The one about the formula for tablet doses


I guess a good place to start is the basics.

The formulae for calculating medication doses are obviously a little different from each other depending on what type of medication is to be given (oral or parenteral) or if it’s an IV infusion requiring rate calculation.  For this post, I’ll just cover the basic formula for tablets.  It works the same way for other oral medications too, such as liquids.

The basics:  a/b x c = d

It’s often presented like this: dose ordered/stock on hand x volume = dose to administer

But what dose that mean?  If it’s a tablet, it goes like this (for example):

Doctor has ordered 50mg of Medication X, so that goes in ‘a’ or ‘dose ordered’.

Stock on hand is 25mg tablets of Medication X, so that goes in ‘b’ or ‘stock on hand’.

Volume in this instance is ‘tablets’ in ‘c’.

Now it looks like this: 50mg/25mg x tablets = dose to administer

so we divide the 25 into 50, getting 2, so 2 x tablets = 2 tablets

If the dose ordered had been 62.5mg, it would look like this: 62.5/25 x tab = dose, therefore 2.5 tablets.

Practicing the maths can be tricky sometimes, but the basic formula stays the same.  Once you master the formula and understand the concept, it’s just the maths.

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