The drop is a unit of measure of volume, the amount dispensed as one drop from a dropper or drip chamber. It is often used in giving quantities of liquid drugs to patients, and occasionally in cooking.
The volume of a drop is not well-defined: it depends on the device and technique used to produce the drop, on the strength of the gravitational field, and on the density and the surface tension of the liquid.
There are several exact definitions of a "drop":
- in medicine, IV drips deliver 10, 15, or 20 drops per mL for macrodrip, 60 per mL for microdrip
- an informal (and incorrect) synonym for minim, the smallest unit of fluid measure in the Apothecaries' systems of the United States customary units, Imperial units, and pre-1824 English units equal to 1/60 of a fluid dram or 1/480 of a fluid ounce
- prior to the adoption of the minim in the early 19th century, the smallest unit of fluid measure in the Apothecaries' systems of the United States customary units and pre-1824 English units, inexact, but presumed to be equal to 1/60 of a fluid dram or 1/480 of a fluid ounce
In the first decade of the 19th century, the minim, the smallest unit of Apothecary Measure, was promoted by the pharmaceutical and medical establishments as an alternative to the drop. It was noted that the size of a drop can vary considerably depending on the viscosity and specific gravity of the fluid, as well as the size and shape of the vessel from which it is poured. (At the time, surface tension was not well-understood.) The minim came with a set of procedures for ensuring accurate measurement, specifically, diluting powerful medicines that had previously been measured by the drop, then using a "minimometer" or "minim glass" (graduated pipette) with minim marks at regular intervals. The minim was defined as one 60th of a fluid dram or one 480th of a fluid ounce. This is equal to about 61.6 μL (U.S.) or 59.2 μL (Britain).
Pharmacists have since moved to metric measurements, with a drop being rounded to exactly 0.05 mL (that is, 20 drops per milliliter). In hospitals, intravenous tubing is used to deliver medication in drops of various sizes ranging from 10 drops/mL to 60 drops/mL. A drop is abbreviated gtt, with gtts used for the plural. These abbreviations come from gutta, the Latin word for drop.
See also 
- "Drop - size". Physics and Astronomy Online. Retrieved 2010-03-29.
- Snyder, Dr. Richard; Schoenborn, Barry (3 May 2011). Medical Dosage Calculations for Dummies. John Wiley & Sons. p. 186. ISBN 978-0-470-93064-9. Retrieved 8 March 2012.
- Nicholson, William (1809). The British encyclopedia, or Dictionary of arts and sciences comprising an accurate and popular view of the present improved state of human knowledge. Whittingham. p. 264. Retrieved 18 December 2011.
- Royal College of Physicians of London; Richard Powell (1809). The pharmacopoeia of the Royal College of Physicians of London, M. DCCC. IX. Longman, Hurst, Rees, and Orme. p. 6-7. Retrieved 18 December 2011.
- Hugh Cornelius Muldoon (1916). Lessons in pharmaceutical Latin and prescription writing and interpretation. John Wiley & sons, inc. p. 147. Retrieved 8 March 2012.