One of the more common questions that comes up when talking about short fills is how to reach a given PG/VG ratio. The short fill bottle has a VG/PG ratio, and you might have your nicotine in PG, VG, or some combination of the two. Let’s work through a few examples to see how you can easily get your desired final ratio.
We’ll start with the simplest case first. Let’s say you have a 60ml short fill bottle with 45ml of 70/30 VG/PG liquid in it to start. Your nicotine is 100% PG.
In this scenario, you cannot end up with a 70/30 finished product unless you pre-mix your nicotine to also be 70/30 or remove some of the flavor base from the short fill bottle.
The first step is determining the ratio of the liquid you already have in the bottle to the liquid you will be adding. Simply divide the starting volume of the short fill by its total volume
45ml / 60ml = 0.75
Next, multiply the percentage of the diluent that is not present in your nicotine (i.e. VG for PG nic, PG for VG nic) by the number you got in the first step. Then, subtract 1 by the number you got in the first step and add.
PG = (0.3 * 0.75) + (1 - 0.75) = 47.5%
Now just subtract 100% from the result of the second step to get the other half of the ratio.
VG = 1 - 0.475 = 52.5%
So if you add 15ml of PG-based nicotine to a 45ml 70/30 VG/PG mix, you'll end up with a 52.5/47.5 mix.
Now, what happens when you have a 70/30 liquid and nicotine that is 40% VG and 60% PG?
First we determine the ratio of liquid being added.
45ml / 60ml = 0.75
Then, multiply both the VG and PG percentages by this ratio.
VG = 0.7 * 0.75 = 52.5%
PG = 0.3 * 0.75 = 22.5%
Now, subtract the ratio from the first step from 1 and multiply by the VG/PG ratio of the nicotine.
VG = (1 - 0.75) * 0.4 = 10%
PG = (1 - 0.75) * 0.6 = 15%
Now add the results of the previous step to the results of the step before it.
VG = 10% + 52.5% = 62.5%
PG = 15% + 22.5% = 37.5%