Abstract Objective. Prediction of body composition from bioimpedance spectroscopy (BIS) measurements using mixture theory-based biophysical modelling invokes a factor (K _B) to account for differing body geometry (or proportions) between individuals. To date, a single constant value is commonly used. The aim of this study was to investigate variation in K _B across individuals and to develop a procedure for estimating an individualized K _B value. Approach. Publicly available body dimension data, primarily from the garment industry, were used to calculate K _B values for individuals of varying body sizes across the life-span. The 3D surface relationship between weight, height and K _B, was determined and used to create look-up tables to enable estimation of K _B in individuals based on height and weight. The utility of the proposed method was assessed by comparing fat-free mass predictions from BIS using either a constant K _B value or the individualized value. Results. Computed K _B values were well fitted to height and weight by a 3D surface (R ² = 0.988). Body composition was predicted more accurately compared to reference methods when using individualized K _B than a constant value in infants and children but improvement in prediction was less in adults particularly those with high body mass index. Significance. Prediction of body composition from BIS and mixture theory is improved by using an individualized body proportion factor in those of small body habitus, e.g. children. Improvement is small in adults or non-existent in those of large body size. Further improvements may be possible by incorporating a factor to account for trunk size, i.e. waist circumference.