Calculate the amount of water in a soap recipe

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

When trying to determine the right amount of water for making soap, you often encounter several different methods. This can be confusing for beginners, as it is easy to mix up the various approaches. It is important to understand that soap makers generally rely on three main methods to determine the water content of a recipe.

Percentage of oil weight

This method involves calculating the water weight 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 practical method for beginners.

Lye solution concentration

This method involves defining the proportion of pure sodium hydroxide (caustic soda) relative to the water-and-soda mixture—known as the lye solution. Beginners generally work with concentrations ranging from 28% to 33%.

The water-to-lye ratio

This method involves calculating the amount of water based on the quantity of pure lye. For example, a 2:1 ratio is used, meaning twice as much water as lye is employed.

Two important concepts to understand

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

Oil percentage: this indicates the proportion of each oil in the recipe.

These two concepts are different 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 caustic soda.

If I decide to use a 2:1 ratio:

• 2 = water
• 1 = caustic soda

This means there is twice as much water as caustic soda.

Example

For 42 g of caustic soda with a 2:1 ratio:

42 × 2 = 84 g

You would therefore need to use 84 g of water for 42 g of pure caustic soda.

This corresponds to a caustic soda concentration of approximately 33.3%.

A quick chart to help clarify things

The lye concentration method

This is the method I most recommend when you really 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 at a concentration of 30%.

Water = 60 × (1 − 0.30) ÷ 0.30

Water = 140 g

We therefore need 140 g of water for 60 g of lye at a concentration of 30%.

This corresponds to a ratio of approximately 2.33:1.

Which concentration should you choose?

There is no single concentration that suits every recipe.

A lower concentration generally means more water, whereas a higher concentration means less water.

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

Conversely, a concentration of 33% or 50% contains significantly less water and can result in a batter that thickens more quickly.

A 50% concentration is particularly high and requires greater skill, as it approaches the solubility limit of the lye.

Key takeaway

Lye concentration should not be confused with the percentage of the recipe's total oil weight.

These are two different methods for determining the amount of water in your soap.

I invite you to learn more with my "Cold Process Soap Making Starter Guide" kit.