Concentration? Percentage? How do I figure the water amount for my lye solution?

When it comes to cold process soapmaking, some things are absolutely necessary and rigid, like how much sodium hydroxide (NaOH) is needed for a given amount of oils. It’s a property of the oils and can’t be changed without affecting the outcome of the soap, sometimes in really bad ways.

Other things are much more flexible, like the amount of water that is used to make the lye solution. The water is only there to dissolve the NaOH. Using more or less may affect how the soap mixes up, but in the end the water evaporates out and the finished, cured soap is what’s left. In fact, many experienced soapmakers use lye solutions that are quite concentrated so that they don’t have to wait so long for their bars to get dried-out and hard. It’s a real time-saver!

If you’re ready to start using your lye solution at a higher concentration, I can help you figure out how much water you will need to get there. The math can be a bit confusing for some. But I have a little formula that should do the trick.

So first, let’s explain percentage strength of a solution.

When we talk about a solution being, for example, at a 25% concentration, we are saying that it is 25% solute (the thing you are making the solution out of) and 75% diluent (the thing that you are dissolving the solute in).

In our case NaOH is the solute and water is the diluent. So a 25% lye solution is 25% NaOH and 75% water. “Per cent” means “per 100”. So the total percentage always adds up to 100.

Want another way to think about it? Imagine you have a 25% lye solution in a bowl. If you have 100 gr of lye solution in that bowl, there is 25 gr of NaOH and 75 gr of water.

OK, got the concept? Now how do you get your lye solution to be the concentration you want?

Well, you could use a lye calculator that can figure it out. The one in Soapmaker 3 software will do the math for you. So will the Soap Calc online.

But what happens when you’re not near your computer? Or you just like doing the calculations yourself. (Yay for math geeks!)

I’ll show you how to work it out. Now don’t get scared off by the math. Just think of it as a simple ratio. All we have to figure out is how many parts of water do we need for the amount of NaOH in our recipe to get a given percentage.

Here’s the formula:

(100 minus “percentage you want”) divided by “percentage you want” = the multiplication factor that you use to calculate the water

Huh? Yeah. Let’s plug in some numbers.

Let’s keep using our 25% solution example. And let’s say that I need 72g of NaOH for the recipe that I’m going to make (I used a lye calculator to figure that out for me.) I need to know how much water to use to make my solution 25%.

Here’s that formula with the numbers plugged in:

(100 minus 25) divided by 25 = 75 divided by 25 = 3.00  <– that is the ratio of NaOH to water

So now we multiply the amount of NaOH times the ratio:

72g X 3.00 = 216g <– the amount of water you need

Mix 72g of NaOH and 216g of water and you get 288g of a lye solution that is at 25% concentration.

Easy, right?

It works for whatever concentration you want. Just make sure your ratio answer has at least 2 decimal places to be more accurate.

Now, I’ll show you what happens if I changed my mind and I want my solution to be 34% concentration. It’s going to be more concentrated, so I’m going to need less water. How much less? Let’s find out:

(100 minus 34) divided by 34 = 66 divided by 34 = 1.94

So, 72g X 1.94 = 140 gr of water, which is what you need to make a 34% solution. Wow! That’s quite a bit less water!

Remember to use the higher concentration of NaOH only when you are comfortable with soapmaking, and are familiar with the formula that you will be using. You don’t want to do this with a brand new fragrance for example, or you may end up reading about soap that seizes!