The study of cell growth and cell size regulation prior to cell cycle entry has been hindered by technical limitations. One recent advance was the development of an approach that dynamically monitors the mass of individual growing cells by using a suspended microchannel resonator (SMR) mass sensor. Here, Manalis and colleagues refine this technique, enhancing the precision of cell mass measurement, and also integrate a microscope, which enables the visualization of cell cycle progression using fluorescent cell cycle indicators. Using this approach, they reveal that there is an increase in growth rate during G1-to-S phase transition, suggesting that cell mass and cell cycle progression are tightly coupled. Importantly, despite initial variability in growth rates, all cells entered S phase with similar growth rates. This indicates that a growth rate threshold, rather than a critical size threshold, controls the G1-to-S phase transition.