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Review of Basic Drug Calculations
TOPIC
Quick Conversion Reference
2
Metric Conversions
3
Methods of Solving Drug Calculations
4
Reconstitution of Powders
5
Calculating Pediatric Dosages
6
Calculating Electronically Regulated IV Flow Rates (ml/hr)
7
Calculating Manually Regulated IV Flow Rates (gtts/min)
MCC
PAGE
7-8
Practice #1: Conversions
9
Practice #2: Reconstitution of Powders
9
Practice #3: Pediatric Dosages
10
Practice #4: Electronically Regulated IV Flow Rates (ml/hr)
11
Practice #5: Manually Regulated IV Flow Rates (gtt/min)
11
Answers to Practice Problems
12
Page 1
QUICK CONVERSION REFERENCE
CONVERSION CLOCK
60mg (gr 1)
45mg (gr 3/4)
15mg (gr ¼)
30mg ( gr ½)
Weight Conversions
1 kilogram (kg) = 2.2 pounds (lb)
1 kilogram (kg) = 1000 grams (g)
1 gram (gm or g) = 1000 milligrams (mg)
1 milligram (mg) = 1000 micrograms (mcg or ug)
Volume Conversions
1 milliliter (ml) = 1 cubic centimeter (cc)
1 liter (L) = 1000 milliliters (ml)
5 milliliters (ml) = 1 teaspoon (tsp)
15 milliliters (ml) = 3 teaspoons (tsp) = 1 tablespoon (T)
30 milliliters (ml) = 1 ounce (oz) = 2 tablespoons
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General Rounding Guidelines:
Adults: round to nearest 10th for weights, drug dosages, and measurable doses
Pediatrics: round to nearest 100th for weights, drug dosages, and measurable doses
Metric Conversions
 TO CONVERT FROM A LARGER UNIT OF MEASURE TO A SMALLER UNIT OF MEASURE (for example:
grams to milligrams OR milligrams to micrograms), MOVE DECIMAL TO THE RIGHT OR MULTIPLY BY
1000.
 TO CONVERT FROM A SMALLER UNIT OF MEASURE TO A LARGER UNIT OF MEASURE (for example:
milligrams to grams OR grams to kilograms), MOVE DECIMAL TO THE LEFT OR DIVIDE BY 1000.
Remember, LEFT is LARGER.
EXAMPLES:
 To Convert Grams (large) to Milligrams (small):
a) multiply by 1000
OR
b) move decimal point 3 places to the right
3 g = 3000 mg
0.5 g = 500 mg
0.08 g = 80 mg

To Convert Milligrams (small) to Grams (large):
a) divide by 1000
OR
b) move decimal point 3 places to the left
125 mg = 0.125 g
50 mg = 0.050 mg
6 mg = 0.006 g

Convert Milligrams (large) to Micrograms (small):
a) multiply by 1000
OR
b) move decimal point three places to the right
5 mg = 5000 ug
0.8 mg = 800 ug
0.05 mg = 50 ug

Convert Micrograms (small) to Milligrams (large)
a) divide by 1000
OR
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b) move decimal point 3 places to the left
2500 ug = 2.5 mg
400 ug = 0.4 mg
10 ug = 0.01 mg
See Practice Problems #1, Page 9
3 Methods of Solving Drug Calculations
RATIO AND PROPORTION
Dosage on Hand : Amount of Hand :: Dosage Desired : Amount Desired
OR
Dosage on Hand
Amount on Hand
=
Dosage Desired
Amount Desired
FORMULA METHOD
Desired x Amount =
Have
D x Amt =
H
DIMENTIONAL ANALYSIS
Starting Factor x Converting Factors = Answer Label
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EXAMPLES:
Order: 650mg of Tylenol PO. On hand: Tylenol tablets 325mg/1tablet. How many tablets would you
administer?
