The thickness of lips refers to the thickness of upper and lower red lips when the mouth is gently closed. Some people divide it into four categories according to the average thickness of the upper and lower layers: ① small thin lips: the thickness is below 4 mm; ② Middle lip: 5 mm; ③ Thick lips: 9 ~12mm; ④ Thick convex lip: greater than 12mm. Because the thickness of the upper and lower lips is not exactly the same, and the lower lip is usually thicker than the upper lip, Japanese cosmetic medical experts believe that the thickness of the upper and lower lips should be observed separately. In their view, the standard value of female beautiful lips should be: upper red lip 8.2 mm, lower red lip 9. 1 mm, male 2 ~ 3 mm.