One of the most fundamental math concepts that every pharmacy technician needs to know is the definition of a percent. I know most of us have a general idea of percents. When we achieve a 95% on an exam of 100 questions, then we correctly answered 95 out of 100 questions. If we receive a 90% on an exam of 50 questions, then we correctly answered 45 out of 50 questions. To get the answer, we divide 45 by 50 and we get 0.9 or 90%. This, so far, is probably something most of us already understand. What about 2.5% hydrocortisone cream or 0.9% sodium chloride solution? What do these percents of medication mean?
In the case of 2.5% hydrocortisone cream, it is a solid (versus being in a solution). This means that the percent is weight in weight. We now know that the cream is 2.5 parts hydrocortisone in 97.5 parts of other stuff (inert ingredients also known as excipients). This is typically expressed in grams or (g). Therefore, if you had 100 grams of 2.5% hydrocortisone cream, then you have 2.5 grams of hydrocortisone and 97.5 grams of a cream base and other excipients. If you had 30 grams of hydrocortisone cream, you would multiply 30 grams by 0.025 (or 2.5%) to obtain 0.75 grams of hydrocortisone and 29.25 grams of cream base and other excipients.
In the case of the 0.9% sodium chloride solution, it is a liquid. This means that the percent is weight in volume. Always consider a percent that is in a solution as grams per 100 milliliters (mL). Our 0.9% sodium chloride is 0.9 grams per 100 mL. People seem to always forget the 100 mL part. There are many calculations where you need to know the concentration in milligram (mg) per mL or gram per liters (L). You could do the simple algebra equation below:
Converting percent to mg/mL
0.9 g/100 mL = x mg/mL
(we can add 1000 mg/1 g because they equal each other)
x mg/mL = 0.9 g/100 mL X 1000 mg/g = 9 mg/mL
Converting percent to g/L
0.9 g/100 mL = x g/L
(we can add 1000 mL/1 L because they equal each other)
x g/L = 0.9 g/100 mL X 1000 mL/L = 9 g/L
Now that you know the actual calculations, you can use a shortcut. Whenever you need to convert a percent to g/L or mg/mL, simply move the decimal to the right by one place. In the case of 0.9%, this means it become 9. You probably noticed above that both answers are 9. This is a fundamental mathematical skill that all pharmacy staff (pharmacists and pharmacy technicians) needs to know. Commit it to memory, and it will serve you well throughout your career.