The question of how many drops constitute a cubic centimeter (cc) or milliliter (mL) is common when measuring medications or flavorings. A cc and an mL are identical, precise, and standardized units of volume in the metric system. However, a “drop” (abbreviated $gtt$ from the Latin gutta) is an inexact, variable measure whose volume is not standardized outside of controlled environments. Relying on the drop’s lack of precision for accurate measurements can be misleading.
The Medical and Pharmaceutical Standard
For standardized measurements, such as pharmaceutical dosing, a conventional value has been established. For calculation purposes in pharmacy, a drop is conventionally rounded to $0.05$ mL, translating to a standard of $20$ drops per milliliter.
This $20$ drops/mL standard is widely accepted when using a calibrated dropper designed to meet these specifications. In hospital settings, intravenous (IV) tubing uses standardized drop factors to control infusion rates. Macro-drip tubing, used for large volumes, commonly employs factors of $10$, $15$, or $20$ drops per mL. Micro-drip tubing delivers $60$ drops per mL for highly potent or small-volume medications, especially in pediatric care.
Physical Factors Affecting Drop Size
The actual volume of a drop varies significantly from the $20$ drops/mL convention due to the physical properties of the liquid and the dispensing device. Drop formation and detachment are governed by gravity versus the forces holding the liquid together. Three primary variables control the final volume of a drop: viscosity, surface tension, and the size of the dropper’s tip.
Viscosity, the liquid’s resistance to flow, plays a major role in determining drop size. Thicker, more viscous liquids, such as oils or syrup, form larger drops because internal friction delays separation from the dropper tip. This increased cohesion requires a greater volume and more gravitational force to overcome the liquid’s stickiness. Consequently, a milliliter of a highly viscous liquid will contain fewer drops than a milliliter of water.
Surface tension is the cohesive force between the liquid’s molecules at the surface. Liquids with high surface tension, like water, create a stronger “skin” that resists breaking, resulting in a larger drop volume. Conversely, liquids with low surface tension, such as alcohol, separate more easily from the tip and form smaller drops.
The diameter of the dropper’s orifice, or the size of the tip opening, is the most straightforward physical variable. A larger tip provides a greater surface area for the liquid to cling to, requiring a larger volume of liquid to build up before gravity overcomes the adhesive forces holding it to the tip. This is why non-standardized household droppers, pipettes, and medicine bottles produce different drop sizes, making an exact count impossible without calibration.
When Precision Matters: Accurate Measurement Techniques
When administering medication or mixing precise chemical solutions, accurate volume measurement tools must be used. Relying on a drop count is suitable only for non-critical applications, such as estimating a flavoring agent in a large mixture. For critical dosing, calibrated instruments are necessary to ensure safety and effectiveness.
Calibrated oral syringes are a reliable alternative, measuring liquid volume by displacement and marked in precise milliliter increments. Laboratory tools like graduated cylinders, burettes, and pipettes are even more accurate. These are designed with narrow, consistent diameters to minimize measurement error and allow for reading the liquid’s meniscus at eye level. These tools provide the consistency that the subjective “drop” unit cannot offer.
A practical way to ensure accuracy with a household dropper is to perform a simple calibration for the specific liquid. This involves counting how many drops it takes to fill a known, precise volume, such as $1$ mL, using a calibrated measuring syringe as a reference. This process establishes a specific drop-to-milliliter ratio for that liquid and dropper, providing a more reliable conversion for future use.