chapter 52 measurement systems, basic mathematics, and dosage calculations copyright ©2012 delmar,...

Chapter 52

Measurement Systems, Basic Mathematics,

and Dosage Calculations

Copyright ©2012 Delmar, Cengage Learning. All rights reserved.

The Metric System

• Primary system of measurement in medicine• Three basic units:

– Gram (g), which measures mass (weight)– Liter (L), which measures volume (liquid)– Meter (m), which measures length (size)


The Metric System

• Based on multiples of 10– Add prefixes to the basic units to indicate the

value (multiple or submultiple) of the unit– Kilo = 1000– Kilo+gram = 1000 grams


The Metric SystemPrefix Unit Decimal ValueMicro- 1/1,000,000 of a unit 0.000001Milli- 1/1,000 of a unit 0.001Centi- 1/100 of a unit 0.01Deci- 1/10 of a unit 0.1No prefix 1 unit 1Deka- 10 units 10Hecto- 100 units 100Kilo- 1,000 units 1,000


Household Measures

• Patients at home typically use household measurements

• You may need to use these measurements to explain to patients how to take their medications after discharge


Household Measures

• Drop (gtt)• Teaspoon (t or tsp)• Tablespoon (T or tbs)• Ounce (fluid) (fl oz)• Cup

• Pint (pt)• Quart (qt)• Ounce (weight) (oz)• Pound (lb)


Household Measures60 gtt = 1 tsp

3 t = 1 T2 T = 1 fl oz

1 cup = 8 fl oz1 pt = 2 cups

1 qt = 4 cups (2 pt)1 lb = 16 oz


The Apothecary System

• The first system of medication measurement, but not commonly used today

• Basic units:– Grain (gr)– Quart (qt)– Pint (pt)– Ounce or fluid ounce (oz)– Dram– Minim


Review of Basic Math

• Basic math skills are necessary for accurately calculating and verifying medication dosages to ensure patient safety before administering any medication



• A fraction indicates a part of a whole number– Numerator: top of fraction– Denominator: bottom of fraction

• A percentage expresses a value that is part of 100Example >> 35% is 35/100 (or 0.35)



• A ratio expresses a relationship between two components– It is another way to express a fractionExample >> 1 : 5 is the same as 1/5



• A proportion expresses the relationship between two ratiosExample >> 3 : 5 = 600 : 1000– Means: 5 and 600– Extremes: 3 and 1000– The product of the means always equals the

product of the extremes– You can solve for X if you know the other 3 values


Dosage Calculations

• Calculating the right dose of medication is a responsibility not to be taken lightly

• The wrong dose of any medication can have profound adverse consequences


Basic Formula

• (Needed/Available) x Vehicle = Dose– Needed > Amount prescribed by provider– Available > Supply on hand– Vehicle > Form and amount of supply


Ratio and Proportion Method

• Dosage on hand : amount on hand = Dosage desired : amount desired (x)

• When using this method, you may need to convert between units of measurement


Converting Between Units of Measurement

• Identify the equivalent• Set up a proportion of two equivalent ratios• Solve for x

• Calculating dosage by weight– Conversion factor: 1 kg = 2.2 lbs


chapter 52 measurement systems, basic mathematics, and dosage calculations copyright ©2012 delmar,...

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