We extend the definition of descriptors from the molecule to the synthetic route. Thus, the route descriptor is "the final result of a logical and mathematical procedure, which transforms chemical information encoded within a symbolic representation of a synthetic route into a useful number or the result of a standardized experiment." This definition encompasses computed, modeled, and experimental descriptors. An example is the LogP, a quantitative representation of the lipophilicity of the molecules that can be measured in an experiment or predicted with a model. Similarly, the overall chemical yield can be the outcome of an experimental synthetic procedure or a route projection of the yield predicted for the single steps . Here, we describe our approach to calculated route descriptors. In a future code release, we will include methods to combine descriptors and construct hybrid experimental/modeled route metrics.