Must weight is a measure of the amount of sugar in grape juice (must) and, hence, indicates the amount of alcohol that could be produced if it is all fermented to alcohol, rather than left as residual sugar.[1]
In France and many other countries, the grape must density is often recalculated to show potential alcohol, the percent alcohol content that would be the result if the must were fermented to a completely dry wine.
Must weights are commonly measured in the vineyard to monitor the ripeness of the grapes, which helps select the time of harvest. For this purpose, it is usually measured by using a refractometer. A few grapes are crushed between the fingers and the must is dripped onto the glass of the refractometer. The refractometer will be calibrated in a must weight scale, but as refractometers actually measure refractive index (of the grape must in this case), the must weight scale on the refractometer is in effect an indirect measurement, with refractive indices having been pre-converted to their must weight equivalents.