What is superfat (lye discount) in soap?
Superfat is the share of the oils you deliberately leave unsaponified, by using less lye than would turn every gram of oil into soap. A 5% superfat (or 5% lye discount) means 5% less lye than full saponification. Around 5% is a common starting point for bar soap; very cleansing recipes such as 100% coconut oil are often made with much more.
Superfat and lye discount are the same number
Calculators work out the lye for full saponification and then multiply it by (1 − superfat). A 5% superfat is the same as a 5% lye discount. The oil that's left over stays in the bar.
Why makers leave some oil unsaponified
- A margin for real oils. SAP values are averages; a given lot of oil can need a little more or less lye than the published figure. The Soap Ledger oil reference (sources for every SAP value) list the published values behind each number.
- A margin for weighing. A small superfat absorbs small scale errors. It is not a substitute for weighing carefully.
- Feel. Many makers use about 5%, and a higher superfat makes a softer bar (The Soapery: What is superfat in soap making?).
Worked example: the same recipe at three superfats
1000 g of oils (Olive oil 50%, Coconut oil 25%, Shea butter 20%, Castor oil 5%), 100% NaOH, 33% lye concentration:
| Superfat | NaOH to weigh | Water |
|---|---|---|
| 0% | 145.8 g | 296.1 g |
| 5% | 138.5 g | 281.3 g |
| 8% | 134.2 g | 272.4 g |
Coconut oil: a common exception
A soap made only from coconut oil is very cleansing, so makers often superfat it at about 20–25% (The Soapery: What is superfat in soap making?). For 1000 g of Coconut oil: 173.7 g of NaOH at 5%, or 146.3 g at 20%.
Things to know
- Superfat is worked out on the oils in the pot. Oils or butters added after the lye has done its work (in hot process, for example) are a separate choice.
- A higher superfat leaves more free oil, which can make a bar softer and less bubbly. Small test batches are the safest way to find what you like.
- Superfat doesn't fix a wrong SAP value or the wrong lye. Check the oil, the lye type and the purity first.
Lye safety. Verify every weight before you make this, and follow lye safety. Not a safety or compliance certification. Eye protection, gloves and covered skin; ventilation and a clear, uninterrupted workspace.
Try it in the lye calculator. Change the superfat and see the lye update.
Sources
- The Soapery: What is superfat in soap making?
- Soap Ledger oil reference (sources for every SAP value)
Written for Soap Ledger in our own words. Numbers in the examples are calculated by Soap Ledger's engine when the page is built. Last updated .