Monday, November 15, 2010

Question: How to convert recipes from percentages to weight

Here's an e-mail question I received from JC:
I don't know anything about chemistry, so I don't understand the % you put for each ingredient. I'm used to read cooking recipes, where they tell you 1 teaspoon of this, 1 cup of that, but I don't know how I would measure the % of the ingredients in your formulations. Maybe you have a blog post explaining this but I haven't found it yet, so can you enlighten me about how should I measure the ingredients for a formulation?  Do I use a scale? a measuring cup? How do the % translate into miligrams, grams, ounces or whatever measurement system is the appropriate one?

All the recipes on my blog are in percentages and they should add up to 100% (although there are some differences in the amount of preservative used). The easiest way to convert the recipe is to think of the percentage sign as the word "grams". So if you see 70% water, you'd use 70 grams of water. 15% oil would become 15 grams of oil and so on. The recipe will total 100 grams of product.

But let's say you want to make a really large batch of lotion to give as Christmas presents. Substitute the percentage sign with grams, then multiply by how much you want to make. If you want 500 grams of lotion, you'd see the 70% water as 70 grams of water x 5 for 350 grams of water. 15% oil would be 15 grams x 5 for 75 grams of water. And so on. You'd have 500 grams of product.

You can do the same thing for weighed ounces, but it really is a pain in the bum. The easiest way is to convert the percentage sign to ounces, move the decimal one space to the left, and use that. So if you have 70% water, you'd convert that to 70 ounces, then move the decimal over once space to the left and call it 7 ounces. The recipe will total 10 ounces of product. (But this is a way more complicated way to do it!) You can multiply the ounces by the amount of product you want to make.

Why do we weigh all our recipes? For accuracy. Using cups and teaspoons aren't accurate, so we might end up with more or less emulsifying wax than we need, which can result in an epic lotion fail or end up with more beeswax than we want in a lotion bar, leading to drag on our skin. It makes it easier to replicate that awesome recipe you made last time, as well. 

Here's an excerpt from a previous post I wrote on weight vs volume! 
Pure water at 4 Celsius is our baseline for specific gravity and everything else is compared to it. Water weighs 1000 grams per litre - 1 kg per litre - or 1 gram per millilitre. If something is listed as being less than 1, it weighs less than water per gram. If something is more than 1, it weighs more than water per gram.

So if we see cetrimonium chloride listed as having a specific gravity of 0.93, we know this means it weighs 0.93 grams per 1 cc or 1 ml. Let's say we want 5% cetrimonium chloride in our conditioner. If we're making a 100 ml batch and add 5 ml, we'd only have 4.65% cetrimonium chloride. Not the biggest deal in the world. If you wanted to make 1 litre of conditioner and added 50 ml, you'd only have 46.5 grams of cetac or 4.65%.

Liquid Germall Plus has a specific gravity of 1.15 to 1.25. If you want 0.5% in a lotion and add it in volume at 0.5 ml to a 100 ml batch of lotion, you might have 0.575 to 0.625 ml preservative, which is above the 0.5% recommended!

What kind of scale should you get? I use an Escali scale like the one above (although mine is a boring silver colour), one that can measure down to 1 gram. You definitely want to go digital.

If you are using a lot of extracts, want to make mineral make-up, or want to be really accurate, you can get digital scales that register to 0.1 grams. I use an epoxy weighing scale I bought at the hardware store because it's sealed and has a nice little cover that closes up to make it portable. (The last one I had wasn't sealed and I'm a bit of a klutz, so I ended up shorting it out because so much goo got under the weighing plate!) I find it works well for weighing my extracts, preservatives, proteins, panthenol, and anything else I'm adding at less than 3%.


Judy Hill said...

So, in essence it can be important to know the specific gravity of an ingrdient. I have the SoapMaker3 program and it asks the specific gravity of all items entered, except packaging of course. Most I don't know what that # is... some suppliers provide it ... some don't.. so I usually enter 1 for the amount.

Lynda said...

Susan, When I saw the title of this post, I thought big deal. I know how to do that. But I've been measuring in ounces - using fractions to convert and what a pain! I use grams when I make soap but not with anything else. Well, youve converted me today - It's grams all of the way. So easy! Thanks so much.

Tara said...

I find the simplest way to convert percentage to grams or ounces is to multiply the percentage's decimal value by your total batch size. If you need 15% of an ingredient and your total batch size is 8 oz, the calculation would be 0.15 x 8 = 1.2 oz (yes, use a calculator).

Shaloreial said...


That just made my life easier! I was using google to convert, but using too many steps to get to that point. Thanks for the tip!

Pier said...

to convert grams more accurately into ounces the easy way is

Susan Barclay-Nichols said...

Thanks for the suggestion, Pier, but it really is easier to just substitute the word "grams" for the % sign. I don't suggest going to ounces, then to grams. It's just too much work!

Pier said...

You're right Susan. Products have different density such as Germall Plus.
0,5 % of Germall is 0,5 gr but not 0,5 mls. If I understood well, every recipe totalizing 100 % should make 100 grams which will fill a bottle or jar of about less than 4 ounces. So if we want to fill a bottle of about 12 ounces, we will have to multiply every products (grams) by 3. Very important also when adding Essentials oils to keep in mind that density are about 0,9 and less. Hope I catch everything well. Thanks a lot. My suggestion was wrong concerning cosmetic products. I am learning a lot with you.