Drop (unit)

For other uses, see Drop (disambiguation).

The drop is a unit of measure of volume, the amount dispensed as one drop from a dropper. 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 and on the physical properties of the fluid.

A water drop is a basic measurment. Create a dripping faucet or hose. Each drop must be a single unit; it cannot be a stream or have multiple drops coming together. Collect and count each drop using a small container calibrated in mL or ounces (1000 mL = 1 Liter and 29.6 mL = 1 fluid oz). The experiment shows that 5 drops of water equals 1 mL or about 150 drops equals 1-oz (US Fluid). A standard unit conversion gives (5.0 drops-water/1.0 mL-water) or (0.2 mL-water/1 drop-water).

There are several exact definitions of a "drop":

• the "metric" drop, 1/20 mL (50 μL).
• the medical drop, 1/12 mL (83⅓ μL).
• the Imperial drop, 1/36 of a fluidram (1/288 of an Imperial fluid ounce, or 1/1440 of a gill) (approximately 99 μL).
• an alternate, possibly apocryphal, definition of the drop is 1/1824 of a gill (approximately 78 μL).
• the U.S. drop, 1/60 of a US fluidram, 1/80 of a teaspoon or 1/480 of a U.S. fluid ounce (approximately 62 μL).
• an alternate definition of the U.S. drop is 1/96 of a teaspoon or 1/576 US fl oz (approximately 51 μL).

According to Webster dictionary, "drop" indicates the smallest volume of a liquid that may be measured.^[1] Drop size varies according to gravity, the density and the surface tension of the liquid. ^[2]

In the past, a drop was another name for a minim. This meaning was used in Pharmacy to describe a volume equal to 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 for drop.^[3]

A drop can also be used less formally as a unit of volume in recipes. According to some older kitchen references, 24 drops = ¼ teaspoon. Using U.S. definitions, this makes the drop equal to about 0.051 mL, making it quite comparable to the pharmacist's drop.^[3]

References

1. ^ "Drop - definition". Webster Dictionary. http://www.merriam-webster.com/dictionary/drop. Retrieved 2010-03-29. 
2. ^ "Drop - size". Physics and Astronomy Online. http://www.physlink.com/education/askexperts/ae681.cfm. Retrieved 2010-03-29. 
3. ^ ^{a b} "Russ Rowlett". How Many? A Dictionary of Units of Measurement. University of North Carolina. 2005-07-11. http://www.unc.edu/~rowlett/units/. Retrieved 2006-09-07.