Pharmaceutical Calculations: Foundations of Pharmaceutical Measurement

Today, we kick off with Foundations of Pharmaceutical Measurement by breaking down the three primary languages of measurement you will encounter: the Metric System, the Household System, and the antiquated Apothecary System.

1. The Metric System: The Legal Standard

The metric system is the most common system used in pharmacy practice and is the legal standard for medicine in the Nigeria. It is favored for its accuracy, precision and ease of use.

The metric system is decimal-based, meaning all units are described as multiples of 10. It measures the following:

• Length: Measured in meters (m), Centimeters (cm) e.t.c.
  
• Volume: Measured in liters (L), Centiliters (cl) Mililiters (ml) e.t.c.
  
• Weight: Measured in grams (g) Micrograms e.t.c..
  

To incorporate the metric system in your practice, you must memorize the prefixes that denote larger or smaller quantities. Here are some of  the most important ones for pharmacy:

• Kilo- (k): One thousand times the base unit (1,000 X). e.g kg (kilogram) 
• Milli- (m): One-thousandth of the base unit (0.001 X). E.g Mg (Miligram)
• Micro- (mc): One-millionth of the base unit (0.000001 X). E.g. Mcg (Microgram)

Examples

CONVERTING TO KG

To convert Kilograms (kg) to Grams (g), multiply by 1,000 (move the decimal 3 places to the right).

Example A: Patient Weight

• Problem: Convert a patient’s weight of 72.5 kg into grams.
• Calculation: 72.5 X 1,000 = 72,500.
• Answer: 72,500g.

Example B: Bulk Compounding

You have 0.25 kg of ointment base. How many grams is this?

Calculation: 0.25 X 1,000 = 250.

Answer: 250g.

CONVERTING TO MG

To convert Grams (g) to Milligrams (mg): Multiply by 1,000 (move decimal 3 places right).

To convert Milligrams (mg) to Grams (g): Divide by 1,000 (move decimal 3 places left).

Example A: (g – mg)

A prescription calls for 1.5 g of Cephalexin. How many milligrams is this?

 1.5 X 1,000 = 1,500. Answer: 1,500 mg.

Example B: (mg – g)

You have a 500 mg tablet. Express this weight in grams.

Calculation: 500 / 1,000 = 0.5. Answer: 0.5 g.

CONVERTING TO Micro- (mc): One-millionth of the base unit.

• Conversion:  mg = 1,000 mcg.
  • To convert Milligrams (mg) to Micrograms (mcg): Multiply by 1,000.
  • To convert Micrograms (mcg) to Milligrams (mg): Divide by 1,000.

Example A (mg – mcg)

A patient takes 0.125 mg of Levothyroxine. How many micrograms is this?

Calculation: 0.125 X 1,000 = 125. Answer: 125 mcg.

Example B: (mcg – mg)

A capsule contains 1000 mcg of Cyanocobalamin. How many milligrams is this?

Calculation: 1000 / 1,000 = 1. Answer:  mg.

2. The Household System

While pharmacists Make use of  “Metric,” patients will often use “Household.” This system includes teaspoons, tablespoons, cups, and gallons.

Household measuring devices vary significantly in actual capacity. A spoon from a kitchen drawer is rarely an accurate medical device.

 Always advise patients to use calibrated administration devices rather than kitchen silverware.

HouseHold System EquiValents

• 1 Teaspoon (tsp) = 5  mL
• 1 Tablespoon (tbsp) = 15  mL
• 1 Fluid Ounce (fl oz) = 30  mL(approx)
• 1 Pint (pt) = 480  mL/ (16 fl oz)
• 1 Quart (qt) = 960  mL/ (2 pints)
• 1 Gallon (gal) = 3,840  mL/ (4 quarts)

3. The Apothecary System 

The apothecary system is an outdated system previously used in science and medicine. Its use is generally discouraged today due to safety concerns and inaccuracies, but you will still see remnants of it in practice.

Exceptions where this system survives include thyroid medications and phenobarbital dosing.

The only equivalent unit of measure between the apothecary system and the modern avoirdupois system is the grain (gr).

• Conversion: 1  grain (gr) = 64.8  mg (often rounded to 65  mg/)18.
  
• Classic Examples: You may see Aspirin 325mg referred to as “5gr” (5 X 65 = 325).
  

4. The Avoirdupois System

This is the common system for measuring weight in the United States (selling goods, food, etc.).

• Pound (lb): Based on 16 ounces.

• Conversion to Metric: 1  kg = 2.2  lb. This conversion is important for weight-based dosing.

Cheat Sheet: Common Pharmacy Equivalents

Memorize these standardized conversions to move fluidly between systems.