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Category: Medical

Topic: Medical Math

Level: AEMT

Next Unit: IV Flow Rate Calculations

14 minute read

A 1968 study recommended to Congress a 10-year conversion plan to metric, but it failed because it was voluntary. Today, it is felt that the benefits would be small, but the costs great, due to the cost of conversion from a system entrenched for hundreds of years.

There is one discipline in everyday use, however, where the accuracy of the metric system was important enough to adopt the system universally (although not mandatory), and that was in the practice of medicine, where lives depend on accuracy. Simplicity breeds accuracy.


The Metric System

The advantage of the metric system is that is a "base-10" system. That is, 10 things make one thing, and ten of those make another. In pharmacies, the old Imperial system, using grains, minims, drams, ounces, etc., has been replaced for the most part by the metric counterparts:

1 ml = 15 minims = 15 drops.

15 ml = 4 drams = 1 tablespoon. (Close enough...1.01442 tablespoons)

30 ml = 1 ounce (fluid). (Close enough...1.0142 oz)

500 ml = 1 pint. (Close enough...1.05669 pints)

1,000 ml = 1 quart. (Close enough...1.05669 quarts)

4,000 ml = 1 gallon. (Close enough...1.056688 gallons)

60 mg = 1 grain (gr) and 15 gr = 1 gram (g). 1 kg = 2.2 pounds.

Metric = Simplicity: the basis of the metric system, using groupings of ten, means that the decimal point can be used, so it works with simple arithmetic.

  • 1 Liter (L) = 1,000 milliliters.

Since 1 liter = 1,000 ml, 1 mL = 0.001 L (1/1000 of a liter).

  • 1,000 microgram (mcg) = 1 milligram (mg.).
  • 1 gram (g) = 1,000 milligrams (mg.).
  • 1,000 grams = 1 kilogram (kg.), which for the sake of demonstration also = 1,000,000 mg.

1 kg = 1,000 gm = 1,000,000 mg = 1,000,000,000 mcg.

WEIGHT: milligrams, grams, kilograms.

VOLUME: mL, liters.

Milli- means one-thousandth. Kilo- means a thousand of something.



MASS (or amount) of a drug is measured in grams (g).

From smallest to largest:

  • microgram (mcg)
  • milligram (mg)
  • gram (g)
  • kilogram (kg)

VOLUME is measured in liters. From smallest to largest:

  • milliliters (mL)
  • liters (L)

FYI: the measurement cubic centimeter, or "cc," is used interchangeably with mL.

â–º1 cc = 1 ml.

Most interval jumps of named metric measurements are in increments of a thousand. Since these jumps are by the thousand, conversions are done by moving 3 decimal places in either direction, which is the same as either multiplying or dividing by 1,000, depending on whether you're going from larger to smaller or from smaller to larger.

TRICK: Go smaller, go to the right. Go larger, go to the left.

It's a matter of moving the decimal point 3 places in either direction.

Going from small to large, direction = 3 decimals the left, which is the same as dividing it by 1,000.

Going from large to small, direction = 3 decimals to the right, which is simply the same thing as multiplying it by 1,000.

EXAMPLES: 1 L = 1,000 ml = 1,000.00 ml.

Converting 1,000.00 ml to L (smaller to larger, therefore 3 decimal places to the left) move the decimal point to the left 3 places. 1. < x < x < x .00 to become 1.000 L.

Converting 5200 g to mg means going from larger to smaller units, so move the decimal 3 places to the right. 5200.(→ x→ x→ x.00) to become 5200000., or 5,200,000 g.

However, the plot thickens: although not used frequently, some odd measurements can be seen, such as the deciliter (one-tenth of a liter, or dl.). The 3-decimal shift won't work for these.

These are primarily used in blood test results and not in medications. Example: a fasting blood sugar level is 100 mg/dL.

HOLDOVERS: Some common Imperial measurements still used, mainly in households:

  • 15 drops (ggt) = 1 ml;
  • 15 ml = 1 tablespoon (Tbsp) (5 ml = 1 teaspoon--"tsp."; 3 teaspoons = 1 tablespoon.)
  • 1 grain (gr) = 60 mg.

Tsp, tbspn, and gr: Household pediatric liquid medications often give directions in teaspoons (tsp) and tablespoons (tbsp); AND

grain is FREQUENTLY used in morphine doses, so you should know these three holdovers.

CAUTION: Many medical mistakes resulting in Adverse Drug Events (ADEs) have been made thinking grains instead of grams and vice versa.

  • 1/4 gr morphine = 15 mg.
  • 1/6 gr morphine = 10 mg.
  • However, 1/4 gram of morphine = nearly 4 grains, so this mistake could be lethal!

1 kg = 1,000 g. 1 mg = 1/1000 A kilogram(kg) would be 1000 grams. A milligram(mg) would be 1/1000th of 1 gram. A microgram(mcg) is 1/1000th of a milligram.



You have 500 mg and you want to convert it to grams (go from smaller to larger):

Moving the decimal point 3 steps left, you find that 500 mg becomes 0.500 g, 0.5 g (half a gram).

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