Look around at products in the pharmacy, and you’ll see volumes and weights measured in milliliters and grams rather than teaspoons and pounds. The U.S. is one of only three countries that has not officially converted to the metric system in all areas of industry; however, the metric system is the official standard of measurement in pharmacy and has been since the 1800’s.
There are several reasons why we use the metric system in pharmacy. The most important reason is for dosing because it is the most accurate. A dose written as 15 mL is a precise amount in a measuring cup; the equivalent dose of 3 tsp may vary in amount depending on the measuring spoon used by the patient.
Another advantage of the metric system is that calculations are easier because the metric system is based on multiples of ten. 1 gram equals 1000 milligrams. 1 liter equals 1000 milliliters. This is more logical than the customary system where 12 inches equal a foot, and 3 feet equal a yard. Being able to multiply or divide by multiples of 10 is like our dollar system where ten pennies equal a dime, and ten dimes equal a dollar.
It’s not often anymore that prescriptions are written for teaspoons rather than mL, but you need to know the conversion rates in case you need to make that calculation (and for the PTCE.) There are two things to understand before you start measuring: a liter is a measurement of volume (such as the volume of a liquid medication) and the gram is a measurement of weight (such as the weight of a solid medication or a person.)
Conversions
| Customary | Metric Equivalent |
| 1 tsp | 5 mL |
| 3 tsp=1 Tbsp | 15 mL |
| 1 oz=2 Tbsp | 30 mL |
| Customary | Metric Equivalent |
| 1 oz | 30 g |
| 1 lb | 454 g |
| 2.2 lb | 1 kg |
It’s not enough to memorize that 1 tsp equals 5 mL or 2.2 lb equals 1 kg; to catch potential medication errors, you need to be able to visualize what 5 mL of medication looks like. If you comprehend what the amount should look like, you’ll be able to recognize when something doesn’t look right. A standard dose cup (the kind that comes with liquid DayQuil for example) is 30 mL. Therefore, 6 tsp or 2 Tbsp should fill the line to the 30 mL mark in a dose cup.
Example: You receive a prescription for Amoxicillin 400 mg/5mL with sig, “Take 1 tsp by mouth 2 x daily for 7 days.” How many mL of liquid medication do you need to dispense?
Conversion: 1 tsp=5 mL
5 mL x 2 times a day = 10 mL/day
10 mL/day x 7 days = 70 mL
Example: A child is to receive 15 mg/kg/dose of acetaminophen. The child weighs 80 lbs. How many mg acetaminophen should the child receive per dose?
Conversion: 2.2 lb=1kg
80 lb x 1 kg/2.2 lb=36.4 kg
36.4 kg x 15 mg = 546 mg per dose
It is especially important to remember that 2.2 lb equals 1 kg. Major under and overdosing errors happen when the weight of a patient is recorded in pounds, but the dosage is calculated based on kilograms.
One additional step for safety you can take as a pharmacy technician is to dispense a measuring cup or syringe with liquid medications. Remember that all teaspoons are not the same, but 5 mL is always a teaspoon!