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How do percentages work?

The word “percent” means “per one hundred”—so, if there was 100 of a thing, what portion of that 100 would be X? If X = 2%, then we know 2/100 are X—or 4/200, 6/300, etc. All of those ratios are 2%.

When you make a formulation, it is always 100%. If you combine four ingredients in equal parts, they are each present at 25% (100 ÷ 4 = 25). If you add a fifth ingredient in an equal amount you do not have 125%. It’s still 100%, but you now have five ingredients present at 20% each instead of 25% (100 ÷ 5 = 20).

We formulate in percentages because it creates universal formulas with easily recognizable proportions. 10% is 10%, regardless of batch size, while 20g or 3 tbsp is fairly meaningless without the full context of the rest of the formula. A common recommendation for personal finance is that you don’t spend more than 30% of your income on rent/housing expenses; this is presented as a percentage rather than a dollar value so it is relevant regardless of income level. We want that same kind of scalability with formulas—if somebody told you they spent 70% of their income on rent you’d know that was too much regardless of how much money they earn. With formulas, if you see it calls for 10% preservative when the maximum recommended rate is 1%, you know that’s way too much, regardless of batch size. You wouldn’t be able to know that immediately if the recipe just called for 3g.

Maximum usage rates are always part of the entire formula. For instance, if an essential oil cannot be used above 0.3%, that is 0.3% of the entire formula, not 0.3% of a certain phase or a certain selection of ingredients.

If you wish to include a new ingredient you will first need to make room for it by removing an equal amount of something else to keep the recipe in balance. I’ve discussed that here. For instance, if you wanted to add another carrier oil to a lotion recipe you would need to remove an equal amount from a carrier oil already present in the recipe. If you don’t, you’ll throw off the balance of the recipe and there might not be enough emulsifier for the newly larger oil phase, meaning the emulsion may fail.