Prior studies often mention lipid packing defects caused by cis lipid acyl tails, particularly when compared with trans acyl tails. The opposing trends between thickness and area per lipid do not make it obvious how the packing is changing. By quantifying the total membrane volume, taken to be the membrane surface area multiplied by the membrane thickness, we find that the total volume is larger for the all-cis membrane, indicating tighter packing and a greater overall density for trans lipids (Fig ). Experiments have also shown that trans lipids to have tighter packing, and are intermediate between typical cis lipids and unsaturated acyl tails. The increased packing density and ordering is anticipated to have far reaching and often deleterious effects, such as changing membrane permeability by potentially orders of magnitude,