The vascular architecture within a perforator flap is different from a conventional muscle or myocutaneous flap. The purpose of this paper is to understand the correlation between flow rate and flap size in perforator flaps. With extrapolation of these data, we have provided an indirect analysis of the venous drainage and its correlation with flap size. A prospective study was planned. Twenty-five patients were enrolled in this study: six patients were operated on using an anterolateral thigh (ALT) flap and 19 using a deep inferior epigastric artery perforator (DIEAP) flap. One month postoperatively, echo-colour-Doppler measurements were performed on pedicle and perforator arteries to calculate blood flow rate in the flaps. A correlation between weight and flow rate was analysed. Spearman rho statistic was calculated. A linear regression model was made from patient data of flow rate/flap weight and predicted values of flow per flap weight were calculated. Then, flow rate values of veins of various diameters were estimated using Hagen-Poiseuille's formula. Our data show that flow rate measured postoperatively on flap arteries is significantly correlated with flap weight [rho(23 d.f.)=0.725, P<0.01 (two-tailed)]. Moreover, we have calculated the minimum size of veins able to drain flaps of increasing weights with different patterns, i.e. our data show that veins of 1.30, 1.50 and 1.75 mm diameter could safely drain flaps of, respectively, 300, 500 and 900 g in weight. This can be useful preoperatively to estimate the risk of flap congestion and in planning additional drainage.