Friday, December 24, 2010

Calculating percentages in lotions

No doubt you've come across recipes that use grams or ounces and you might be asking yourself, "How do I convert this into a percentage based recipe?" The other question you might ask yourself is, "Why do I want to convert this into a percentage based recipe?" and I'll answer that one first.

It's best to work in percentages for two reasons - it's easier to make larger batches and you can check to see if all the ratios are in order and if all the ingredients are being used at the safe or suggested levels (for instance, too little or too much e-wax). This last bit is the most important one, because you could multiply a non-percentage based lotion easily.

So how do you convert that amazing sounding recipe into percentages? With the wonders of math! Here's a sample recipe we can work with (do not actually make this recipe, it's an example, although it wouldn't be a bad recipe but I'd want to add 1% Vitamin E due to the short shelf life of the grapeseed oil...)

40 grams water
10 grams aloe vera
3 grams glycerin

15 grams grapeseed oil
10 grams shea butter
4 grams emulsifying wax
3 grams cetyl alcohol

1 gram liquid Germall Plus
1 gram fragrance oil
1 gram green apple extract

This recipe totals 88 grams. So let's take a look at how to figure out what each ingredient works out to in the recipe.

To figure out percentage, divide the amount of the ingredient by the total weight of the recipe. So you'd divide everything in this case by 88.

40 grams of water / 88 grams total = 45.45 (round to 45.5)
10 grams of aloe vera/ 88 grams total = 11.37 (round to 11.5)
3 grams of glycerin / 88 grams total = 3.4 (round down to 3.0)
15 grams grapeseed oil / 88 grams total = 17.05 (round to 17)
10 grams shea butter / 88 grams total = 11.37 (round down to 11.0)
4 grams emulsifying wax / 88 grams total = 4.54 (round to 4.5)
3 grams cetyl alcohol / 88 grams total = 3.4 grams (round down to 3.0)
1 gram liquid Germall Plus / 88 grams total = 1.14 (rounded)
1 gram fragrance oil / 88 grams total = 1.14 (rounded)
1 gram green apple extract / 88 grams total = 1.14 (rounded)

Total: 99.72

Are the ratios and suggested usage rates right for each ingredient? 
We need our preservative to be at 0.5% for liquid Germall Plus, so we need to decrease that.

We want 1% fragrance or essential oil, so that's about right.

We want 0.5% of an extract, so let's decrease that.

And we're not using enough emulsifier. We like to use Polawax at 25% of the oil phase of a recipe. In this case, we have 28% oil phase, which means we'll want to use 7% Polawax. If we'd made this recipe in the way suggested, we'd have an epic lotion fail!

So let's re-write this recipe as a percentage based recipe...


45.5% water
11.5% aloe vera
3% grams glycerin

17% grapeseed oil
11% shea butter
7% emulsifying wax
3% cetyl alcohol

0.5% liquid Germall Plus
1% fragrance oil
0.5% green apple extract
Total = 100% 

You can make a nice percentage converter in Excel (get the total recipe amount then divide each ingredient by the total to get the result) or you can do it on a calculator. I would create one, but I can't seem to save the darn thing in a format that other people can use! (Excel and I don't get along very well!) 

Happy formulating! 


p said...

Hi Susan, you've got an error in your percentage calculation for the ingredients at 1 gram - the liquid Germall Plus, fragrance oil, and green apple extract. The percentage is 1 g of water / 88 g total = approx 1.14%, not 0.14%.

Susan Barclay-Nichols said...

Hi p! Thanks for catching that error. I took the decimal place one space too far! I guess it just proves my point - no, wait, it's the opposite of my point. I was trying to say it was easy to do, and I went and messed it up!

I've fixed it now!

Zenobiah said...

If the calculator I posted on the Dish would help, feel free to post it here if you want.

Susan Barclay-Nichols said...

Hi Zenobiah! Thanks for the link!

Mary said...

This was incredibly helpful!! Thank you so much!!

HB said...

Zenobiah's calculator will be incredibly useful. I'm still stuck, though, as I have a recipe that's written in ml/tsp. How do I convert volume measurements to percentages?

HB said...

The recipe in question:
10g / 1/4 oz beeswax
20ml / 4 tsp almond oil
75ml / 2 tsp honey
few drops of EO (optional)
1 vitamin E capsule (optional)

The book says to follow either the metric measurements or the imperial, not a mixture.

Nan Murray said...

I wanted to ask a question. Is there a way to calculate how much e-wax is needed in a formula. In my 1st batch the lotion came out perfect and that was about 1% and it was great. However I made other were the percentage of oils was much higher so I did 3% and the lotion has a really waxy feel. So is there a rule of thumb to use with all emulsifiers and thickeners. Please let me know

Susan Barclay-Nichols said...

Hi Nan! Have you looked at the FAQ?