Calculate the amount of water in a soap recipe

How do you calculate the amount of water in a natural soap recipe?

When determining the right amount of water for making handmade soap, you may encounter several different methods. This can be quite confusing for beginners, as it is easy to mix up the various approaches. To begin with, it is important to understand that soapmakers generally rely on three methods to determine the amount of water to use in a recipe.

Percentage of the total oil weight

This method involves calculating the weight of the water based on the total weight of the oils in the recipe. Typically, the amount of water used is between 30% and 38% of the total oil weight. It is a simple and highly practical method for those just starting out.

Lye solution concentration

This method defines the proportion of pure sodium hydroxide (caustic soda) relative to the total mixture of water and sodium hydroxide, a combination known as the "lye solution." Beginners generally work with concentrations between 28% and 33%, while more experienced soapmakers may go as high as 50%.

Water-to-lye ratio

This ratio is used to calculate the amount of water based on the quantity of pure sodium hydroxide. For example, using a 2:1 ratio means that the amount of water is twice the amount of sodium hydroxide. Therefore, if your recipe calls for 80 g of NaOH, you would add 160 g of water.

Two important concepts to understand

Caustic soda concentration: this refers to the percentage of pure caustic soda contained in the water-and-soda mixture, known as a lye solution.

Percentage of oils: this indicates the proportion of each oil used in the recipe.

These two concepts are distinct and should not be confused.

Determining the water content for your soap recipe

The water-to-lye ratio method

This method involves determining the amount of water based on the amount of lye.

If I decide to use a 2:1 ratio:

• 2 = water

• 1 = lye

This means there is twice as much water as there is lye.

Example

For 42 g of lye with a 2:1 ratio:

42 × 2 = 84 g

Therefore, you would need to use 84 g of water for 42 g of pure lye.

This corresponds to a lye concentration of approximately 33.3%.

A small table to help clarify

The Lye Concentration Method

This is the method I most recommend when you truly want to understand how to determine the amount of water for a recipe.

The formula is:

Water = NaOH × (1 − concentration) ÷ concentration

Example

Let’s take 60 g of lye (sodium hydroxide) at a concentration of 30%.

Water = 60 × (1 − 0.30) ÷ 0.30

Water = 140 g

Therefore, we need 140 g of water for 60 g of lye at a 30% concentration.

This corresponds to a ratio of approximately 2.33:1.

Which concentration should you choose?

There is no single concentration that is suitable for every recipe. Selecting a lower concentration means using more water, while a higher concentration means using less water.

For beginners, a lye concentration of around 28–30% is often easier to manage, as the soap batter remains more fluid, giving you more time to work with it.

Conversely, a concentration of 33% or 50% contains significantly less water and can cause the batter to thicken more quickly. Furthermore, a 50% concentration is very high and requires a great deal of skill, as it approaches the solubility limit of lye.

Key Takeaway

Lye concentration should not be confused with the percentage based on the weight of the oils in a recipe. These are two distinct methods for determining the amount of water in your soap. I have noticed that many beginners struggle to tell them apart. I invite you to learn more by checking out my kit, which includes an introductory guide to cold-process soap making.