Calculating Medical Dosages
Calculating Medical Dosages
Calculating Medical Dosages
and more. Conversion math isnt hard to do as long as you know the basic conversion factors. Here are
the most useful ones:
Converting lb to kg and kg to lb
lb = kg 2.2
kg = lb 2.2
Converting mL to L and L to mL
mL = L 1,000
L = mL 1,000
Converting mg to g, g to mg, mg to mcg, and mcg to mg
mg = g 1,000
g = mg 1,000
mcg = mg 1,000
mg = mcg 1,000
QD
QOD
MSO4
U or IU
Mistaken Meanings
Does it mean discontinue or
discharge?
Does it mean half-strength or at
bedtime?
Does it mean every day or right eye?
QD looks like OD, which means right
eye. (OS means left eye.)
Does it mean every other day or
daily?
Does it mean magnesium sulfate or
morphine sulfate?
Does it mean unit or zero? Could it
be mistaken for 0 or 10?
IV
SQ or SC
Better Choice
Write discontinue or discharge.
Write at bedtime or a designated time.
Also write out the specific dosing strength
and/or quantity
Write every day.
Write every other day or daily, according to
patients needs.
Write magnesium sulfate or morphine
sulfate.
Write units.
IV is an acceptable abbreviation for
intravenous, but the doc could write
international units or intravenous to be
clearer.
Or 4
Write Subq, subcut, subcutaneous, or 5
Determine ml/minute.
Divide ml per hour by 60 minutes (or number of minutes of infusion).
ml/hr / 60 minutes = ml/minute.
What you want, over what youve got, times what its in
Type A calculations
When the dose you want is not a whole ampoule.
For example:
Converting units
All weights, volumes and times in any equation must be in the same units. With weights the unit
changes every thousand. For example, you need 1000 micrograms (mcg) to make 1 milligram (mg) and
1000 milligrams to make one gram (g) (Box 2).
Type B calculations
These are infusion rate calculations.
For example:
Prescription states 30 mg/hour
You have a bag containing 250mg in 50ml
At what rate (ml/hr) do you set the pump?
These are the same as type A calculations, only once you have worked out the volume that contains the
amount of drug you need, you set the pump to give that amount per hour.
In this instance, work out how many ml contain ONE mg of drug
Using the WIG equation:
30 x 50 / 250 = 6ml
Therefore the calculation shows that, to give 30mg per hour, the infusion pump rate would need to be
set at 6ml per hour.
This calculation is straightforward when the rate you want (30mg/hour) and the amount of the drug in
Type C calculations
Infusion rate is required, but dose is mg per kg.
For example:
Prescription states 0.5mg/kg/hour
You have a bag of 250mg in 50ml
Your patient weighs 70kg
At what rate (ml/hr) do you set the pump?
To do this calculation you still use the WIG equation as above, but with one extra step to work out the
what you want.
First you need to convert the mg per kg into total mg by multiplying it by the patients weight.
So for a person who weighs 70kg, 0.5mg per kg is the same as 35mg. Once you have calculated this,
the infusion rate can be worked out as in the Type B calculations.
In this instance:
0.5mg/kg/hr x 70kg x 50ml / 250mg = 7ml/hr
Type D calculations
Infusion rate required, but dose is in mg/kg/min.
For example:
Prescription states 0.5mg/kg/min
You have a syringe of 250mg in 50ml
Your patient weighs 70kg
At what rate (ml/hr) do you set the pump?
As before, you will need to calculate what you want by multiplying the amount per kg by the patients
weight. In this case:
0.5mg x 70kg = 35mg
This time, however, the prescription states the rate per minute. The pump demands that the rate be set
in ml per hour, therefore the rate per minute will need to be converted before the equation can be
completed, by multiplying 35 by 60; that is, 35mg/min (35 milligrams per minute) is converted to
Type E calculations
Infusion rate is required, but the dose is in mcg/kg/min.
For example:
Prescription states 3 micrograms (mcg)/kg/min
You have a syringe of 100mg in 50ml
Your patient weighs 70kg
At what rate do you set the pump (ml/hr)?
As before, what you want is calculated by multiplying the amount per kg by the patients weight, that
is:
3mcg/kg for a 70kg person is 210mcg
Next the prescription rate needs to be converted into rate per hour, that is,
210mcg/min = 12 600mcg/hr
The prescription is in micrograms, but in your syringe you have milligrams. Both need to be in the
same units, so you must convert one to the other, in this case mcg to mg. 12 600mcg/hr is the same as
12.6mg/hr.
The calculation is then as follows:
12.6 x 50 / 100 = 6.3ml/hr
Abbreviations
Abbreviations rule the medical world. It is important to know the recognized abbreviations in clinical
setting so you will not be lost when preparing medications for your patient. Here are the most
commonly used abbreviations when preparing drugs:
cc- cubic centimeter
DD- Desired Dose
IM- Intramuscular
IO- Intraosseous
SL Sublingual
IV- Intravenous
IVP- Intravenous Push
Kg- Kilogram
gm- gram
mg- milligram
mcg- microgram
mEq- milliequivalent
L- Liter
mL- milliliter
g- microgram
gtt drop
gtt micro drop
tbsp tablespoon
tsp teaspoon
Conversions
Before mastering drug dosage formula, you should first learn how to convert the commonly used units
of measurements in drug preparation. It is essential so you will not get lost between drug calculations
as physicians commonly order a medication available in a different preparation.
