Medical Math 2

The Quick and Dirty Guide to Medical Math Module II


In our last installment, we just started simple with changing grams to milligrams, micrograms and back! In today's medical math lesson, we're going to add weight and concentration in 3 easy steps!  If you haven't looked at module 1 in awhile, go back and check it out first!

Pounds to Kilograms

Step 1: Pounds to Kilograms

We don’t do dosages in pounds! To change pounds to kilograms, simply divide the number of pounds by 2.2
There are plenty of apps out there that do this for you on the fly, but nothing is better than just having a dollar store pocket calculator in the back of the truck. Doing it like this will also give you the ability to do it without the calculator. So, you’ll be able to express how it works in words, reports, and when teaching others. It keeps the skill up! If you’re stuck to an app and someone asks you how to calculate dopamine, you’re just going to have to plug the app. Don’t do that. Practice practice practice!

So, lets start with the easiest. 110 lbs = ____kg.
We divide 110 by 2.2, which gives us 50.
110 lbs = 50kg
220lbs = 100kg
330lbs = 150kg
All we did was plug in the number of pounds, and divide it by 2.2
Practice converting pounds to kilograms!


milligrams per kilogram

Ok, now we’re going to do the very basics of weight-based drugs.

When there’s a ‘mg/kg’ it is read as ‘milligrams per kilogram’ which means ‘FOR EVERY KILOGRAM that the patient weighs, you’re going to give X amount of milligrams.’ If the patient weighed 10kg, and the order was for 1mg/kg, the total desired dose would be 10 mg. Get it?

Your order is to give Succinylcholine at 1mg/kg. Your patient weighs 220 pounds.

First, change your weight to kg: 220/2.2 = 100. So your patients weight is 100kg, and your order is for 1 mg for every kg.
1 mcg x 100 = 100 mg.

So, your desired dose for this patient is 100mg.

Its not always milligrams. It can be grams, micrograms, milliequivalents or any other measurement, but it works the same way.

Sodium bicarbonate standard dose is 1 meq/kg  (1 milli-equivalent per kilogram)

You’re still going to multiply the dose times the number of kilograms and bring down the measurement…

So for the same 220 lb patient, you’re going to change the pounds to kg. So, 220 / 2.2 = 100. 100 kilograms.
Then, 1 meq * 100 = 100 meq.

So now you know that pushing ‘1 amp’ of Bicarb in a 220 lb patient who needs it is only HALF of the dose he needs! AND YET THIS IS FREQUENTLY all they will ever get, because his paramedic didn’t know what you now know!
This stuff is important! Practice until you’re a bad ass…



How Much to Deliver

In this next exercise, we’re going to draw up some medications and give you some practical situations to practice on!


Just go slow, repeat these exercises as often as you need, and continue until you really have this down! It isnt something you learn once and you’re done…Its something you will want to review on occasion to maintain proficiency.

Your medical control orders you to give 25mg of Diphenhydramine to your patient having a mild allergic reaction. It comes packaged as 50mg/1ml in a vial. How many mls do you draw up?

In this example, the concentration is 50mg/ml. So, we don’t have to convert it. You just divide the desired dose by the amount on hand. In this case, you’re dividing 25 by 50. The answer is 0.5, so you would draw up 0.5ml in order to administer the desired dose of 25mg.

Example 2: You’re ordered to give 8mg of a drug. The vial states that there is 10mg/5ml. So, the vial contains 5ml of fluid, and that 5ml altogether contains 10mg of the drug.
First, you have to find out how much of the drug there is PER ML.
You divide the number of mg by the number of ml to find out how many mg there is in 1 ml.

So… 10/5 = 2.
Therefore, you can simplify the concentration to 2mg/ml. That means for every milliliter of fluid, there are 2mg of the drug.
Then, you divide your desired dose by the amount on hand. So… 8mg divided by 2mg/ml. Your mg on both sides cancel and you end up with 4ml.

Got it? Lets try!