Let's strip this down to the basics. Every e-liquid bottle has two numbers that drive everything: the total volume in milliliters (ml) and the target nicotine strength in milligrams per milliliter (mg/ml). Multiply those two together and you get the total nicotine needed for the batch.

Example: You want to make 120ml bottles of 3mg/ml juice for your shop. That's 120 × 3 = 360mg of nicotine per bottle. If your nicotine base is 100mg/ml, you'd need 3.6ml of that base per bottle. That's it. That's the core of bottle math.

Most of the time, you're not starting from zero—you're adding a nicotine base to a pre-made or house juice. So the math becomes: total nicotine needed minus the nicotine already in your base. If your base is 100mg/ml and you add 5ml, that's 500mg of nicotine. In a 100ml bottle, that gives you 5mg/ml. Simple subtraction.