Nursing Drug Calculations

            Four Methods for Drug Calculation 

Before drug dosage can be calculated, units of measurement must be converted to one system. If the drug is ordered in grams and comes in milligrams, then grams are converted to milligrams or milligrams are converted to grams.
Four methods for calculating drug dosages include basic formula, ratio and proportion, fractional equation, and dimensional analysis. The ratio and proportion and fractional equation methods are similar. For drugs that require individualized dosing, body weight and body surface area are used. When body weight and body surface area calculations are used, one of the first four methods for calculation is necessary to determine the amount of drug needed from the container.

Method 1: Basic Formula 

The basic formula is the most commonly used method, and it is easy to remember.

D or desired dose: drug dose ordered by physician or health care providers (HCPs).

H or house dose: drug dose on label of container (bottle, vial, ampule)

V or vehicle: form and amount in which the drug comes (tablet, capsule, liquid)

Order: Acetaminophen 500 mg every 4 hours.

Drug available: 250 mg per tablet

How many tablet(s) would you give per dose?

Basic formula:      

Method 2: Ratio and Proportion

Ratio and proportion (RP) is the oldest method used for calculating dosage problems:

H and V: On the left side of the equation are the known quantities, which are dose on hand and vehicle.

D and X: On the right side of the equation are the desired dose and the unknown amount to give.

Multiply the means and the extremes. Solve for X.

PROBLEM 1: Order: erythromycin  0.5 g, po, q4h.

Drug available: 250 mg per 1 tablet

 To convert grams to milligrams, move the decimal point three spaces to the right'

Method 3: Fractional Equation

The fractional equation (FE) method is similar to RP, except it is written as a fraction.

• H: the dosage on hand or in the container
• V: the vehicle or the form in which the drug comes (tablet, capsule, liquid)
• D: the desired dosage
• X: the unknown amount to give

Cross multiply and solve for X.

Order: valproic acid (Depakene) 100 mg, po, tid.
Drug available: valproic acid (Depakene) 250 mg/5 mL suspension.

No unit conversion is needed.

Answer: valproic acid (Depakene) 100 mg = 2 mL

Method 4: Dimensional Analysis

The advantage of DA is that it decreases the number of steps required to calculate a drug dosage. It is set up as one equation.

• 1. Identify the unit/form (tablet, capsule, mL) of the drug to be calculated. If the drug comes in tablet, then tablet = (equal sign)
• 2. The known dose and unit/form from the drug label follow the equal sign.Example order: Amoxicillin 500 mg. On the drug label: 250 mg per 1 capsule.
• 
  
• 
  
  
• 
  
•  The milligram value (250 mg) is the denominator and it must match the NEXT numerator, which is 500 mg (desired dose or order). The NEXT denominator would be 1 (one) or blank.

Cancel out the mg, 250 and 500. What remains is the capsule and 2. Answer: 2 capsules.

 Metric and Household Conversions

METRIC
Grams (g)	Milligrams (mg)
1	1000
0.5	500
0.3	300 (325)
0.1	100
0.06	60 (64 or 65)
0.03	30 (32)
0.015	15 (16)
0.010	10
0.0006	0.6
0.0004	0.4
0.0003	0.3
Liquid (Approximate)
30 mL = 1 oz = 2 tbsp (T) = 6 tsp (t)
15 mL = ½ oz = 1 T = 3 t
1000 mL = 1 quart (qt) = 1 liter (L)
500 mL = 1 pint (pt)
5 mL = 1 tsp (t)

Household to Metric Conversions (Approximate)

Standard Household Measure	Apothecary	Metric Volume	Metric Weight
⅛ teaspoon	7-8 gtt/1/48 OZ	0.6 mL	0.6 g
¼ teaspoon	15gtt/1/24 OZ	1.25 mL	1.25 g
½ teaspoon	30 gtt/1/12 OZ	2.5 mL	2.5 g
1 teaspoon	60 gtt/1/6 OZ	5 mL	5g
1 tablespoon/3 teaspoons	½ OZ	15 mL	15 g
2 tablespoons/6 teaspoons	1 OZ	¼ dL/30 mL	30 g
¼ cup/4 tablespoons	2 OZ	½ dL/60 mL	60 g
⅓ cup/5 tablespoons	2½ OZ	¾ dL/75 mL	75 g
½ cup	4 OZ	1 dL/120 mL	120 g
1 cup	8 OZ	¼ L/250 mL	230 g
1 pint	16 OZ	½ L/480-500 mL
1 quart	32 OZ	1 L/1000 mL
2 quarts/½ gallon	64 OZ	2 L/2000 mL
1 gallon	128 OZ	3¾ L/3840-4000 mL