The drop is a unit of measure of volume. It is a variable amount of fluid and depends on the device and technique used to produce the drop and on the physical properties of the fluid. This is similar to units like the cup, tablespoon, and teaspoon that depend on the spoon or cup and are not exact either.

• the "metric" drop is defined as 1/20 mL (50 μL)
• the medical drop is defined as 1/12 mL (83 1/3 μL)
• the Imperial drop is defined as 1/36 of a fluidram (1/288 of an Imperial fluid ounce, or 1/1440 of a gill) (approximately 98.656 μL)
• an alternate, possibly apocryphal, definition of the drop is 1/1824 of a gill (approximately 77.887 μL)
• the U.S. drop is defined as 1/60 of a teaspoon or 1/360 of a U.S. fluid ounce (approximately 82.149 μL)
• an alternate definition of the U.S. drop is 1/76 of a teaspoon or 1/456 US fl oz (approximately 64.854 μL)