Easy Way to Remember Metric Conversions for Medical Math

Module 3 - The Metric System of Measurement and Conversions Using the Metric system

What's in this module?

Most of the world uses the metric system of measurement.  The United States is one of the very few exceptions.  We tend to use the household system of measurement in everyday situations. In the United States, the metric system tends to be used primarily in scientific and medical settings.

As a nurse, you will use the metric system in almost everything you do.  Medication orders are stated using the metric system with very few exceptions.  If you learn the relationships between the units of the metric system, converting units of measurement will be easy for you.

Summary of problem types in this module

In this module you will be working problems to change from one unit of measurement in the metric system to another unit of measurement.

Let's look at the basics of the metric system:

The metric system has three measurement bases, or basic units: meters (length), liters (volume), and grams (weight or mass).

The prefixes in front of the base measurement tell you whether the amount is larger or smaller than the base measurement and how much larger or smaller.  Kilo- represents the largest unit on the chart, while milli- represents the smallest unit on the chart.  Conversions within a base of measure can be made by moving the decimal place.

Need an easy way to remember the relative sizes of the metric units?  Remember King Henry Died by Drinking Chocolate Milk.

Equivalents to know

Conversion between the metric system and the system commonly used in the United States will be introduced as needed in later modules.  Illustrations of metric measurements are shown for reference.

These equivalents are the most commonly used in nursing:

WEIGHT MEASUREMENTS (Gram is the measurement base)

1 kg (kilogram) = 1000g (Note: g, G, Gm, gm are all abbreviations for gram)

1 g (gram) = 1000 mg (milligrams)

1 mg (milligram) = 1000 mcg (micrograms)

VOLUME MEASUREMENTS (Liter is the measurement base)

1 L (liter) = 1000 ml (milliliters)

LENGTH MEASUREMENTS (Meter is the measurement base.)

1 meter = 100 cm (centimeters)

1 cm (centimeter) = 10 mm (millimeters)

EQUIVALENTS ACROSS SYSTEMS

1 kg (kilogram) = 2.2 lb (pounds)

5 ml (milliliter) = 1 tsp (teaspoon)

30 ml (milliliter) = 1 oz (ounce)

2.5 cm (centimeters) = 1 inch

Converting temperature

Hospitals and emergency services often use the Celsius system of measuring temperature instead of the Fahrenheit system more familiar to us in the United States. The most frequent example of the use of Celsius in the medical setting is the measurement of body temperature.  Storage temperatures for medication and other substances may also be stated in Celsius.

This type of conversion is a situation in which you will need to remember a formula.

Converting to military time

Military time is used by many hospitals and emergency services.  Military time reduces ambiguity because the A.M. and P.M. designations are not needed.  Colons are also not needed in recording military time.  Times are always recorded as four digits.

Times before 1:00 P.M. need no conversion.  Simply omit the colon and the A.M. designation.

Afternoon and evening times starting with 1:00 P.M. are found by adding 1200 to the conventional time used in the United States.  Example:  1200 + 0100 = 1300; 1:00 P.M. is written as 1300.

The colon and the P.M. designation are omitted.  Times at or after 1300 are understood to be afternoon.  Midnight may be written as 0000 or 2400.  Follow the convention of the institution where you work when recording midnight.

Here are some easy examples:

12:00 noon  =  1200  (nothing is added to noon)

12:01 A.M. =  1201  (nothing is added to minutes after noon)

2:30 P.M. + 1200 = 1430 (the military time equivalent)

11:59 P.M. + 1200 = 2359 (the military time equivalent)

Rounding rules to know

Only these general rounding rules for decimals will apply to this module.

1. If the answer is less than one (1), take the math out three (3) places past the decimal point (the thousandth position) and round to two (2) places past the decimal point (the hundredth position).

2. If the answer is greater than one (1), take the math out two (2) places past the decimal point (the hundredth position) and round to one (1) place past the decimal point (the tenth position).

3. Do not include trailing zeros. (Ex: 12.0 ml would simply be expressed as 12 ml and 0.40 mg would be expressed as 0.4 mg)

4. Always use a leading zero for numbers less than one. (Ex: .25 ml should be expressed as 0.25ml)

Starting factors and answer units

The starting factor (SF) is the amount you start with  - the quantity and the units you know.  It is the quantity and unit of measurement to be converted.

The answer unit (AU) is the equivalent quantity expressed in the units that you have available.  The AU is the unit of measurement you have in an amount equivalent to the SF (the amount you know).

Problem Type 1 –Conversions within the Metric System

A medication strength is listed as 0.25 mg per ml.  How many mcg are in one ml?

Here's the problem set up in the dimensional analysis format:

SF = 0.25 mg

AU = mcg

Equivalent:

1 mg = 1000 mcg

Equation:

Note that conversions within a base unit of the metric system can be done by simply moving the decimal point:

0.25 mg = 250 mcg (The decimal is moved three places to the right)

Here's another example:

A medication is ordered at 500 mg per dose.   The dosage strength is 150 mg per ml.  How many ml will be given per dose?

This problem requires a conversion between weight and volume bases in the metric system.

SF = 500 mg

AU = ml

Equivalent:

1 ml = 150 mg

Equation:

Note that the rounding rule for numbers >1 is used.  If you check your answer,

3.3 ml X 150 mg per ml  = 495 mg per dose

This answer is a close approximation to the actual dose ordered.  Measuring instruments for liquid medication can usually not be measured more closely than a tenth of a ml.

Problem Type 2 – Conversions Between the Metric System and the Household System

A nurse has measured the length of a wound as 4 inches.  How many cm long is the wound?

Here's the problem set up in the dimensional analysis format:

SF = 4 in

AU = cm

Equivalent:

           2.5 cm (centimeters) = 1 inch

Equation:

Here's another example:

A child weighs 56 lb.  A medication is ordered by weight in kg.  How many kg does the child weigh?

SF = 56 lb

AU = kg

Equivalent:

1 kg (kilogram) = 2.2 lb (pounds)

Equation:

Use the rounding rule for numbers >1.

Problem Type 3 –Conversion of temperature

Examples:

A nurse has measured a client's temperature as 99.5 degrees F using a thermometer with only the Fahrenheit scale.  How many degrees Celsius is the temperature?

Formula:

(Degrees F – 32) X 0.5556 = Degrees C

Equation:

Use the rounding rule for numbers >1.

Here's another example:

A solution needs to be stored at 20 degrees C.  At what temperature should the storage unit be using the Fahrenheit scale?

Formula:

(Degrees C X 1.8) + 32 = Degrees F

Equation:

Use the rounding rule for numbers >1.

Problem Type 4 –Conversion of time

An intravenous solution started running at 8:00 A.M.  The solution will take 10 hours to complete.  At what time will the solution be finished (in military time)?  Convert the time back to standard time as used in the United States in order to inform the client's family when the solution will be finished.

Here's another example:

A medication is to be given every 6 hours.  The first dose was given at 6:00 P.M.  When should the next four doses be scheduled?

For as long as the client takes this medication, the doses will be scheduled to be given at 1800, 2400 (0000), 0600, and 1200.  These times will be recorded on the client's medication schedule in a hospital setting.

The outpatient client will be told to take the medication at 6:00 P.M., 12:00 A.M., 6:00 A.M., and 12:00 P.M.

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Source: https://getlibraryhelp.highlands.edu/c.php?g=937883&p=6759243