Differences in bark and wood content in woody biomass directly affect its quality and economic value as an energy source. In order to estimate the quality of biomass before harvesting, an allometric regression of bark percentage of total aboveground biomass and DHB (diameter at 1.30m height) was developed in a 2-year poplar short-rotation coppice system in Central Italy. Firstly, a relationship between mean diameter and bark content percentage was established in 1cm-wide sections belonging to all diametric classes. The model of best fit for these stem cylindrical sections was an equation y=ax^−b. Following this, sample stems (of which we measured DHB) were collected and divided into sections belonging to a diameter class. Fresh and dry matter were determined for each class. Using the first equation, bark content was calculated separately for all classes. Thereafter, a second equation between bark content in the whole stem and DHB was developed. The best fitting equation for the whole stem was y=cx^−d. Bark content in the whole stem ranged from 33.9–31.4% in large-sized DHB stems to 15.1–12.5% in smallest stems, depending on their moisture content. Bark content decreased rapidly in the small diametric classes until DHB reached 4cm. Thereafter, the ratio of reduction of bark percentage dropped.