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RMIT University Library - Learning Lab

Need over have


This short video is the third of three videos in the Nursing calculations - Finding the volume required section. It explains the formula for calculating the quantity of a drug in solution for correct dosage by determining the stock strength and volume (have), then calculating a fraction of that stock volume for prescribed dosage (need).

This is a sachet of GOFUEL drink hydrate, it is in powder form, and the sachet contains 40 grams of powder... This is a bottle of water and this bottle contains one litre of water... We are going to empty the sachet into the water so that it dissolves evenly.

“If I need to ingest 15 grams of GOFUEL… How much solution should I drink?”... Most pure drug calculations will be of this form. Our sports coach has recommended that we take 15 grams of the GOFUEL powder. How much liquid do I need to have out of the ready made 1 litre solution? Let’s clarify our variables.

What we need is 15 grams of GOFUEL. What we have in the bottle is 40 grams of GOFUEL. The total volume of liquid in the bottle is a 1000 mls… And the amount of liquid we are required to drink is the quantity we are looking for.

So the volume of the liquid that we have is given this equation: the amount of GOFUEL we need over the amount of GOFUEL we have, as a fraction of the bottle’s volume. So plugging those values into the formula gives us fifteen-fortieths of the 1000 millilitres.

15 and 40 is divisible by 5, so they can cancel down to three-eighths... The volume we’re required to drink is three-eighths of the 1000 millilitres. Three-eighths converted to a decimal form is 0.375, and multiplying that by 1000 gives us 375 mL. We need to drink 375 mL of the liquid to give us 15 millilitres of GOFUEL.

Now, returning to our formula of what we need over what we have as a fraction of the bottled volume. This is normally expressed in this form. What we need is strength required and what we have available is the stock strength. The volume of the bottle is normally the stock volume.

So the volume of liquid we need to drink or receive is given by... The amount of drug powder that we want, divided by the amount of drug powder we have available in the bottle, as a fraction of the bottle volume that it comes in… And this is the formula in abbreviated form.

VR stands for Volume Required, this is usually the amount being sought in the problem. SR represents the Strength Required as in how much go fuel powder you want to ingest, or how much medication you have been prescribed. Stock Strength is given by SS, which is how many grams or milligrams is in the bottle to begin off with. And SV is Stock Volume, which is how much liquid is in the bottle or, in other cases, is perhaps what is in vial or an IV bag.