Counting the uncountable For many years, textbooks often quoted figures ranging from 10 trillion to 100 trillion cells, but these estimates were based on rough approximations. Their analysis suggests that an average adult weighing around 70 kilograms (154 pounds) contains roughly 30 trillion human cells. Most of your cells are blood cells Here’s one of the biggest surprises. Red blood cells are also unusual because mature human red blood cells lack a nucleus, leaving more room to transport oxygen. It is a living ecosystem made up of around 30 trillion human cells, each specialized for a particular task.