The athymic (nude) mouse is a useful model for studying human tumors in vivo. A survey of recent literature revealed 19 different formulas for determining the size of subcutaneous tumors in nude mice. These formulas were compared with actual tumor weights from 50 tumors. The ellipsoid volume formulas (π/6 × L × W × H and ½ × L × W × H) best estimated tumor mass (r = 0.93), while diameter measurements correlated poorly with tumor mass (r < 0.66). Tumor area correlated well with mass in small tumors (r = 0.89), but poorly in large tumors (r = 0.41). The study concluded that ellipsoid volume calculations from three axes consistently yield the most accurate estimations of both relative and actual tumor mass. Human tumors can grow as xenografts in nude mice, providing a useful model for studying tumor biology and response to therapies. Tumor size measurements over time can construct growth curves, which provide data on tumor biology in vivo. The natural growth pattern of a tumor can be used as a standard to measure the effects of various treatments. To ensure accuracy in growth curves, a reliable method for determining tumor size is needed. A survey of recent literature showed that investigators used various methods to determine tumor size. Some measured one dimension or used the average of two, while others calculated area from two diameters or used formulas for volume from two or three dimensions. The study evaluated 19 formulas to determine which best estimated tumor size and mass. In this study, 50 tumors were measured just before mouse sacrifice. Tumors with irregular shapes were divided into lobes. The length, width, and height of each lobe were measured with calipers. Tumors were then weighed. The median tumor mass was 9.35 g (range, 0.46–22.0 g). Tumor volumes were calculated using formulas from Table 1. For multi-lobed tumors, volumes of each lobe were calculated separately and summed. Three tumors of the same cell line were measured at bi-weekly intervals. Tumor volumes were also determined by water displacement, and their masses were measured. The mass and volume were found to be identical, indicating a density of 1 g/ml.The athymic (nude) mouse is a useful model for studying human tumors in vivo. A survey of recent literature revealed 19 different formulas for determining the size of subcutaneous tumors in nude mice. These formulas were compared with actual tumor weights from 50 tumors. The ellipsoid volume formulas (π/6 × L × W × H and ½ × L × W × H) best estimated tumor mass (r = 0.93), while diameter measurements correlated poorly with tumor mass (r < 0.66). Tumor area correlated well with mass in small tumors (r = 0.89), but poorly in large tumors (r = 0.41). The study concluded that ellipsoid volume calculations from three axes consistently yield the most accurate estimations of both relative and actual tumor mass. Human tumors can grow as xenografts in nude mice, providing a useful model for studying tumor biology and response to therapies. Tumor size measurements over time can construct growth curves, which provide data on tumor biology in vivo. The natural growth pattern of a tumor can be used as a standard to measure the effects of various treatments. To ensure accuracy in growth curves, a reliable method for determining tumor size is needed. A survey of recent literature showed that investigators used various methods to determine tumor size. Some measured one dimension or used the average of two, while others calculated area from two diameters or used formulas for volume from two or three dimensions. The study evaluated 19 formulas to determine which best estimated tumor size and mass. In this study, 50 tumors were measured just before mouse sacrifice. Tumors with irregular shapes were divided into lobes. The length, width, and height of each lobe were measured with calipers. Tumors were then weighed. The median tumor mass was 9.35 g (range, 0.46–22.0 g). Tumor volumes were calculated using formulas from Table 1. For multi-lobed tumors, volumes of each lobe were calculated separately and summed. Three tumors of the same cell line were measured at bi-weekly intervals. Tumor volumes were also determined by water displacement, and their masses were measured. The mass and volume were found to be identical, indicating a density of 1 g/ml.