Metric and standard measurements, Dosage Calculations, and mathematical conversions and Formulas (Mod 1)

Most medication prescriptions and dosages will be in the metric system of weights and volume. However some medication formulations in the apothecary and standard systems require conversions. Also, some prescriptions require dosage calculations based on a patients weight in kilograms, especially pediatric dosages.

The metric system quantifies weight in kilograms (kg), grams (g), milligrams (mg), and micrograms (mcg). It measures volume in deciliters (dL), liters (L), and milliliters (mL). Length is in kilometers (km), meters (m), centimeters (cm), and millimeters (mm).

Unit

Relationship to base unit

Decimal value/ whole number

Micro

divide by 1,000,000

0.000001

Milli-

divide by 1,000

0.001

centi

divide by 100

0.01

base unit

1

1

Kilo

multiply by 1,000

1,000

Standard Systems

Household value

Metric Equivalent

15 drops (gtt)

1 ML

1 Teaspoon (Tsp)

5 mL

1 tablespoon (tbsp)

15 mL

1 fluid ounce (oz), 2 tbsp

30 mL

1 cup

240 mL

1 pint

480 mL (about 500 mL)

1 quart

960 mL (about 1 L)

1 gallon

3,830 mL

2.2 pounds (lb)

1 kilogram (kg)

Dosage Calculations

With all dosage calculations, always take time and recheck calculations. If there is any doubt, ask the provider or another medical assistant to check the calculations. The patient’s well-being depends on accuracy in all calculations.

For example, a provider prescribes diphenhydramine 50 mg for a patient who is having a mild allergic reaction. Available are 25 mg capsules. Here is how to determine how many capsules to give the patient.

If 25 mg equals 1 capsule (cap), then 50 mg equals how many (X) capsules?

25mg/ 1 Cap = 50mg/ X cap

Cross multiply and get the following

1× 50 = 25x

50= 25x

Then divide both sides of the equation by 25 and the result is 2 capsules

Another common method for dosage calculation is the formula method, or desired over have. This involves thinking of the calculation as to what to give divided by what you have times the quantity you have. So, for that same prescription for diphenhydramine, the equation looks like this.

desires/ have X Quantity = x

50mg/25mg X 1 cap = x

50/25× 1 = 2 capsules

Simpler explanation:

Lesser to greater

MCG< MG<G

ML<L

When going from smaller to larger, divide by 1,000

When going from Larger to smaller, multiply by 1,000

1kg= 2.2 LBS

To convert Kg to LBS, Multiply by 2.2

To convert LBS to KG, divide by 2.2

Desired X Quantity = D/H X Q

Have

  1. Ordered 3.6

On hand: 1,200mg

3.6× 1000= 3600mg

3600mg/1200mg= 3 tabs

  1. Ordered: 750mcg

    On hand: 0.25mg

750mcg divided by 1000= .75mg

.75mg divided by .25mg= .30 tab

Pediatric Dosage Calculations

The most accurate method to determine medication dosage calculations for children is to use weight calculations.

Dosage By Weight

-Watch video on dosages***