Despite playing a significant role in the global water cycle, ocean volume has not been re-examined in over 25 years. The main uncertainty associated with ocean volume is the mean ocean depth. The earliest studies tended to overestimate ocean depth due to undersampling of seamounts and ocean ridges. The advent of the echosounder in the 1920s and subsequent ship-borne technologies rapidly increased aerial coverage of the ocean; hence, over time there has been a gradual decrease in calculated mean ocean depth. Today, however, in situ measurements span only ~ 10% of the ocean’s surface area. Here, we use satellite altimetry data to estimate the ocean’s volume, which is lower by a volume equivalent to 500 times the Great Lakes or five times the Gulf of Mexico when compared to the most recent published estimates.