With tarps you have to look at both the mil (thickness) and the oz (material weight per square yard). So it might be 18 mil thick, but if the oz weight is low the tarp will be lightweight and probably not as durable.
Tarp Material Weight and Material Thickness
When comparing tarps it is important to compare using the same classification. Some websites rate their tarps by thickness. This is somewhat meaningless since it is mainly the density of the material that gives it its durability, not the thickness. To add to the confusion, some websites show material thickness in millimeters (1,000th of a meter) while others show the thickness in mils (1,000th of an inch).