Microsoft word - how to solve drug dosage problem rev 1 2009.docx

How to Solve Drug Dosage Problems
*General Information ----------------------------------------- ------ ------------------ pg 2 *Converting between units ------------------------------------------------------------ pg 3 *Converting between metric units ---------------------------------------------------- pg 4 *Calculating Drug Dosages ------------------------------------------------------------ pg 5 *Other Formulas --------------------------------------------------------------------- pg 6-7 * Calculating a child’s dosage from an adult dosage * Calculating flow rate in ml/h * Calculating flow rate in gtt/min * Converting from °F to °C or °C to °F *Helpful Websites ----------------------------------------------------------------------- pg 8 How to Solve Drug Dosage Problems General Information
There are 3 different types of measurements you will encounter when dealing with
medications: Household, Apothecary, and Metric.
Type Number
Solids Liquids
Fractions before
Roman Numerals
after unit.
Ex: gr 15 ½ or dr iss
decimals before unit Meter (m)
(always put a 0 in
front of the decimal.
Ex: 0.15 mL
Note: When more than one equivalent is learned for a unit, use the most common equivalent for the measure or use the number that divides equally without a remainder. Common Conversion Factors
Roman Numerals
How to Solve Drug Dosage Problems Converting Between Units

In order to convert between units, you first begin with the unit you are given. You then find
a conversion factor
that relates the unit you have to begin with and the unit you need to
convert to. Next, you multiply the unit you began with by the conversion factor you found.
This calculation will give you the new unit.
Ex: Convert 120 mg to gr________ .

1st: think of a conversion factor that relates mg and gr. 60 mg = gr 1 (This can be
used as either 60 mg/gr 1 or gr 1/60 mg)
2nd: set up your multiplication equation.

120 mg x gr 1 = gr _____
60 mg
factor, always place the unit you need on top and the unit you begin with on the bottom. Multiply 120 mg x gr 1, then divide by 60 mg. Therefore: 120 mg = gr 2

If you do not have a conversion factor for the two units you are dealing with you may be
able to go through another unit to get to the unit you need.
Ex: Convert 3 t to _____oz.
1st: Try to find a conversion factor that relates teaspoons to ounces. If you look at
your list, you probably won’t find one. However, we can convert from teaspoons to
milliliters and then from milliliters to ounces. Therefore, we need two conversion
factors: 3 t = 15 mL and 1 oz= 30 mL.
3 t x 15 mL = _____mL
____ mL x 1 oz = m_____
Therefore: 3 t = 0.5 mL
How to Solve Drug Dosage Problems Converting Between Metric Units

When you convert between metric units, you do not have to multiply anything; you may
simply move the decimal place.
Kilo . UNIT . Milli . Micro
Kilogram . Gram . Milligram . Microgram
Kiloliter . Liter . Milliliter . Microgram
Kilometer . Meter . Millimeter . Micrometer
1 . 1,000 . 1,000,000. 1,000,000,000

There are 3 decimal places between each; therefore you move your decimal places 3 spaces
for each conversion.
Ex: Convert 25.3 g to __________mg
1st: There is only one space between g and mg, so that means I move the decimal place 3 spaces in the same direction as the chart: to the right. 2nd: 25.3 (move decimal → 3 places) = 25,300 Therefore: 25.3 g = 25,300 mg

Ex: Convert 346,720.2 mg to ________kg

1st: You use the same rules for converting from milligrams to kilograms. There are 2 spaces between kg and mg so you move your decimal 6 places (2 spaces x 3 decimal places each). In this case, according to the cart, we move the decimal to the left. 2nd: 346,720.2 (move decimal ← 6 spaces) =0.3467202 kg Therefore: 346,720 mg = 0.3467202 kg
How to Solve Drug Dosage Problems Calculating Drug Dosages

When performing drug calculations, you may use one of 2 methods: the Ratio-Proportion
or the Formula method. Each of these works as well as the other. However, once
you decide which you are more comfortable with, you should stick with that way and not try
to switch back and forth between the two.
1st: set up your proportion. 2nd: Multiply means and extremes 3rd: solve for “x” algebraically. Formula:

You use drug calculations when you are calculating the dose of a medication you need to
give a patient when you already know the strength of medication that the patient needs.
Ex: If the doctor orders 20 mg of Benadryl and you have 10 mg tablets, how many
will you give the patient?

We know that 10 mg = 1 tablet, and we need 20 mg in an unknown number 1st: set up equation.
Notice that on both sides of the equation, mg comes first, then tablets. This is very important. It doesn’t matter which unit comes first, as long as they are in the same order on both sides of the equal sign. 2nd: Multiply means and extremes
3rd: Solve algebraically.
So: 20 mg x 1 tablet = 2 tablets
Therefore: you would give the patient 2 tablets.
How to Solve Drug Dosage Problems Other Formulas

Calculating BSA (m²)

Ex: If you know a patient weighs 140 lb and 62 inches tall, to calculate the BSA, you simply plug the numbers into the formula, then solve. 8,680 ÷ 3131 = 2.77 √2.77 = 1.66 m²

Calculating a child’s dosage using an adult dosage

Child’s BSA x adult dosage = child’s dosage
Ex: The normal adult dosage of a medication is 150 mg. You have a child that weights 30 kg and is 120 cm. How much medication do you give the child? 1st: You must find the child’s BSA. To do so, use the formula given above. 1 m² x 150 mg = 88.24mg

Calculating Flow Rate in mL/h

Total mL ordered = mL/h (must round to a whole number)
Total h ordered

Ex: Calculate the flow rate for an IV of 1,820 mL Normal Saline IV to infuse in 15 h by controller. Flow rate = _________ ml/h mL = 121.33 = 120 mL/h
How to Solve Drug Dosage Problems
Calculating Flow Rate in gtt/min

Volume (mL) x drop factor (gtt/mL) = Rate (gtt/min)
Ex: The physician orders Lactated Ringer’s IV at 150 mL/h. The drop factor is 15 gtt/min. Find the flow rate in gtt/min. 150 mL x 15 gtt/min = 37.5 = 38 gtt/min
60 min

Converting from ºF to ºC or ºC to ºF
ºF = 1.8 (ºC) + 32
*Carry to hundredths and round to tenths How to Solve Drug Dosage Problems Helpful Websites
There are many helpful websites when it comes to drug calculations. The following all have practice problems and solutions. We encourage you to use them to your advantage. After all, the best way to get good at something is to PRACTICE! How to Solve Drug Dosage Problems


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