drug calculation

Many nurses are weak with drug calculations of all sorts. This article will help to review the major concepts related to drug calculations, help walk you through a few exercises, and provide a few exercises you can perform on your own to check your skills. There are many reference books available to review basic math skills, if you find that you have difficulty with even the basic conversion exercises.

Common Conversions:
1 Liter = 1000 Milliliters
1 Gram = 1000 Milligrams
1 Milligram = 1000 Micrograms
1 Kilogram = 2.2 pounds

Methods of Calculation
Any of the following three methods can be used to perform drug calculations. Please review all three methods and select the one that works for you. It is important to practice the method that you prefer to become proficient in calculating drug dosages.
Remember: Before doing the calculation, convert units of measurement to one system.
I. Basic Formula: Frequently used to calculate drug dosages.
D (Desired dose)
H (Dose on hand)
V (Vehicle-tablet or liquid)
D
H x V = Amount to Give
D = dose ordered or desired dose
H = dose on container label or dose on hand
V = form and amount in which drug comes (tablet, capsule, liquid)
Example:
Order-Dilantin 50 mg p.o. TID
Drug available-Dilantin 125 mg/5ml

D=50 mg
H=125 mg
V=5 ml

50
125
x 5 =
250
125
= 2 ml
II. Ratio & Proportion: Oldest method used in calculating dosage.
Known

Desired
H
:
V
::
D
:
X

Means

Extremes
III. Left side are known quantities
IV. Right side is desired dose and amount to give
V. Multiply the means and the extremes
HX = DV

VI.
X =
DV
H
VII.
Example:
Order-Keflex 1 gm p.o. BID
Drug available-Keflex 250 mg per capsule

VIII. D=1 gm (note: need to convert to milligrams)
1 gm = 1000 mg
IX. H=250 mg
X. V=1 capsule
250
:
1
::
1000
:
X
XI. 250X = 1000
X =
1000
250
XII. X = 4 capsules
XIII. Fractional Equation
H
V
=
D
X
XIV. Cross multiply and solve for X.
H
V