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How to Better Understand Water Discounts When You Make Soap

soap science and info
pouring lye into soaping oils

One of the most common issues I run into with soapmakers is not understanding how the lye solution in soapmaking works. For instance, a soapmaker might take one of our recipes and incorrectly use the "water as a percentage of oils" option instead of the "lye concentration" option and think there is something wrong with the formula. Or misunderstand how to partially replace the water in a recipe, like the Lemon Zest & Blueberry Yogurt Recipe.

Even bigger issues crop up when a soapmaker tries to scale up and doesn't understand the lye solution when trying to masterbatch or when they reach out for recipe help because the recipe is soft, traces too quickly, or takes a long time to cure!

Many soap calculators compound the issues by presenting the default options of calculating the water as a percentage of the oils, so newer soapmakers are missing out on this vital information!

 Understanding Your Lye Solution in Soapmaking!

Let's talk about the elephant in the room first: what's wrong with calculating your water amount based on the oils?

Simply put, you end up with a large variety of solution strengths! Different oils need varying amount of lye of to saponify, but the amount of water won't change based on the lye. Since the amount of water present helps determine both your speed of trace and your cure time, this can create inconsistent results from batch to batch.

For instance, if calculating your water amount based on a percentage of the oils, these two formulas are wildly different:

100% Olive Oil Soap

  • 16 ounces of Olive Oil
  • 2.06 ounces of Lye
  • 38% of oils: 6.08 ounces of water
  • results in a 25.3% lye solution

100% Coconut Oil Soap

  • 16 ounces of Coconut Oil
  • 2.79 ounces of Lye
  • 38% of oils: 6.08 ounces of water
  • results in a 31.42% lye solution

The olive oil soap will trace slower and cure slower than the coconut oil soap, due to the varying lye solution strength from batch to batch, even if you made the soaps on the same day! However, the coconut oil soap will saponify even faster and enter gel phase at a much higher temperature (though the gel phase will be shorter duration).

The bigger problem with this is that olive oil saponifies slower than coconut oil anyways! It would be better to have a stronger lye solution and less water for the olive oil soap (so that it doesn't take a year to cure and an hour to trace). Since coconut oil saponifies more quickly on its own, it would be more beneficial to have a weaker lye solution and more water so the lack of water isn't compounding the issue.

If this is news to you, you might want to read up on controlling trace and the various factors that affect trace!

My biggest concern with calculating your water as a percentage of oils is that you might accidentally discount your water too much! If you aren't keeping your lye in mind by calculating your water based on your lye, you could do something like this:

100% Olive Oil Soap

  • 16 ounces of Olive Oil
  • 2.06 ounces of Lye
  • 15% of oils: 2.40 ounces of water
  • results in a 46.19% lye solution

100% Coconut Oil Soap

  • 16 ounces of Coconut Oil
  • 2.79 ounces of Lye
  • 15% of oils: 2.40 ounces of water
  • results in a 53.72% lye solution

The coconut oil soap's lye solution is too concentrated! Lye cannot dissolve into a solution with less liquid than its own weight. If you were to make this soap, it would be lye heavy and unevenly saponified, with free particles of lye in the bar.

In order to master water discounts and water replacements or alternative liquids, you must be able to understand your water in relation to the lye instead. The easiest way to do this is to look at your lye solution as a whole ingredient and calculate your water as a ratio of the lye.

The Most Common Lye Solution Strengths in Soapmaking

Most early soapmaking books and recipes use a “full water” amount, which is a misnomer as there is not a maximum amount of water you can add to a soaping formula. (Yes, you could use more!)

When most recipes refer to full water, they are usually calculating a lye solution strength between 25% and 28%, which means that 25% to 28% of the solution is lye and the remainder (72% to 75%) is water.

 A 25% lye solution is made of 25% lye and 75% water.

As we know, the higher the amount of water, the slower the trace (but also the longer the temperature phase during saponification). Using "full water" will give you the maximum amount of time at a workable consistency, but will also ensure a fully gelled soap in most cases (which may not be ideal!) It will also take the longest for a soap with a 25% lye solution to cure because there is a lot of water to evaporate.

The next most common lye solution strength is a 33% lye solution, which is ideal for a slightly faster curing time (less water to evaporate), reduced length of heat phases during saponification, and slightly harder bar straight out of the mold.

 A 33% lye solution is made of 33% lye and 67% water.

Most recipes and tutorials here on Modern Soapmaking use a 33% lye solution, as it's typically a manageable water discount during the soapmaking process, but also hardens up a little quicker to get the soap out of the mold!

A strong water discount is typically referred to as a 40% lye solution. Reducing the water amount in your lye solution will help the soap get out of the mold quicker and cure faster. Compared to a soap made with a 25% lye solution, soap batches made with a 40% lye solution can cure in about half the time.

A 40% lye solution is made of 40% lye and 60% water.

Keep in mind that a soap made with a stronger lye solution like a 40% solution will trace more quickly!

The least amount of water you can use to fully dissolve lye (at optimal temperatures and conditions) is a 50% solution. Remember, this means that 50% of the solution is lye and the other 50% is water. This is considered the strongest lye solution strength and the highest water discount possible.

A 50% lye solution is made of 50% lye and 50% water.

It’s important to remember that your lye solution can be a maximum strength of 50% lye and 50% water in ideal conditions. However, if your 50% lye solution is used or stored in an environment that is too cold (less than 25° C or 77° F), the lye can precipitate out of the solution.

Using a lye solution where the lye is no longer dispersed in the liquid or water can cause uneven saponification and lye pockets. As such, I tend recommend using a weaker solution strength (40% lye solution) as the maximum water discount in most cases.

