Soap properties: what soapmakers expect vs what soap calculators actually calculate

Soap calculators display familiar property values such as hardness, cleansing, and conditioning. These numbers are widely used when formulating recipes, but they are often misunderstood.

In practice, there is a clear difference between how soapmakers interpret these properties and how soap calculators actually calculate them - and this article explains why that difference exists.

Key takeaway: Soap property values in calculators are based only on fatty acid percentages - not on how the soap actually behaves in use. This explains why they often differ from real-world experience.

Soap properties: what soapmakers think vs what soap calculators calculate

Soap property values in soap calculators are numerical values derived from the soap's fatty acid profile.
They can serve as rough indicators of how a soap might behave, but they do not tell the full story of how the soap actually performs in use.

Let's look at a few examples.

Conditioning

For soapmakers, "Conditioning" refers to how soothing, moisturizing, and gentle the soap feels on the skin.
For soap calculators, "Conditioning" is simply the sum of the percentages of Oleic, Ricinoleic, Linoleic, and Linolenic acids in the recipe.

Conditioning and superfat

In practice, soapmakers know that a higher superfat generally results in a more "conditioning" soap, gentler to the skin.
However, from the perspective of a soap calculator, two soaps made from the same recipe but with different superfat levels will have exactly the same "Conditioning" value.

Hardness

Soapmakers think of "Hardness" as the ability of a soap to remain firm and not dissolve quickly in use.
For soap calculators, "Hardness" is simply the sum of the percentages of Lauric, Myristic, Palmitic, and Stearic acids in the recipe.

Hardness and additives

It is well known that certain additives, such as sodium lactate or salt, can significantly increase the hardness of a soap, making it easier to unmold and longer-lasting.
Soap calculators do not take this into account. For two soaps using the same recipe - one with and one without sodium lactate - the calculated "Hardness" value will be exactly the same.

Other soap properties

The same pattern applies to other properties. Cleansing, bubbly lather, creamy lather, and longevity are all calculated as sums of specific fatty acids.
Any factor that does not alter the relative percentages of these fatty acids - such as superfat, water amount, or additives - will leave the calculated property values unchanged.

Soap property values are just conventions

It is important to understand that these values - and even the property names themselves - were defined somewhat arbitrarily in the early days of soap calculators.

As more soap calculators were developed, they adopted the same conventions for naming and calculation. This created consistency, allowing users to compare results across different calculators.

However, this consistency also has a downside: these simplified definitions and naming conventions continue to be used, which can lead to confusion - especially for newer soapmakers who may assume that these values directly reflect real-world soap behavior.

Conclusion

Soap property values can be useful as general guidelines, especially when comparing different recipes. However, it is important to remember what they represent - and what they do not.

These values are based entirely on fatty acid composition and ignore other important factors such as superfat, additives, water content, and curing conditions. As a result, they cannot fully describe how a soap will behave in real use.

Understanding this limitation helps you interpret calculator results more accurately and rely more on experience, testing, and formulation intent when designing your soaps.