The most common conversion factors you will use for drug dosage calculations are the following:
Solid Conversions
gr 1 = 60 mg
gr 15 = 1 g
2.54 cm = 1 in
2.2 lb = 1 kg
Liquid Conversions
30 mL = 1 oz
1 tsp = 5 mL
1 standard measuring cup = 240 mL
Mass:
mcg mg g kg ( x by 1,000 )
mcg mg g kg ( by 1,000 )
lb kg ( x by 2.2 )
lb kg ( by 2.2 )
Volume:
mcL mL L kL ( x by 1,000 )
mcL mL L kL ( by 1,000 )
Time:
min hr ( x by 60 )
min hr ( by 60 )
Example:
Convert 0.008 kg to mg
mcg mg g kg ( x by 1,000 )
0.008 L x 1,000 = 8 mg
8 mg x 1,000 = 8,000 mg
Convert 1,000 mcL to mL
mcL mL L kL ( by 1,000 )
1,000 L 1,000 = 1 mL
Convert 480 minutes to hour
min hr ( by 60 )
480 60 = 8 hr
The desired dosage is the ordered dosage of the physician while the stock strength is the amount of
drug present in each tablet. Stock strength is also known as stock dose.
Example:
The physician orders 1,500 mg of calcium carbonate for the patient. The drug is available in 250
mg tablets. How many tablets should be given to the patient?
Desired dosage stock strength = number of tablets
1,500 mg 250 mg = 6 tablets
The patient is ordered to have 2 g of potassium chloride. The drug is available in 500 mg
tablets. How many tablets should be given?
Convert 2 g to mg = 2 x 1,000 = 2,000 mg
The desired dosage is the ordered dosage of the physician. The stock strength is the amount of drug
present in the preparation while the stock volume is the amount of the solution where the drug is
diluted.
Example:
The physician orders 375 mg of cefuroxime for the patient. The drug is available in 750 mg
vial. You plan to dilute it in 10 mL of sterile water. How much should you give to your patient?
[Desired dosage stock strength] x stock volume = amount of solution to be given
[375 mg 750 mg] x 10 mL = amount of solution to be given
0.5 mg x 10 mL = 5 mL
A pediatric patient recovering from accidental fall is about to be given with 130 mg paracetamol
syrup. The drug is available in 250 mg per 5 mL preparation. How much should you give to
your patient?
[Desired dosage stock strength] x stock volume = amount of solution to be given
[130 mg 250 mg] x 5 mL = amount of solution to be given
0.52 x 5 mL = 2.6 mL
The total IV volume is the amount of fluid to be infused while the time is the number of running hours
or minutes.
Example:
Start venoclysis with D5 0.9 NaCl 1 L to be infused for 16 hours. How many mL of the IV fluid
should you infuse per hour?
Total IV volume time (hour or minute) = mL per hour
1 L 16 hours = mL per hour
[1 L x 1,000] 16 hours = mL per hour
1,000 mL 16 hours = 62.5 mL per hour
You are going to start IV infusion with Plain Lactated Ringers Solution 1 L. By regulating it for
11 hours, how much fluid are you going to infuse per minute?
Total IV volume time (hour or minute) = mL per minute
1 L 11 hours = mL per minute
[1 L x 1,000] [11 hours x 60] = mL per minute
1,000 mL [11 hours x 60] = mL per minute
1,000 mL 660 = 1.5 mL per minute
The total IV volume is the amount of IV fluid to be infused while time is the duration of how long the
IV fluid should be infused in terms of minutes.
The drop factor is the drops per milliliter delivered to the patient and it depends on the macrodrip
used for the infusion. The common drop factors used in different hospitals are 10, 15 and 20.
Example:
The physician orders to start venoclysis with D5 0.3 NaCl 1 L solution. The IV fluid will be
infused for 14 hours and the drop factor of the macrodrip used is 20. It should be regulated to
how many drops per minute?
[Total IV volume time (minute) ] x drop factor = drops per minute
The volume remaining is the amount of IV fluid remaining for the infusion while the drops per minute
is the regulation of the IV infusion. The drop factor can be determined in the macrodrip used in the
hospital.
Example:
You see that your patient has D5 0.9 NaCl IV infusion at 400 mL level. It is regulated to run for
22 drops per minute using a macrodrip set with drop factor 20. How many minutes are
remaining before you are due to change the IV fluid?
jh
[Volume remaining (in mL) drops per minute] x drop factor = minutes remaining
[400 mL 22] x 20 = minutes remaining
18.18 x 20 = 363 minutes or 6 hours
A patient has 350 mL of Plain 0.9 NaCl solution as IV infusion regulated at 20 drops per
minute. Drop factor 10 was used for the patients macrodrip set. Considering the IV fluid level
of the patient, for how many minutes will it run?
[Volume remaining (in mL) drops per minute] x drop factor = minutes remaining
[350 mL 20] x 10 = minutes remaining
17.5 x 10 = 175 minutes or 3 hours