Soapy Math Part 3: Calculating Your Water

Why bother doing soapy math by hand when we have the use of lye calculators to do the calculating for us? Well, because that lye calculator will only get you so far! There are a variety of math calculations you need to understand to formulate, create, and troubleshot recipes.

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0:00 Calculating Your Water
0:32 Water Solution Strength
4:30 Option 1 Formula
8:21 Option 2 Formula
10:00 Practice Problem

I've divided this lecture into a four part series. If you missed Part 1 & 2 be sure to go back and watch those first! The goal of these videos is to teach you how to calculate a recipe from scratch and in turn help you understand what each element of a lye calculator does. So, let's get started!

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PART 1: CALCULATING YOUR Water
Water is needed to dissolve the lye (sodium hydroxide) and for the hydrolysis of the fats. Hydrolysis is a reaction involving the breaking of a bond in a molecule using water. The reaction mainly occurs between an ion and water molecules and often changes the pH of a solution.

When making soap too much water will result in a soft bar of soap that will need extra drying time and could even encourage rancidity (depending on how much water and the oils you used). Too little water could result in an inability for your lye to completely dissolve (worst case) or significantly speed up your trace (giving you less time to make the soap).

As you learn and experiment you will figure out a water “amount” that you want to use. The more water used the more fluid the mixture will remain. Higher water discounts will trace much quicker. However they will also cure quicker. It’s really a matter of preference. There’s no right or wrong water amount unless you go below a 50-50 solution. Sodium hydroxide needs at least equal amounts of water to dissolve. A good starting point is a lye solution strength between 25 and 33%.

What does 25% solution strength mean? When a water strength is referred to in a percentage the percentage is representing the lye. The remainder is water. So, a 25% solution means that 25% of your lye solution is lye and the remaining 75% is water. You cannot go any higher than a 50% solutions strength if you want the sodium hydroxide to fully dissolve.

You’re going to work with a solution strength between 25% and 50%. If you’re unsure what strength to use or are new to soap making stick with a solution strength between 28% and 33% until you have a better understand of how water works in the soap making process.

To calculate your water discount use the following formula:

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

Let’s look at an example: Let’s make a 2 pound batch of soap that uses 100% avocado oil using a 30% solution strength.

Step 1
Avocado’s SAP value is 0.1337 so to saponify 32 ounces of avocado we need 4.28 ounces of lye (32 x 0.1337 = 4.2784).

Step 2
Next we take the amount of lye we need (4.28 oz) and divide it by the desired solution strength 33%. Then multiply that to the following formula: 100% - 33%.

(4.28 oz / .30) x (100% - 30%) = Water Amount

(14.26) x (70%) = 9.98 ounces

Modern Soapmaking has an amazing post on water discounts and lye solution. https://www.modernsoapmaking.com/lye-...

Alternative Water Calculation Method
You can reach the same results by completing the following math equations. If you haven’t already done so, first determine the total amount of lye you will use. Then divide that number by the lye solution strength you want to use and subtract the amount of lye from the result.

1. (Amount of Fat) × (Saponification Value of the Fat) = (Amount of Lye)
2. (Amount of Lye) ÷ (Lye Solution %) = (Total Weight of Lye Water Solution)
3. (Total Weight of Lye Water Solution) − (Amount of Lye) = (Amount of Water)

Let’s look at an example: Let’s make a 2 pound batch of soap that uses 100% avocado oil with a 30% Lye Solution Strength.

Step 1
Avocado’s SAP value is 0.1337 so to saponify 32 ounces of avocado we need 4.28 ounces of lye (32 x 0.1337 = 4.2784).

Step 2
Next we take the amount of lye we need (4.28 oz) and divide it by 0.3 (our lye solution strength). That gives us: 14.26 ounces.

Step 3
We take the total amount of the weight of the water and lye solution (14.26) and subtract the amount of lye (4.28) and the amount of water we need to dissolve the lye is: 9.98 ounces

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