The following "density quiz," part of a longer presentation by Redwood City, CA planner Dan Zack, highlights the inadequacy of the units per acre ratio as a measure of density given that the physical form in which similar levels of density are contained can vary widely:
One point which comes across for me in this segment is how building height, although it may be the most immediately noticeable building trait, and the one we instinctively associate with "high density," is far from the only, or even most important, contributor to overall density. The compounded effect of high lot coverage, modestly-sized units and limited on-site parking can produce exceedingly high densities in the absence of great height, while the Corbusian towers-in-a-park design approach produces much lower density even where buildings are very tall.
The pro-density urban advocate who focuses on the relaxation of height limits to the exclusion of other factors such as parking requirements, mandatory setbacks, minimum street widths and minimum unit sizes addresses only one component of overall density, and one which, from a city-wide perspective, may not have an especially large impact on it.
Finally, a mindset which holds up the increase of density, by whatever means, as an absolute good, ignores important non-economic costs and benefits of each means of increasing densification. The negative impacts of very tall buildings on urban life, beyond the obstruction of views and natural light immediately noticed by city residents, have been covered at length by Nikos Salingaros and Léon Krier, among others. Greater lot coverage and narrower streets, meanwhile, confer numerous benefits, both economic and not, in addition to their positive impact on density.