Drop (unit)
The drop is an approximated 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 and in organic synthesis. The abbreviations gt or gtt come from the Latin noun gutta ("drop").
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 viscosity, density, and the surface tension of the liquid.
Several exact definitions exist:
- In medicine, IV drips deliver 10, 15, 20, or 60 drops per ml. Micro-drip sets deliver 60 drops per ml and 10, 15, or 20 drops per ml for a macro-drip set.
- Prior to the adoption of the unit 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 was, while inexact, presumed to be equal to 1/60 of a fluid dram or 1/480 of a fluid ounce.
- Under the modern US customary measurement system, 1 drop is 1/72 of a US customary fluid dram.
| 1 US customary drop | = | 5/6 | US customary minim |
| = | 1/72 | US customary fluid dram | |
| = | 1/288 | US customary tablespoon | |
| = | 1/192 | US customary dessert spoon | |
| = | 1/96 | US customary teaspoon | |
| = | 1/48 | US customary coffee spoon | |
| = | 1/24 | US customary salt spoon | |
| ≈ | 0·87 | British imperial minim | |
| ≈ | 0·0036 | UK tablespoon | |
| ≈ | 0·0072 | UK dessert spoon | |
| ≈ | 0·014 | UK teaspoon | |
| ≈ | 0·029 | UK salt spoon | |
| ≈ | 0·87 | UK drop | |
| ≈ | 0·051 | millilitre | |
| ≈ | 0·0034 | international metric tablespoon | |
| ≈ | 0·0026 | Australian metric tablespoon | |
| ≈ | 0·0051 | metric dessert spoon | |
| ≈ | 0·01 | metric teaspoon |
- In the United Kingdom, subsequent to the adoption of the minim and the creation of the British imperial system of units in the 1820s, a drop is defined as 1 British imperial minim, the equivalence of 1/60 of a British imperial fluid drachm or 1/480 of a British imperial fluid ounce.
| 1 UK drop | = | 1 | British imperial minim |
| = | 1/60 | British imperial fluid drachm | |
| = | 1/240 | UK tablespoon | |
| = | 1/120 | UK dessert spoon | |
| = | 1/60 | UK teaspoon | |
| = | 1/30 | UK salt spoon | |
| = | 1/480 | British imperial fluid ounce | |
| ≈ | 0·96 | US customary minims | |
| ≈ | 0·016 | US customary fluid dram | |
| ≈ | 0·004 | US customary tablespoon | |
| ≈ | 0·006 | US customary dessert spoon | |
| ≈ | 0·012 | US customary teaspoon | |
| ≈ | 0·024 | US customary coffee spoon | |
| ≈ | 0·048 | US customary salt spoon | |
| ≈ | 1·15 | US customary drops | |
| ≈ | 0·059 | millilitre | |
| ≈ | 0·0039 | international metric tablespoon | |
| ≈ | 0·003 | Australian metric tablespoon | |
| ≈ | 0·006 | metric dessert spoon | |
| ≈ | 0·012 | metric teaspoon |
In organic synthesis, a synthetic procedure will often call for the addition of a reagent "dropwise" with the aid of a syringe or a dropping funnel. The rate of addition for such a procedure is taken to be slow but is otherwise vague: one chemist might consider dropwise to be one drop per second, another five to ten drops per second (almost a stream). Furthermore, needle gauge or the dimensions of the glassware also affect drop volume. To improve reproducibility, experimental procedures also note the total amount of time required to add the liquid or another measure of addition rate. In a related usage, the amount of a reagent, whose precise quantity is unimportant, will sometimes be given in terms of the number of drops, often from a glass pipette. In this usage, a drop is typically considered to be approximately 0.05 mL. The practice of giving quantities this way has declined in usage.