For the accurate dosage of medication it is important that nurses must have knowledge of calculation of prescribed drug to the patient so that proper medication can be given at accurate dose and at prescribed time . Here is the simple way to calculate drug dosage formula and Iv flow rate with some practice questions .
Conversion between matric system
Before starting with drug calculation formula it is important to have some basic knowledge of matric system conversion . Here are some matric system which are commonly used during drug prescription and administration
1 microgram (mcg) = 0.000001 gm
1 milligram ( mg) = 1000 mcg or 0.001 gm
1 gram (gm) = 1000 mg
1 kilogram ( Kg) = 2.2 lb
1 ml = 0.001 L
Q. Convert 3 gm to milligram
solution : First change larger unit to a smaller unit
3 gm = 3000 mg ( moving decimal point three places to right )
or multiply larger unit by 1000 to convert it to smaller unit .
Q. Convert 350 ml to litres.
solution : First change smaller unit to a larger unit
350 ml = 0.25 L ( moving decimal point three places to left )
or smaller unit by 1000 to convert it to larger unit
Drug dosage calculation Formula
Universal formula for drug calculation :
D (Desired amount ) x V (volume per ml)
————————————————————— = Dose
H ( Amount on Hand or available dose)
Q. A physician prescribes 400000 units of penicillin G Intravenously . The label on 10 ml ampule sent from pharmacy includes 300000 units/ ml . How much medication should a nurse should prepare ?
Solotion : Use the drug calculation formula
D (Desired amount ) x V (volume per ml)
————————————————————— = Dose
H ( Amount on Hand or available dose)
Here Desired amount (D) = 400000 units
volume per ml ( V) = 1 ml
Available amount (H) = 300000 units/ml
Apply all these values in formula
400000 units x 1 ml
————————– = 1.33 ml
300000 units
Q. A physician prescribes potassium chloride 30 mEq to be added to 1000 ml normal saline and to be administered over 10 hours . The label on medication reads 40 mEq /20ml. The Nurse prepares how many milliliters of potassium chloride to administer correct dose of medication ?
Solution :
Desired amount (D) = 30 mEq
volume per ml ( V) = 20
Available amount (H) = 40 mEq
Using drug calculation formula :
D (Desired amount ) x V (volume per ml)
————————————————————— = Dose
H ( Amount on Hand or available dose)
30 mEq x 20 ml
———————– = 15 ml
40 mEq
Calculating Iv flow rate
To calculate Iv flow rate use the formula :
Total volume to infuse X Drop factor
————————————————— = Drops per min
Time in minutes
Q. A physician prescribes 1000 mL of normal saline to infuse over 12 hours . The drop factor is 15 drops/mL . The Nurse prepares to set the flow rate at how many drops per minute ?
solution : Total amount to infuse = 1000 mL
Drop factor = 15 gtt
Time = 12 hours , convert it into minutes
12 X 60 = 720 min
Use the formula :
Total volume to infuse X Drop factor
————————————————— = Drops per min
Time in minutes
1000 ml X 15
—————— = 20.8 drops / min
720
Calculation millililters / per hours
Universal formula :
Total volume in milliliters
———————————— = Milliliters per hours
Numbers of hours
Q. Physician prescribes 3000 mL of normal saline to be administered over a 24 hours period . The Nurse determines that how many milliliters per hours will be administered to the patient ?
Solution :
Total volume in milliliters = 3000 mL
Numbers of hours = 24 hours
use the formula
Total volume in milliliters
———————————— = Milliliters per hours
Numbers of hours
3000 ml
————— = 125 mL/hours
24 hours
Two steps question for practice
Q. Heparin sodium is prescribed at a rate of 1300 units/hour by continues IV infusion. Iv bag labels 20,000 units / 250 mL D5W . The nurse should set infusion pump at how many mL per hour to deliver 1300 units/hour ?
Solution :
Step 1
Determine amount of Heparin sodium in 1 mL
Known amount of medication in solution
————————————————– = Amount of medication
Total volume of diluent
200000 units
—————– = 80 units / mL
250 mL
Step 2
Determine the infusion rate or milliliters per hours
Dose per hour desired
————————————— = mL / hour
concentration per milliliters
1300 units
——————- = 16 mL / hour
80 units/mL
