Compounding Math

Pharmacy math is often intimidating, but it is really important that we get it right, so here's a little post aimed at making you more comfortable doing compounding math.



1.  Make 10ml of Methimazole with 5mg tablets at 5mg/ml.

We need 10ml at 5mg/ml.  That's 5mg for every ml. 
So, if we want 10ml, we multiply top and bottom by 10. 
That's 5mg X 10 = 50mg in 1ml X 10 =10ml.

We need 50mg of Methimazole and our Methimazole is in 5mg tablets.
So we divide 50mg by 5mg to get the number of tablets we need:
50 / 5 = 10.  We need 10 tablets.

And our total volume is 10ml, so we need 10 tablets in 10ml of solution.


2.  Make 30ml of Doxy from 50mg capsules at 25mg/ml.

We need 30ml at 25mg/ml.  So, multiply the top and bottom by 30:
25mg X 30 = 750mg.  1ml X 30 =30ml 
So we need750mg of Doxy in 30ml of solution.

Since our Doxy is 50mg we need to know how many capsules to use to gt 750mg:
750mg/50mg capsules = 15 capsules
So, we need to use 15 capsules and 30ml of solution.


3.  Make 20ml of 15mg/ml Meloxicam from 7.5mg tablets.

We need 20ml at 15mg/ml.  So, multiply the top and bottom by 20:
15mg X 20 = 300mg.  1ml X 20 = 20ml
We need 300mg and we have 7.5mg tablets, so:
300mg/7.5mg tablets = 40 tablets in 20ml of solution.


4.  Make 12ml of 25mg/ml Baytril from 100mg/ml (injectable) Baytril.

This is a little different since our medication is liquid.  In the above problems our total volume and amount of solution needed were always the same, but for liquid medications the medication's volume has to be taken into account as part of the final solution, so we need to use the formula: 
(C1) X (V1) = (C2) X (V2)

Original concentration (C1) = 100mg/ml Baytril
Original volume (V1) = unknown amount of Baytril
Final concentration (C2) = 25mg/ml Baytril
Final volume (V2) = 12ml of solution

So:  100 X (x) = 25 X 12
Multiply each side to get:  100x = 300
Solve for x:  300/100 = x, so x = 3

So, we need 3ml of Baytril.
And we need 12ml total.
So, 3ml Baytril and the rest of the volume will be solution.
Find the amount of solution by subtracting the amount of Baytril from the total volume:
12ml total - 3m Baytril = 9ml of solution


5.  Make 4ml of 10mg/ml Baytril from 100mg/ml (injectable) Baytril.

4 X 10mg = 40mg
4 X 1ml = 4ml

C1 = 100mg/ml Baytril
V2 = unknown amount of Baytril
C2 = 10mg/ml Baytril
V2 = 4ml

100 X ? = 10 X 4
100X = 40
X = 40/100
X = 0.4

0.4ml Baytril

Total solution = 4ml
4ml total = 0.4ml Baytril means 3.6ml of solution will be used.