"Zero retention" is a purchase claim, not a useful measurement by itself. Ask what was weighed, whether the grinder was already seasoned with coffee, and whether the quoted result used bellows or a purge.
Two problems that are easy to confuse
Retention describes coffee remaining inside the grinder. Exchange concerns old grounds leaving while some new grounds stay behind. If you put 18 g in and get 18 g out, the grinder may still have exchanged part of the new dose for old coffee. Equal weights do not prove that every particle is fresh.
This distinction matters when you switch from caffeinated to decaf or between very different coffees. A basic home weight check can reveal a missing dose, but it cannot establish how much of the output came from the previous coffee.
Run a limited check honestly
Follow the grinder's cleaning and adjustment instructions. With a reliable scale, weigh a dose before grinding and the output afterwards. Record the coffee, setting and whether you used any manufacturer-approved clearing routine. Repeat several doses under the same conditions.
For example, 18.0 g in and 17.6 g out records a 0.4 g difference on that dose. This is arithmetic, not a product test performed here. Static, spills and scale resolution can affect the result. Do not convert one reading into an exact permanent-retention specification.
Compare the entire daily routine
Single dosing is attractive if you switch coffees and want a weighed input each time. Hopper dosing can be convenient when one coffee stays in use. Neither approach excuses ignoring the actual basket dose.
Ask what happens after a grind change, how accessible the chute is, whether clearing creates mess, and how much noise the routine makes. A grinder that delivers a small weight difference but requires several awkward steps may not fit your morning better than one with a predictable clearing routine.
Give claims a useful boundary
A worthwhile review identifies the coffee, grind range, number of trials, clearing procedure and scale resolution. Without those details, treat a decimal retention figure cautiously. It may describe one setup rather than your likely result.
This publication has not bench-tested a ranked list of grinders. Before purchasing, request a demonstration of the workflow you intend to use and check service availability. If you are mainly struggling with grind adjustment, burr choice and adjustment behaviour may be more relevant than a retention slogan.
Keep the input-output check as part of setup. After that, weigh the dose that reaches the basket. That is the number your espresso recipe actually uses.
