The adult human body is made up of an estimated 37 trillion cells.

Counting the cells in a human body is far harder than it sounds, since cells vary enormously in size and new ones are constantly being created and destroyed. A widely cited 2013 estimate, arrived at by combining known cell counts and densities across major tissue types, put the number at approximately 37.2 trillion cells in an average adult body.

Red blood cells alone make up a large share of that total, despite contributing relatively little to total body mass, simply because they’re so numerous and so small. The estimate is necessarily approximate — later studies have proposed somewhat different totals depending on methodology — but it remains one of the most commonly referenced figures for the scale of cellular organization inside a single human body.