A new equation for volume estimation of tree stems was developed using median diameter of stem profile. This work is based on the volume equation for solids of revolution defined originally by Mathiesen in 1925. The study is based on previously published data for 5,403 Norway spruce trees from Romania grouped in 218 height/diameter categories. For each of these categories, median diameter and area of stem profile as well as volumes by Mathiesen’s formula and Huber’s formula were computed. The volume equation was obtained using quick methods for determining the median diameter and the area of stem profile. The proposed model provides more accurate results than the original equation developed by Mathiesen. Furthermore, it allowed the development of an easy-to-use equation aiming to predict the over-bark stem volume, based on the median diameter of stem profile.

   Keywords: model, Norway spruce, median diameter, stem area, stem volume