11 Apothecaries’, Avoirdupois & Metric Measurement Systems

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Metric Measurement System

In the metric system,
we use the gram to measure units of weight,
the liter to measure units of volume,
and the meter to measure units of length.

Measurements can vary in scale depending upon the application. We use prefixes to precede these basic units of measurement to indicate a multiple or fraction of that unit.

The most often used prefixes are

  • micro ( 1 / 1,000,000 of 1 unit )
  • milli ( 1 / 1,000 of 1 unit )
  • centi ( 1 / 100 of 1 unit )
  • kilo ( 1,000 times 1 unit )

Picture something familiar to mostly anyone — a 2 liter bottle of soda. This will aide you in visualizing what 2 milliliters of a liquid looks like. 2 milliliters (or 2 mL for short) is equivalent to 2 TIMES (1 / 1,000 of 1 liter). We may see this written in terms of the base unit itself as 0.002 liters (or 0.002 L for short) because 2 TIMES (1 / 1,000 of 1 liter) is equal to 0.002 liters, arithmetically.

Tablet and capsule dosage strengths are commonly written in milligrams. A 325 milligram (325 mg for short) Tylenol ® (generic name acetaminophen) capsule contains 0.325 g of the active ingredient.

PRACTICE: Rewrite the following weights in grams:

  1. 10 kg
  2. 700 mg
  3. 500 micrograms

Apothecaries’ Measurement System

In the apothecaries’ system, we do not have a unit to measure length. Instead, we use the grain (or gr for short) to measure units of weight and the minim to measure units of volume.

As opposed to using prefixes, the apothecaries’ system uses equivalents based upon the grain or the minim. Be mindful that the apothecaries’ 12 ounce pound is not equivalent to the more common 16 ounce avoirdupois pound.

Apothecaries’ Weight: US (UK pre 1824)

  • 20 grains = 1 scruple
  • 60 grains = 3 scruples = 1 dram
  • 480 grains = 24 scruples = 8 drams = 1 ounce
  • 5,760 grains = 288 scruples = 96 drams = 12 ounces = 1 pound

Apothecaries’ Volume: US (UK pre 1824)

  • 20 minims = 1 fluid scruple (fl scruple)
  • 60 minims = 3 fluid scruples = 1 fluid dram
  • 480 minims = 24 fluid scruples = 8 fluid drams = 1 fluid ounce
  • 7,680 minims = 384 fluid scruples = 128 fluid drams = 16 fluid ounces = 1 liquid pint
  • 2 liquid pints = 1 quart (qt) = 32 fluid ounces
  • 8 liquid pints = 4 quarts = 1 gallon (gal) = 128 fluid ounces

Avoirdupois Measurement System

In the avoirdupois system, there is neither a unit to measure length nor a unit to measure volume. We only have the grain (or gr for short) to measure units of weight. You may see a tablet or the markings on a bottle labeled with these units. Please note that the avoirdupois grain differs in value from the apothecaries’ grain and that the latter is antiquated so unless asked otherwise, always use the following values on the exam. You may find that some of the following values have been rounded to the tenths or hundredths place on the exam.

Avoirdupois Weight: US (UK pre 1824)

  • 1 grain (gr) = 0.06479891 grams (g) = 64.79891 mg
  • 1 gram = 15.4323584 grains
  • 1 dram (dr) = 27.34375 grains = 1.7718452 grams
  • 1 ounce (oz) = 16 drams = 437.5 grains = 28.3495 grams
  • 16 ounces = 256 drams = 7000 grains
    = 453.592 grams = 1 pound (lb)
  • 1 pound (lb) = 0.453592 kilogram (kg)
  • 1 kilogram (kg) = 2.204622 pounds (lb)

Ways to Prevent Medication Errors

  • If a decimal is smaller than 1, ALWAYS place a zero to the left of the decimal point.
    Example: Write 0.325 not .325
  • If you’re writing a whole number NEVER add a decimal point and a zero afterwards.
    Example: Write 25 not 25.0
  • Stop using mixed numbers. If you have a fraction write it with the decimal point.
    Example: Write 7.5 not 7 1/2
  • You need NOT write extra zeroes unnecessarily.
    Example: Write 105.01 not 105.0100
  • Stop and think.. does your conversion make sense?
    Example: If you’re converting from g to mg, the number should be bigger not smaller.
  • Dividing by 10? Move the decimal to the left.
    Multiplying by 10? Move the decimal to the right.
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