How big are fleas?

cat flea on human leg

Denni Schnapp

Img 1 An adult cat flea on a human leg. See the length of the leg hair for a size comparison.


Adult cat fleas are 1.5 to 3.2 millimeters long. Females are larger than males. Though small, fleas are big enough to see with the human eye. For comparison, a black pepper peppercorn has a diameter of 5 millimeters.


Flea Size

Body Length

Adult cat fleas typically measure 1.5 to 3.2 millimeters in length. The females are bigger than the males, with larger and heavier abdomens. The average length of female fleas is 2.5 mm, while males can be less than 1 mm long. Though tiny, fleas can be seen by the naked eye.

Head Size

An elongate head is one of the distinguishing characteristics of cat fleas. In females, the head’s length is twice the height. In males, the head is as long as it is tall.

Size Comparisons

  • Black pepper peppercorn: 5 mm diameter
  • Tip of a sharp #2 pencil: 1 mm
  • Dimes: 17.9 mm diameter
  • Side of a penny: 1.5 mm thick
  • Large sesame seed: 3.5 mm diameter
  • Poppy seed: 1 mm diameter

cat flea size


Img 2 An adult cat flea’s size compared to the tip of a sharp #2 pencil.

Flea Growth

Fleas don’t molt once they reach adulthood. Thus, their body size remains fixed. Growth only occurs when the abdomen swells during the first blood meal. Segments of the abdomen initially overlap, but spread out during feeding. Fully engorged fleas are double their unfed size. Fleas reach their maximum body weight within 48 hours of feeding. Females increase in weight by 140% and males by 19%.


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  • Cheese Man October 24, 2016, 1:10 am

    Fleas are so tiny