Ratio Proportion Method
325mg : 1 tab : : 650mg : Xtab
325X = 650
X = 2 tabs
Formula Method
650mg X 1tab = 2 tabs
325mg
Dimensional Analysis Method
650mg X 1 tab = 2 tabs
325mg
RECONSTITUTION OF POWDERS
Many medications come in a dry form that remains stable and can be stored for varying periods of time.
The drug is in a solute form (dry powder) and must be reconstituted by adding a diluent (solvent). Once
the drug is reconstituted it will only remain stable for a limited time and must be used within that time or
be discarded.
LEARN TO READ THE LABEL FOR THIS INFORMATION, FOR EXAMPLE:
 NAME OF THE DRUG
EXAMPLE: TAZICEF (ceftazidime); Brand name and generic name will be given

AVAILABLE DOSAGE OF THE DRUG
EXAMPLE: Equivalent to 1gram ceftazidime
This is the total amount of the medication available in the vial.

RECOMMENDED DILUENT
EXAMPLE: reconstitute with Sterile Water for injection (according to table)
The three most common sterile diluents are:
(1) bacteriostatic water
(2) sterile water
(3) sodium chloride (0.9%)
If this information is not on the label it WILL be in the package insert. Bacteriostatic solvents contain
preservatives that may not be compatible with a specific drug. If no information is given sterile water is
the diluent of choice.

QUANTITY OF DILUENT
EXAMPLE: Diluent to be added = 3.0ml for Intramuscular or Intravenous Direct (bolus) Injection
Some labels will offer more than one choice, dependant upon the dosage desired.
EXAMPLE: Diluent to be added = 10ml for Intravenous Infusion
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
RATIO OF SOLUTE TO SOLVENT AFTER RECONSTITUTION
EXAMPLE: When reconstituted, approximate average concentration = 280mg/ml
(95mg/ml for Intravenous Infusion)
A ratio will be given in some form, other examples: yielding concentration is 3g/ml or resulting
concentration is 100mg/ml
SAMPLE RECONSTITUTION PROBLEM:
Ordered: Tazicef 500mg IM
Available: 1gm vial of Tazicef in powdered form
Directions for reconstitution: add 3.0ml Sterile Water per gram; approximate average
concentration = 280mg/ml
500mg X 1ml = 1.78 or 1.8ml
280mg
OR
280mg : 1ml :: 500mg : x ml
x = 1.78 or 1.8ml
See Practice Problems #2, Page 9
CALCULATING PEDIATRIC DOSAGES
Convert Weight from Pounds to Kilograms
Example: total weight is 12 lbs 8 oz
1. Convert ounces to pounds by dividing ounces by 16
8 oz  16 oz = 0.5 lb
2. Divide total pounds (to the nearest hundreth) by 2.2
Total pounds = 12 lbs + 0.5 lbs = 12.5 lbs
12.5  2.2 = 5.681 or 5.68 kg
Calculate Safe Dose Ranges (SDR)
Example: SDR is 30-100mg/kg/day in four divided doses
Ordered dose is 150mg q 6hr
1.
low range calculation: 5.68 x 30mg = 170.4mg/day
high range calculation: 5.68 x 100mg = 568mg/day
SDR = 170.4-568 mg/day or 42.6-142mg/dose
2. evaluate appropriateness of ordered dose:
150mg x 4 doses/day = 600mg/day
3. decision: ordered dose exceeds SDR, hold and clarify See Practice Problems #3, Page 10
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CALCULATING ELECTRONICALLY REGULATED IV FLOW RATES – ML/HR
Electronically Regulated Flow Rates are calculated in ml/hr. The nurse programs the infusion device to deliver the
ordered solution in milliliters per hour (ml/hr)
Formula for calculating ml/hr:
TV (ml) = ml/hr
TT (hr)
OR
If < 1 hr infusion: TV (ml) X 60 (min) = ml/hr
TT (min)
TV = total VOLUME to be infused in ml
TT = total TIME of infusion
EXAMPLES
*IVPBs (often < 1 hr infusion)
1. Infuse 250ml over 2 hours
*3. Infuse 75ml in 30 minutes (IVPB)
75ml
30min
250ml = 125ml/hr
2hr
X 60 = 150ml/hr
*4. Infuse 100ml in 20 minutes (IVPB)
2. Infuse 2L over 4 hours
100ml X 60 = 300ml/hr
20min
2000ml = 500ml/hr
4hr
See Practice Problems #4, Page 11
CALCULATING MANUALLY REGULATED IV FLOW RATES – DROPS/MINUTE (gtt/min)
Manually Regulated Flow Rates are calculated in drops/minute. The nurse calculates the number of drops the IV
should flow per minute according to the ordered infusion time. The drip rate (gtt/min) is then set by adjusting the
roller clamp and timed by counting the number of drops dripping per minute into the IV drip chamber.
