1. Body mass is a key life history trait in animals. Despite being the largest animals on the planet, no method currently exists to estimate body mass of free-living whales. 2. We combined aerial photographs and historical catch records to estimate the body mass of free-living right whales (Eubalaena sp.). First, aerial photogrammetry from unmanned aerial vehicles was used to measure the body length, width (lateral distance) and height (dorso-ventral distance) of free-living southern right whales (E. australis; 48 calves, 7 juveniles and 31 lactating females). From these data, body volume was estimated by modelling the whales as a series of infinitely small ellipses. The body girth of the whales was next calculated at three measurement sites (across the pectoral fin, the umbilicus and the anus) and a linear model was developed to predict body volume from the body girth and length data. To obtain a volume-to-mass conversion factor, this model was then used to estimate the body volume of eight lethally caught North Pacific right whales (E. japonica), for which body mass was measured. This conversion factor was consequently used to predict the body mass of the free-living whales. 3. The cross-sectional body shape (height-width ratio) of the whales was slightly flattened dorso-ventrally at the anterior end of the body, almost circular in the mid region, and significantly flattened in the lateral plane across the posterior half of the body. Compared to a circular cross-sectional model, our body mass model incorporating body length, width and height improved mass estimates by up to 23.6% (mean=6.1%, SD=5.27). Our model had a mean error of only 1.6% (SD=0.012), compared to 9.5% (SD=7.68) for a simpler body length-to-mass model. The volume-to-mass conversion factor was estimated at 754.63kg m-3 (SD=50.03). Predicted body mass estimates were within a close range of existing body mass measurements. 4. We provide a non-invasive method to accurately estimate body mass of free-living whales while accounting for both their structural size (body length) and relative body condition (body width). Our approach can be directly applied to other marine mammals by adjusting the model parameters (body mass model script provided).