Pharmacy Calculations & Measurements

01Household-to-metric conversions

For pharmacy dosing, 1 teaspoon (tsp) = 5 mL, 1 tablespoon (tbsp) = 15 mL (3 teaspoons), and 1 fluid ounce = 30 mL. These conversions are used constantly when compounding or counseling on liquid doses.

02Days supply formula

Days supply = total quantity dispensed ÷ amount used per day (dose × frequency). The same formula applies to tablets, liquids (in mL), and topicals (in grams).

03Percentage strength (w/v and w/w)

A percentage strength states grams of active drug per 100 mL (w/v, for solutions) or per 100 g (w/w, for creams/ointments). For example, 5% dextrose solution contains 5 g of dextrose per 100 mL.

04Insulin unit math

Standard U-100 insulin contains 100 units per mL, so a 10 mL vial holds 1,000 units total. Days supply for insulin is calculated the same way as other liquids: total units ÷ units used per day.

05Basic dose math

To find tablets or volume per dose, divide the total daily dose by the number of doses per day, then divide by the strength per tablet or per mL — always double-check that units match before dividing.