Stearic Acid in soap: SAP value, lye amount & what it does
Stearic Acid has a SAP value of 0.143 g NaOH per gram, and it leans toward hardness in the bar. Here are the exact lye numbers and how it behaves.
SAP value & how much lye stearic Acid needs
The saponification (SAP) value of stearic Acid is 0.143 g NaOH per gram of oil (0.201 g KOH per gram for liquid soap). So 500 g of stearic Acid needs about 72 g of NaOH at 0% superfat, or roughly 68 g at a 5% superfat. Always confirm your full recipe in a lye calculator — SAP is an average and lye is unforgiving.
What it brings to the bar
At 100% (a single-oil bar), its fatty-acid profile works out to these SoapCalc-style qualities (typical range in brackets, per Kevin Dunn):
- Hardness: 97 [29–54] — high
- Cleansing: 0 [12–22] — low
- Conditioning: 1 [44–69] — low
- Bubbly lather: 0 [14–46] — low
- Creamy lather: 97 [16–48] — high
How to use it
Stearic Acid is very hard and long-lasting but can feel brittle or draggy alone; it's typically 10–30% of a recipe to firm up softer oils.
FAQ
What is the SAP value of stearic Acid?
Stearic Acid has a SAP value of 0.143 g NaOH per gram of oil (0.201 g KOH per gram). Multiply your oil weight by that to get the lye at 0% superfat.
How much lye do you need for stearic Acid?
About 72 g of NaOH for 500 g of stearic Acid at 0% superfat, or ~68 g at 5% superfat. Always verify the whole recipe in a lye calculator.
Is stearic Acid good for soap?
Yes — used correctly. Stearic Acid is very hard and long-lasting but can feel brittle or draggy alone; it's typically 10–30% of a recipe to firm up softer oils.
What you need to soap with it
Cold-process soap is lye chemistry — accuracy and protection matter more than the mold.
- Weigh oils and lye to the gram on a 0.1 g digital scale — soap is made by weight, never volume.
- Use fresh sodium hydroxide (NaOH) lye for hard bar soap; KOH is for liquid soap.
- Always wear splash goggles and nitrile gloves — lye is caustic until the soap fully saponifies.
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Sources
SAP values and fatty-acid profiles are typical published averages; every batch of oil varies slightly, so always run your final recipe through a lye calculator. Lye is caustic — soap at your own risk with proper protection.