flaw, and it's part of why even well-engineered single-dose grinders still retain some small amount rather than achieving a perfect zero.
The main reasons are avoiding waste and avoiding cross-contamination between beans. If you switch between different coffees, a grinder that retains a gram or more will mix some of the old bean into your next grind, which is a real, tasteable issue if the two beans are meaningfully different. Single dosing also means you're grinding beans that haven't sat pre-ground in a hopper, which some people feel preserves freshness better, though the practical difference depends heavily on how quickly you go through a hopper's worth of beans in the first place.
Single dosing asks for more hands-on involvement every time you grind, weighing out beans, loading them, and often cleaning or purging some residual static-clung grounds, rather than just pressing a button on a grinder that's already full. For someone brewing multiple times a day or grinding for a household, that adds up. For someone brewing once or twice a day who cares about switching between beans cleanly, it's a small, repeatable step rather than a real burden.
You can load just one dose worth of beans, but you won't get the same low-retention result as a grinder actually engineered for single dosing. Hopper-fed grinders often have internal geometry, corners, chutes, chambers, where grounds can sit and linger regardless of how many beans you loaded.
Mainly static cling and internal geometry. Freshly ground coffee, especially from lighter roasts, builds up a static charge that makes it stick to the inside of the grinder rather than falling straight through. Chutes, corners, and chambers a hopper-fed grinder wasn't designed to avoid also give grounds places to linger.
RDT stands for Ross Droplet Technique, adding a small amount of water, often just a couple of drops, to whole beans before grinding. It reduces the static charge that causes ground coffee to cling to the inside of the grinder, which is a common practical trick for people dealing with retention and mess during single dosing.
It depends on how often you switch beans and how much you care about minimizing waste and cross-contamination between different coffees. If you stick with one bag at a time, the extra hands-on involvement each time you grind may not be worth it for you.