Introduction to Pharmaceutical
Calculation
Units of measurement and
metric conversion
Measurement
• is the process of obtaining quantitative
information about the physical world.
Unit of measurement
• any division of quantity accepted as a
standard of measurement or exchange.
Units of measurement used in
Pharmacy
1. Avoirdupois System
• mainly used in measuring bulk medication
encountered in manufacturing.
• Commonly used to measure weight
• Standard measure for weight is grain (gr)
Pound
Ounces
Grains
(lb)
(oz)
(gr)
1=
16 =
7000
1=
437.5
Units of measurement used in
Pharmacy
2. Apothecary System
• Commonly used for prescribing and
dispensing medication
• Commonly used for measuring volume
• Standard of measure for weight is grain
(gr)
• Standard of measure for volume is minim
( )
Apothecary System
• Weight
Pound
(lb)
1=
Ounces
( )
12 =
1 =
Drams
(ʒ)
96 =
8 =
1 =
Scruples
(϶)
288 =
24 =
3 =
1 =
Grain
(gr)
5760
480
60
20
Apothecary System
• Volume
Gallons
(gal)
1=
Pints
(pt)
8=
1=
Fluid
Fluid
Ounces
Drams
(f )
(f ʒ)
1024 =
128 =
8=
1=
128 =
16 =
1=
Minims
(
)
61,440
7,680
480
60
Units of measurement used in
Pharmacy
3. Metric System
•
•
•
is based on the decimal system in which
everything is measured in multiples or fractions
of 10.
major systems of weights and measurement
used in medicine.
Uses standard measures:
 Gram for weight (g)
 Liter for volume (l)
 Meter for length or distance (m)
Metric conversion
• process of restating a metric measurement in a
larger or smaller metric unit of measurement.
Ex: converting from kilograms to grams
Metric units are denoted by prefixes
• Greek prefixes- multiplier
• Latin prefixes - fractions of the base unit
Metric conversion
Tips:
1. Understand the different units of
measurement and what they measure
(example: gram for weight).
2. Memorize the relationship between the
units of measurements 1 kg = 1000 g.
3. Know where to move the decimal point to
convert accurately.
Unit
Abbreviation
Fraction
Exponent
Decimal
Word
Micro
mc
1/1,000,000
10-6
0.000001
Millionth
Milli
m
1/1,000
10-3
0.001
Thousandth
Centi
c
1/100
10-2
0.01
Hundredth
Deci
d
1/10
10-1
0.1
Tenth
Base unit
g,m,l
1
100
1
One
Deka
da
10
101
10
Ten
Hecto
h
100
102
100
Hundred
Kilo
k
1000
103
1000
Thousand
Prefixes with Standard
Measures
Length: standard measure is meter
1 kilometer (km) = 1000 meters (m)
1 meter (m) = 100 centimeters (cm)
1 centimeter (cm) = 1 millimeter(mm)
1 meter (m) = 1000 millimeter (mm)
Prefixes with Standard
Measures
Volume: the standard measure is liter (L)
1 Liter (L) = 1000 milliliter (ml)
1 Deciliter = 100 milliliter (ml)
1 Centiliter = 10 milliliter (ml)
1 Liter = 1, 000, 000 microliter (mcl)
10 Deciliter = 1 liter (L)
100 Centiliter = 1 liter (L)
Prefixes with Standard
Measures
Weight: Standard measure is gram (g)
1 Kilogram (kg) = 1000 grams (g)
1 Gram (g) = 1000 milligrams (mg)
1 Centigram (cg) = 10 milligrams (mg)
1 Decigram (dg) = 100 milligrams (mg)
1 milligram (mg) = 1,000 micrograms (mcg)
1 gram (g) = 1,000,000 micrograms (mcg)
Prefixes with Standard
Measures
Length : Standard measure is meter (m)
1 Kilometer = 1000 meters (m)
1 Meter (m) = 100 centimeter (cm)
1 Meter (m) = 10 decimeter (dm)
1 Meter (m) = 1000 millimeter (mm)
Conversion
Example 1:
How many minims are there in 25 fluid
ounces?
25 fl x 8 fl ʒ x 60
= 12,000
1 fl
1 fl ʒ
Example 2:
How many ounces are 465 scruples?
465 ϶ x 1ʒ x 1 = 19.375
3϶ 8ʒ
Convert the following
1.
2.
3.
4.
5.
6.
7.
30 ml = _________ fluid ounces
855 ml = ________ pints
92 gr = _________ ounces (apothecary)
2.8 kg = ________ mg
75 cm = ________ dm
6 km = _________ cm
453 mm = _______ m
Convert the following
8. 0.00085 dg = _________ mcg
9. 100 mm = ___________ cm
10. 15 km = ___________m
11. 2,846,230 mcl = ______L
12. 2 kg = _____________cg
13. 56,000 mm = ________km
14. 5 kg = ______________dg
Sample Problems
1. The prescription order calls for
preparation of 1.5g of menthol and
salicylic acid (equal quantity) in an
ointment base. Express the quantity of
each ingredients in mg.
Sample Problems
2. Abdul Azziz needs to take Amoxicillin
500mg 1 capsule 3 times per day for 7
days to treat his chest infection. How
much amoxicillin will he consume during
the course of antibiotic treatment? Express
the answer in grams (g).
Temperature
Scales of Temperature Measurement
1. Celsius – °C
- also known as Centigrade
- scale was devised by dividing the
range of temperature between the freezing and
boiling temperatures of pure water at standard
atmospheric conditions (sea level pressure) into
100 equal parts
2. Fahrenheit – °F
-non-metric temperature scale was
devised and evolved over time so that the
freezing and boiling temperatures of water
are whole numbers, but not round
numbers as in the Celsius temperature
scale.
3. Kelvin – K
- temperature scale is an
extension of the degree Celsius scale
down to absolute zero, a hypothetical
temperature characterized by a complete
absence of heat energy.
Mathematical Formula for
Conversion
Celsius to Fahrenheit: F = C x (9/5) + 32
Fahrenheit to Celsius: C = (F - 32) x (5/9)
Celsius to Kelvin: K = degree Celsius + 273.15
Kelvin to Celsius: Celsius = K - 273.15
Sample Problems
1. Convert 35°C to °F
Formula : F = C x (9/5) + 32
° F = 35 x (9/5) + 32
= 35 x 1.8 + 32
= 95 °F
2. Convert 112 °F to °C
° C = (F - 32) x (5/9)
= (112 F – 32) x 0.55
= 44 ° C
Common Pharmaceutical
Equivalents
Solids
• 1 ounce (oz) = 28.35 g
• 1 pound (lb) = 454 g
• 1 kilogram = 2.2 lbs
Liquids
•
•
•
•
•
•
•
1 gallon = 3785 ml
1 quart = 946 ml
1 pint = 473 ml
1 fluid ounce (fl) = 29.57 ml, rounded to 30 ml
1 teaspoonful (tsp ) = 5 ml
1 tablespoonful (tbsp )= 15 ml
1 milliliter (ml) = 20 drops (gtts)