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The tip of your hair is about 0.1 millimeters (mm), or 100 micrometers (µm), wide. If you assume that the tip is roughly circular, and the area of a circle is pi times radius squared (3.14 X 50µm^2), then the area of the tip is about 7,850 µm. We're assuming a 2 µm long E. coli cell is 1.25 µm wide, so its area is length times width, or 2 µm x 1.25 µm, which equals 2.5 µm squared. Divide 7,850 µm by 2.5 µm and you find out that roughly 3,140 E. coli could fit on the tip of your hair.