Chapter 8
Converting Between Measurement
Systems
Copyright © 2012, 2009, 2003 by Saunders, an imprint of Elsevier Inc. All rights reserved.
1
Learning Objectives
Reading the time of day on the international
standard 24-hour clock, converting to the 12hour clock, and converting between the two
time standards.
Converting between Fahrenheit and Celsius
scales.
Computing and converting approximate
volume equivalents within and between
metric, household and apothecary systems.
Copyright © 2012, 2009, 2003 by Saunders, an imprint of Elsevier Inc. All rights reserved.
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Learning Objectives (cont’d.)
Computing and converting approximate
weight equivalents between the metrics and
household systems.
Computing and converting from one length
unit to another within the metric and
household systems.
Copyright © 2012, 2009, 2003 by Saunders, an imprint of Elsevier Inc. All rights reserved.
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Introduction
To change from one form to another.
Necessary in medical field; no world
standard.
When numbers are converted from one
system to another, they will be approximately
equal.
Conversions that are approximately equal
may differ by as much as 10%.
Copyright © 2012, 2009, 2003 by Saunders, an imprint of Elsevier Inc. All rights reserved.
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Time Conversions
Figure 8-1 A, Standard clock and B, international standard clock.
Copyright © 2012, 2009, 2003 by Saunders, an imprint of Elsevier Inc. All rights reserved.
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Check Your Understanding – Time
Conversion 8-1
4:40 AM = 0440
6:25 PM = 1825
11:02 AM = 1102
2:56 AM = 0256
10:45 PM = 2245
8:10 PM = 2010
12:33 AM = 0033
10:00 AM = 1000
3:33 PM = 1533
7:17 AM = 0717
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Measuring Temperature
In the metric system, temperature is
measured using the Celsius scale
0°Celsius = 32°Fahrenheit
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Temperature Conversion
Fahrenheit to Celsius
EXAMPLE: 104°F = _____°C
FORMULA: C° = 5/9 (F° – 32)
SOLUTION:
5/9(104° – 32) = °C
5/9 x 72 = °C
(72 x 5) ÷ 9 = °C
360 ÷ 9 = 40
104°F = 40°C
(Alternative: subtract 32 from F °and divide by 1.8)
Copyright © 2012, 2009, 2003 by Saunders, an imprint of Elsevier Inc. All rights reserved.
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Temperature Conversions
Celsius to Fahrenheit
FORMULA: F° =(9/5 C°) + 32
38.3°C = ______°F
(9/5 x 38.3) + 32
38.3 x 9 = 344.7
344.7 ÷ 5 = 68.94
68.94 + 32 = 100.94
Round to 101.0°F
38.3°C = 101°F
(May also multiply C° by 1.8 and add 32)
Copyright © 2012, 2009, 2003 by Saunders, an imprint of Elsevier Inc. All rights reserved.
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Temperature Conversions
From C° to F°
 Multiply C° by 1.8
 Add 32
From F° to C°
 Subtract 32 from F°
 Divide by 1.8
When performing temperature conversions,
remember to complete the steps in the order
given by first removing the parenthesis.
Copyright © 2012, 2009, 2003 by Saunders, an imprint of Elsevier Inc. All rights reserved.
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Converting Between Measurement
Systems Using Ratio and Proportion
Used to convert when the dosage available is
not in the same measurement unit as the
doctor prescribed.
Ratio: expression comparing two quantities
Proportion: comparing two equivalent
fractions or ratios
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Converting Between Measurement
Systems Using Ratio and Proportion
Solving Unknown Variables
Numerators must be of the same units.
Denominators should be of the same units.
1 mL : 15 gtts is the same as 2 mL : 30 gtts
1 mL/15 gtts = 2 mL/30 gtts
Copyright © 2012, 2009, 2003 by Saunders, an imprint of Elsevier Inc. All rights reserved.
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Converting Between Measurement
Systems Using Ratio and Proportion
Copyright © 2012, 2009, 2003 by Saunders, an imprint of Elsevier Inc. All rights reserved.
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Converting Between Measurement
Systems Using Ratio and Proportion
When setting up both ratios, be sure to label
all of the terms, including x with the correct
measurement to ensure that the equations
are equivalents.
Copyright © 2012, 2009, 2003 by Saunders, an imprint of Elsevier Inc. All rights reserved.
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System Conversions – Volume
Copyright © 2012, 2009, 2003 by Saunders, an imprint of Elsevier Inc. All rights reserved.
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Converting Volume Measurement Units
between Systems – Apothecary to Metric
fʒ x = ______ mL
 Remember, this is 10 fluid drams
 4 mL = fʒ i
 4 mL / fʒ i = x mL / fʒ x
 4 mL
x mL
fʒ x 1
fʒ 10
 4 x 10 = 40
1xx=x
 1 x = 4 x 10 = 40
 fʒ x = 40 mL
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Converting Volume Measurement Units
between Systems – Metric to Household
Known amounts – 1 pint = 500 ml
Insert equation in Example 9 p. 153
Add cross multiplication
500 x = 750
x = 750/500
x = 1.5 pt
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Converting Volume Measurement
Units between Systems
Figure 8-2 Conversion clock for liquid medications between metric and apothecary systems.
(Fulcher R, Fulcher E: Math calculations for pharmacy technicians: a worktext, St Louis, 2007, Saunders.)
Copyright © 2012, 2009, 2003 by Saunders, an imprint of Elsevier Inc. All rights reserved.
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Converting Volume Measurement
Units between Systems
Example of Apothecary Volume Measurements
ɱ 90 = ʒ ____
Equation needed – 60 minims = one dram
90 minims : x drams :: 60 minims : 1 dram
60 x = 90
x = 90/60
x = 1 ½ drams
Look at the clock and figure that 60 minims (on the
inside of the clock) = one dram (on outside of the clock)
Copyright © 2012, 2009, 2003 by Saunders, an imprint of Elsevier Inc. All rights reserved.
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Weight
Copyright © 2012, 2009, 2003 by Saunders, an imprint of Elsevier Inc. All rights reserved.
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Converting Weight Measurement Units
between Systems
Figure 8-3 Conversion clock for weight or solid measurements. (Fulcher R, Fulcher E: Math calculations for pharmacy
technicians: a worktext, St Louis, 2007, Saunders.)
Copyright © 2012, 2009, 2003 by Saunders, an imprint of Elsevier Inc. All rights reserved.
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Converting Weight Measurement Units
between Systems – Apothecary to Metric
Convert quantities into same unit of measure as
you convert within the metric system.
300 mg = gr ______
 Remember 60 mg = 1 gr or 60 mg / gr i
 60 mg / gr i = 300 mg / x gr
 60 mg
300 mg
gr i






x gr
60 x x = 60x
300 x 1 = 300
60x = 300
60x/60 = 300/60
x=5
300 mg = gr v
Copyright © 2012, 2009, 2003 by Saunders, an imprint of Elsevier Inc. All rights reserved.
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Converting Weight Measurement Units
between Systems – Household to Metric
The conversion for metric system to
household system is
 2.2# = I kg
 206# = ____ kg
 2.2#/ 1 kg = 206# / x kg
 2.2 x = 206
 x = 206/2.2
 x = 93.6 kg
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Length
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Converting Length Measurement
Units between Systems
An abdominal cavity was opened with a 14-inch incision. Convert this
measurement into cm.
• 2.5 cm = 1 inch
• 2.5 cm / 1 in = x cm / 14 in
• 2.5 cm
x cm
1 in
14 in
• 2.5 x 14 = 35
• 1xx=x
• x = 35
• 14 inches = 35 cm
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