The IV solution may be a continuous infusion or an intermittent/bolus infusion.
Formula #1: Use for large volume infusions; plug in hourly rate of infusion (ml/hr = ml/60minutes)
D X V = gtt/min
M
=
D
X hourly rate of infusion (ml/hr)
60
D = DROP factor of tubing (gtt/ml)
M = Time of infusion per 60 MINUTES (hourly rate of infusion); 60 is a constant
V = Total VOLUME to be infused in ml each hour (hourly rate of infusion)
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EXAMPLES
1. Order is to infuse D5NS at 125ml/hr. The drop factor is 15gtt/ml
15 X 125 = 31.25 or 31gtt/min
60
2. Order is to infuse LR at 150ml/hr. The drop factor is 10gtt/ml
10 X 150 = 25gtt/min
60
3. Infuse 1L NS over 4 hours. The drop factor is10gtt/ml
10 X 250 = 41.66 or 42gtt/min
60
4. Order is to infuse 200ml 0.9% NaCl over 2 hours. The drop factor is 20gtt/ml.
20 X 100 = 33.33 or 33gtt/min
60
Formula #2: Use for small volume infusions (IVPBs) that are often < 1 hr
D X V = gtt/min
M
D = DROP factor of tubing (gtt/ml)
M = Time of infusion in total number of MINUTES
V = Total VOLUME to be infused in ml
*IVPBs / Small Volume Infusions (often < 1 hr infusion)
*5. Order is for Clindamycin 600mg in 100ml D5W to infuse in 30 minutes. Drop factor is 20gtt/ml
20 X 100 = 133.33 or 133gtt/min
15
*6. Order is to infuse IVPB “X” in 30 minutes. The bag contains 50ml; the drop factor is 10gtt/ml.
10 X 50 = 16.66 or 17gtt/min
30
7. Infuse Drug X 1gm in 250ml in 1 ½ hrs. The drop factor is 10gtt/ml.
10 X 250 = 27.77 or 28gtt/min
90
OR Formula #2: 10 X 167 = 27.83 or 28gtt/min
60
Practice Problems #5, Page 11
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PRACTICE #1: CONVERSIONS
1. 60mg = _____gr
2. 66lbs = _____kg
3. 500mg = _____gm
4. 250mg = _______mcg
5. 4oz
= _____ml
6. 1tsp
= _____ml
7. 1 liter = _____ml
8. 90ml = _____oz
9. 15ml = _____tsp
10. 45kg = _____lbs
PRACTICE#2: RECONSTITUTION OF POWDERS
1. Chlorothiazide 750 mg IV is ordered. The drug label states that after reconstitution with 18
ml sterile water, there are 500 mg/10 ml. How many ml should be given?
ml.
2. Amobarbital 75 mg is ordered IM. When the 250 mg vial is reconstituted, each ml = 100 mg.
How much should be administered?
ml.
3. Give Methicillin 1 g IM q6h. When the vial is diluted with 5.7 ml of diluent, each 500 mg = 1
ml. How many should be given to the patient?
ml.
4. Give Nafcillian 500mg IM q6h. When 3.4 ml diluent is added to the 1 g vial,
250mg/1ml is the result. What volume should be given.
ml.
