Body length 2.6–3.1 mm in the female and 2.4–2.6 mm in the male. Usually smaller than Pediculus humanus. The body colour is often used a character to separate the species from Pediculus humanus (dark brown in capitis, whitish brown often with a hue of pink in humanus), but according to Jancke (1938) the colour depends mainly on the food and thus is not suited for identification. Pediculus capitis is more densely hairy on the underside of the abdomen than Pediculus humanus. The main morphological difference is the sclerotisation of genital area in the males. In Pediculus humanus there is a strongly sclerotized cuticle plate behind the genital cleft that bears long bristles arranged in rows, whereas in Pediculus capitis this area is only weakly sclerotized and the bristles are arranged irregularly. In addition, the antennal segments in Pediculus capitis are shorter than in Pediculus capitis, especially in the male. The larvae are extremely similar and cannot be reliably identified by morphological characters.
Of course, the two species differ in their ecology: while Pediculus capitis lives in the hairs on the human head, Pediculus humanus is usually found in the clothes, bed linen etc. but lives on humans only briefly for feeding.