Learning Lab

From time-to-time, a medication dosage is expressed in terms of a number of milligrams or millilitres, per kilogram of body weight. This means that a larger person needs a larger dose of the drug. It means that the amount you give is not a standard dosage but, rather, depends on the body mass of the patient.


In this example, the patient will need 12 milligrams of the drug for each kilogram of their body, and don’t forget that this is what they need per day. Do n’t neglect this last unit! Other examples may specify that this has to be given per dose or per hour, so please keep this one in mind.


Let’s just focus on this information first. We know that the patient is 60 kilograms, and as soon as you have a patient with a specific body weight, you need to determine how much of the drug we will give to this particular patient. We know from the dosage label that for every kilo of their body mass, we need to give 12 milligrams of the medication. Clearly, that will be 60 lots of 12 milligrams. 60 times 12 is 720, and remember that is per day.

You can visualise this in the following way: Think of the 60 kilogram patient, and then, visualise them consisting of 60 separate kilograms of body mass. To each kilogram, we need to deliver 12 milligrams of the medication. 60 times 12 gives us a total of 720 milligrams. And, as I said before, that is per day.


You can show your working in the following way: 60 kilograms multiplied by 12 milligrams per kilogram. The kilograms cancel, so that the unit you are left with is milligrams. That is, 720 milligrams.


Once you have achieved this single amount, it is unnecessary to think of the dosage in terms of milligrams per kilogram anymore… Until you are faced with another patient, of course. Your dosage for this patient is simply 720 milligrams and, as we have said previously, that will be 720 milligrams per day.