However, a 50% lye solution does become quite handy when you want to use alternative liquids. For instance, you can create a 50% lye solution with water and lye, and then add an additional liquid (such as goat's milk) to the lye solution or oils. This will increase the water amount (slowing down trace), add additional properties, and reduce any issues with creating a lye solution with alternative liquids (freezing milk or scorching).

How to Calculate Your Water Amount for Your Lye Solution

When you calculate your formula, you usually choose your oils first, which then dictates your lye amount depending on each oils’ weight and saponification value. Then, you can determine your water amount by choosing a solution strength. A quick and easy way to do this is to multiply your lye amount by a corresponding multiplier.

Let's say you would like to create a 50% lye solution and your oils dictate that you need 3.5 ounces of lye. You would multiply 3.5 ounces x 1 = 3.5 ounces of water. If you would like to create a 33% solution instead, you would multiply 3.5 ounces x 2 = 7 ounces of water.

If you aren't sure what the multiplier is and don't have the chart below handy, you can find out by doing a little math! Let's say that you want a 25% lye solution, so you would:

(100% - Desired Solution Strength) / Desired Solution Strength = Multiplier

(100% - 25%) / 25% = 75 / 25 = 3

If you know you need 2 ounces of lye (because your lye is determined by your oils), and want to use a 25% lye solution, you would simply multiply your lye amount by 3 to find your water amount.

2 ounces of lye x 3 = 6 ounces of water

The table below covers the full range of multipliers from a 25% solution to a 50% solution, and highlights the most commonly used concentrations:

Lye Amount Water Multiplier Lye Solution Notes
Weight of Lye x 1  = Water Amount 50% lye solution maximum water discount
Weight of Lye x 1.1  = Water Amount 47.6% lye solution  
Weight of Lye x 1.2  = Water Amount 45.5% lye solution  
Weight of Lye x 1.3  = Water Amount 43.5% lye solution  
Weight of Lye x 1.4  = Water Amount 41.7% lye solution  
Weight of Lye x 1.5  = Water Amount 40% lye solution strong water discount
Weight of Lye x 1.6  = Water Amount 38.5% lye solution  
Weight of Lye x 1.7  = Water Amount 37% lye solution  
Weight of Lye x 1.8  = Water Amount 35.7% lye solution  
Weight of Lye x 1.9  = Water Amount 34.5% lye solution  
Weight of Lye x 2  = Water Amount 33.3% lye solution moderate water discount
Weight of Lye x 2.1  = Water Amount 32.3% lye solution  
Weight of Lye x 2.2  = Water Amount 31.3% lye solution  
Weight of Lye x 2.3  = Water Amount 30.3% lye solution  
Weight of Lye x 2.4  = Water Amount 29.4% lye solution  
Weight of Lye x 2.5  = Water Amount 28.6% lye solution mild water discount
Weight of Lye x 2.6  = Water Amount 27.8% lye solution  
Weight of Lye x 2.7  = Water Amount 27% lye solution  
Weight of Lye x 2.8  = Water Amount 26.3% lye solution  
Weight of Lye x 2.9  = Water Amount 25.6% lye solution  
Weight of Lye x 3  = Water Amount 25% lye solution no water discount


Okay, what about if I want to just figure out my water amount without all this multiplier nonsense? Can do! Take your amount of lye and your desired solution strength and plug it into this calculation:

Amount of Lye / Desired Solution Strength x (100% - Desired Solution Strength) = Water Amount

Let's say that you need 3.51 ounces of lye and want to use a 40% lye solution:

3.51 ounces of Lye / 40% Lye Solution x (100% - 40% Lye Solution) = Water Amount

3.51 / 40 x 60 = 5.265 ounces of Water

It doesn't matter which way you decide to conquer the beast, as long as you are calculating your water amount based on your lye!

Why It's Important to Understand Your Lye Solution in Soapmaking

Besides being able to control trace and curing time, understanding your lye solution in soapmaking will open the doors to being able to replace some or all of your water in a formula without concerns. You'll be able to use multiple liquids or do a partial water replacement, without having to worry about scorching lye solutions or using too little or too much water.

For instance, in the Avocado & Argan Oil Soap Recipe, I used a 33% lye solution for the total formula. However, I substituted part of the water for pureed avocado.

Here's the formula and the lye amount dictated by my lye calculator:

  • 8 ounces Avocado Oil
  • 8 ounces Babassu Oil
  • 7 ounces Olive Oil
  • 6 ounces Coconut Oil
  • 6 ounces Argan Oil
  • 3 ounces Castor Oil
  • 5.3 ounces Sodium Hydroxide

To find how much liquid I need for a 33% lye solution, I multiplied the amount of the lye by 2. I knew that the lye needed at least its own weight to dissolve in water (5.3 ounces). As such, I was free to use up to 5.3 ounces in avocado puree. My avocado was small, weighing in at 4 ounces, so I used a bit more water instead!

When you understand your lye solution in soapmaking, you'll also be able to scale up your production easier with masterbatching. For instance, you could make a large amount of 33% lye solution to use in all of your recipes. (Just be sure to store your lye solution in safe, appropriate containers!) If you calculated your water as a percentage of oils, you wouldn't be able to use one lye solution in different formulas!

To dive into really understanding your lye solution strengths, give Auntie Clara's ghost swirl a try! It will show you exactly how varying lye solution strengths affect your soap during the production process, saponification, and cure.

When it comes to my own soapmaking, I prefer 33% lye solutions in smaller batches (five pounds or less) and 40% lye solutions in larger batches (more than five pounds). Do you have a favorite lye solution strength in your soapmaking? If so, leave a comment below and tell me why! I think it'll be interesting to see what other soapmakers use as their go-to!

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