Dosage Calculation Types

Before starting on dosage calculations, be sure you are familiar with reducing fractions, ratios and proportions, and simple algebra.

Number of Doses

Measurement Conversion

Paxton Pharmacy has compounded 1g of a topical cream dispensed in a tube for patient, Cleo. The directions state that Cleo should apply 20mg of the cream daily to the affected area. How many doses are in the tube?

First, note that both grams and milligrams are used in the problem so we need to do a measurement conversion. There are 1,000 milligrams per gram. The first ratio is one dose per 20mg so ¹⁄₂₀. The second ratio contains an unknown so initially it is ˣ⁄₁₀₀₀. Set these two ratios in a proportion.

Next, cross-multiply to set up the problem algebraically.

We have determined that there are fifty doses for Cleo to use.

What if Cleo’s medication was dispensed in a 1.5g tube rather than a 1g tube? Set up the problem the same way, but mind the ratio. There are 1,500 milligrams in 1.5 grams.

With a 1.5g tube, Cleo now has a 75 day supply of cream.

Calculate Days Supply Needed

Using the 1g tube example above, Cleo would use the prescription for fifty days because there are fifty doses and he is applying the cream once a day. Now, what if the prescription called for BID? QID?

First, know your Sig codes! BID is twice a day. QID is four times a day.

Using the cream twice a day cuts the number of days used in half.

50 / 2 = 25 days

50 / 4 = 12.5 days

Total Amount Needed Dosage Calculations

The problems will either be set up as total amount to dispense or the total amount that the patient has already received.

Total to Dispense

Merle drops off his prescription for Metformin HCL 1,000mg at his local pharmacy. The prescription calls for the drug to be taken twice a day for thirty days. The pharmacy does not have enough 1,000mg tablets to fill the prescription, but they do have a full bottle of 500mg in stock. How many 500mg tablets will need to be dispensed to fill this prescription? How many milligrams total will be dispensed?

(Two 500mg tablets * 2) * 2 = Four 500mg tablets daily

Four 500mg tablets * 30 days = 120 tablets of 500mg strength dispensed

(Two 500mg tablets * 2) * 2 = 2,000mg daily

2,000mg * 30 days = 60,000mg total mg dispensed

Total to Dispense Parenteral Example

Parenteral means medication is delivered by other routes other than orally/gastrointestinal tract. In many cases, this means an injection (remember there are several types of injections!) Commonly, injectable medication strengths are in terms of milligrams per milliliter e.g. clindamycin 300mg/2mL. Understandably, these math problems can be a little more difficult than a problem that has clindamycin 300mg tablets.

Nurse Frank has been instructed to give a patient a clindamycin injection 150mg IM q8h. The drug in stock is clindamycin 300mg/2mL. You might notice that reduces to 150mg/mL, but if not, set up the problem as a proportion to get the milligram per milliliter.

Nurse Frank will be injecting 1mL of the 300mg/mL medication every eight hours.

Let’s take it a step further. How many mL total daily will the patient receive? There are 24 hours in a day so the patient will receive the injection three times daily.

1mL * 3 = 3mL total clindamycin daily