Math Tools for Nutrition
You will use a few mathematical concepts in studying nutrition. Besides the typical addition, subtraction.... you need to
know how to convert standard English units to metric and vice versa, as well as calculate ratios and percentages.
CONVERSIONS A conversion factor is a fraction in which numerator (top) and the denominator (bottom) express the
same quantity in different units. For example, 12 inches and 1 foot are equivalent..... they express the same length. The
conversion factor used to change inches to feet or feet to inches is:
12 inches
1 foot
or
1 foot
12 inches
To perform a conversion, use one of the factors with the unit you want for an answer in the numerator (top). Here is an
example which undoubtedly you could have done in your head. The important feature is that this will work for any unit
conversion as long as you have the conversion factors.
Example 1
If Josh was measured to be 68 inches tall, how tall is he in feet?
68 inches
x
1 foot
=
12 inches
68 feet
12
=
5.7 feet
Some folks get a bit spooked when the metric system is thrown into mix, but it is much more convenient, albeit you might
not be as familiar. For volume, there are many units in the standard English system...... teaspoon, tablespoon, ounce, cup,
pint, quart and gallon.....do you remember how they are all related? How many teaspoons in a tablespoon? How many pints
in a quart? A bit messy. The metric uses one unit; the liter (l). It’s about the size of a quart. To change the “size” of the
liter, we use base-10 prefixes.
centi
milli
micro
kilo
c
m
μ
k
one hundredth the size
one thousandth the size
one millionth the size
one thousand times the size
Using the same thinking as the conversion of inches to feet, we can convert meters (m, unit of length) to centimeters (cm), as
long as you can identify and understand the conversion factor for this.
100 cm
1m
Example 2
or
1m
100 cm
How many centimeters are in 2.7 meters?
2.7 m x
100 cm
1m
=
270 cm
1
=
270 cm
Many times we will need to convert a standard English measurement to a metric value. Again conversion factors will be
applied. A few should be remembered; the IPQ (inch, pound and quart) equivalencies.
one inch
one pound
one quart
Example 3
is the same as
is the same as
is the same as
2.54 centimeters
454 grams
946 milliliters
Bev is 173 cm tall. What is her height in inches?
173 cm x
1 inch
2.54 cm
=
173 inches
2.54
=
68 inches
RATIOS Ratios are nothing more than the comparison between two quantities and reported in decimal form. A classic
example is what is known as the P to S ratio (P/S or P:S). The P represents the amount of polyunsaturated fat in a food; the
S represents the saturated fats in a food.
Example 4
Lets assume that a food item was reported to have 3 g of polyunsaturated fat and 13 g of saturated fat,
then the P/S ratio would be:
3gP
13 g S
=
3
13
=
.23
1
or simply .23
What this indicates is that there is less than a quarter the amount of polyunsaturated fat compared to the saturated fat in this
particular food item, or for every one gram of saturated fat, there is .23 g of polyunsaturated fat.
PERCENTAGES The term percent (%) refers to a part of the total when the total represents 100 parts. For example, if
you earn 80% on your first nutrition exam, you will have answered the equivalent of 80 out of 100 question correctly. This
equivalent could be 8 correct answers out of 10; 80% also describes 16 out of 20 (16/20 = .80 or 80%). The best way to
master this concept is to calculate some percentages. Here are some examples......
Example 5
You consume 1500 Calories in a day. The RDA for your age and gender is 2200 Calories. What percent
of the RDA did you consume?
1500 Calories (your intake)  2200 Calories (RDA) = .68
to make this a percentage, .68 x 100 = 68%
Example 6
Rita ate 15% of the adult Recommended Dietary Allowance (RDA) for vitamin C at lunch. How many
milligrams did she eat? (RDA = 60 milligram.)
since 15% in decimal form is .15
60 mg vitamin C (RDA) x .15 = 9 mg vitamin C (Rita’s intake)
Another use of percentages is the comparison between parts of a whole.
Example 7
In one day you consume 2400 Calories. Remember that the energy supplying nutrients are carbohydrates,
proteins and fats. If you ate 65 grams of protein that day, what percent of the total Calories came from the
protein?
65 grams protein x
4 Calories
=
1 gram protein
260 Calories
1
=
260 Calories from protein
260 protein Calories ÷ 2400 total Calories = .11
to make this a percentage, .11 x 100 = 11% of Calories from protein
If you knew the grams of carbohydrate and fat, the % of Calories from these could be determined in much the same way.
And it would be reasonable to recognize that all these % of Calories should add up to approximately 100%.
It will be difficult to succeed in this nutrition course unless you know what a percentage means and how to calculate
one. Percentages are used frequently when referring to menus, nutrient composition and nutrition labels.
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