We advance Urban Density—the ratio of the total population of a city and its overall urban footprint—as the single most useful metric for measuring the overall compactness of cities as well as for measuring progress in their densification, now a recognized sustainability objective. Yet we also realize that adopting Urban Density as a single measure of compactness hides more that it reveals. The key contribution of our study is in strengthening its usefulness by exposing its anatomy, showing that it can be factored into separate metrics. By factoring Urban Density into two, three, four, and then seven factors, we provide a structure for ordering most of the familiar density metrics used by planners, thus contributing to increased ‘density literacy.’ We show, for example, that Urban Density= Floorspace Occupancy × Floor Area Ratio × Residential Share, where Floorspace Occupancy is the average number of people per hectare of floor area in the city; the Floor Area Ratio is the ratio of the total residential floor space and the total residential area in the city; and Residential Share is the share of the residential area in the city’s overall urban footprint. Our second contribution is a rigorous and replicable methodology for measuring Urban Density and all its factors in a representative group of ten cities from all world regions. Factoring Urban Density shows, for the first time, how different cities acquire their density: Hong Kong, for example, gets its high Urban Density from its high Floor Area Ratio; Kinshasa from its high Floorspace Occupancy; and Dhaka from its high Floorspace Occupancy and Residential Share. Our anatomy of density offers a novel outline for a comprehensive strategy for city densification, one that focuses on possibilities for increasing each of the seven factors that constitute Urban Density through selective policy interventions.