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PRACTICE #3: PEDIATRIC DOSAGES
Convert the following weights from pounds to kilograms:
1.
_____ 8lb
3. _____ 4lb, 6oz
2. _____ 5lb, 8oz
4. _____ 18lb, 9oz
5. Child with nausea post-operatively
Order: phenergan 20mg IM q6h
Child’s weight: 99lbs
Drug reference: 0.25-0.5mg/kg/dose, repeat q 4-6h
Drug available: phenergan 25mg/ml
a. What is the SDR for this child?
b. How much will the nurse draw up to administer a single dose?
6. 9 day old infant with R/O sepsis; IV access lost – order to give last dose IM
Order: Gentamicin 10mg IM now (original order for 10mg Gentamicin IV q 8hrs)
Child’s weight: 8lbs 13oz
Drug reference: > 7 days old: 5-7.5mg/kg/day in 3 divided doses
Drug available: gentamicin 40mg/2ml
a. What is the safe dose range for this child (include SDR per day and per dose)?
b. How much will the nurse draw up to administer a single dose?
7. Child receiving pre-operative medication
Order: atropine 0.2mg IM
Child’s weight: 12kg
Drug reference: 0.2mg/24-40 lbs
Drug available: atropine 0.4mg/ml
a. Is this a safe dose for this child (include supporting information)?
b. How much will the nurse draw up to administer the ordered dose?
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PRACTICE #4: ELECTRONICALLY REGULATED FLOW RATES (ml/hr)
*IVPBs (often < 1 hr infusion)
Calculate the IV flow rate in ml/hr for each of the following problems:
1.
Infuse 1.5 liters of NS over 3 hours
*2. Unasyn 0.5gm in 75ml D5W IVPB to infuse over 20 minutes
*3. Zantac 50mg in 100ml NS to infuse over 30 minutes
4. Infuse 500ml LR over 2 hours
*5. Ancef 1gm in 100ml D5W IVPB to infuse in 45 minutes
PRACTICE #5: MANUALLY REGULATED FLOW RATES (gtts/min)
*IVPBs (often < 1 hr infusion)
Calculate the IV flow rate in gtt/min for each of the following problems:
1.
Infuse 1L NS bolus IV over 2 hours. Drop factor of tubing is 20gtt/ml.
2 Infuse 1000ml of lactated ringers IV in 6 hours with a 10gtt/ml administration set.
*3. Order is for Ancef 2gm IVPB diluted in 100ml D5W to infuse in 45 minutes.
Drop factor of the tubing is 15gtt/cc
*4. Order is for Zosyn 3gm in 100ml D5W to be infused in 40 minutes. Drop factor is 10gtt/ml.
How many drops/minutes do you infuse the IVPB?
*5. Order is for carbenicillin disodium 2gm IVPB diluted in 50ml D5W to infuse in
30 minutes. Drop factor is 60gtt/ml.
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ANSWERS
Practice #1 Conversions
1. 1gr
2. 30kg
3. 0.5gm
4. 250,000mcg
5. 120ml
6. 5ml
7. 1000ml
8. 3oz
9. 3tsp
10. 99lbs
Practice #2 Reconstitution
1. 15 ml
2. 0.75 ml
3. 2 ml
4. 2ml
Practice #3 Pediatric Dosages
1.
2.
3.
4.
3.64kg
2.5kg
1.99kg
8.44kg
5a. 11.25-22.5mg /dose
5b. 0.8ml
6a. 20-30mg /day OR 6.67-10mg/dose
6b. 0.5ml
7a. yes, 26.4lbs falls w/in safe
dose/kg
7b. 0.5ml
MCC
Practice #4 IV Flow Rates (ml/hr)
1. 500ml/hr
2. 225ml/hr
3. 200ml/hr
4. 250ml/hr
5. 133ml/hr
Practice #5 IV Flow Rates (gtts/min)
1. 167gtts/min
2. 28gtts/min
3. 33gtt/min
4. 25gtts/min
5. 100gtts/min
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