# Superfat and Lye Discount, Explained With a Worked Example

URL: https://rasatheory.in/blog/superfat-and-lye-discount-explained/
Published: 2026-07-29
Updated: 2026-10-05

Answer: Superfat and lye discount are the same adjustment from opposite ends: a 5% superfat and a 5% lye discount both mean using about 95% of the stoichiometric lye. The deliberate shortfall is what guarantees no free alkali remains in the bar.

**Superfat** is the percentage of oil deliberately left unsaponified, and **lye discount** is the percentage by which the lye is cut below what the oils could consume. They are the same adjustment seen from either end. The Handcrafted Soap & Cosmetic Guild's [Kevin Dunn](https://www.soapguild.org/tools-and-resources/resource-center/166/superfatting-lye-discount/) found little difference between the two in practice.

## What is superfat?

Superfat is the share of oil in a recipe that is deliberately left unreacted. As [Dunn explains](https://www.soapguild.org/tools-and-resources/resource-center/166/superfatting-lye-discount/), a soapmaker superfatting by 5% adds 5% more oil than was used to calculate the lye. That extra oil is never turned into soap and stays in the bar.

## What is lye discount, and is it the same as superfat?

Lye discount is using less lye than the full calculation says. Dunn's example: if 100 oz of lye are indicated and 95 oz are used, the lye was [discounted by 5%](https://www.soapguild.org/tools-and-resources/resource-center/166/superfatting-lye-discount/). The two terms differ in wording and slightly in arithmetic, but the finished soap is effectively the same. The reaction behind both is covered in [what saponification is](/blog/what-is-saponification/).

## Why would anyone use less lye than the recipe needs?

Soapmakers use less lye because the saponification value of an oil is only an average. Dunn measured a palm oil at 196 mg KOH per gram, [about 3% lower](https://www.soapguild.org/tools-and-resources/resource-center/166/superfatting-lye-discount/) than the supplier's 203, and notes it could fall anywhere between 190 and 209. A margin absorbs that spread. Without it, a measurement slip lands on the harsh side.

With the margin in place, the alkalinity of the finished bar comes from the soap itself, not leftover lye, as [why real soap is alkaline](/blog/why-real-soap-is-alkaline/) explains.

## What is a saponification value?

A saponification value, or SAP, is the amount of alkali needed to turn one gram of a given oil completely into soap. Dunn expresses it as [milligrams of potassium hydroxide per gram of oil](https://www.soapguild.org/tools-and-resources/resource-center/166/superfatting-lye-discount/). [SoapCalc](https://soapcalc.net/oil-list) lists both forms: olive oil is 0.190 for KOH and 0.135 for NaOH. Bar soap uses the NaOH column.

## How do you calculate superfat? A worked example

To calculate superfat, multiply each oil's weight by its saponification value, add the results, then multiply the total by one minus the superfat percentage. Water comes last. The values below are [SoapCalc's NaOH figures](https://soapcalc.net/oil-list), applied to a 1000 g batch of oil.

| Oil | Amount | SAP (NaOH) | Lye required |
|---|---|---|---|
| Olive oil | 500 g | 0.135 | 67.5 g |
| Coconut oil | 350 g | 0.183 | 64.1 g |
| Castor oil | 150 g | 0.128 | 19.2 g |
| **Total** | **1000 g** | | **150.8 g** |

That 150.8 g is the full saponification value. With a 5% superfat:

```
150.8 g × (1 − 0.05) = 143.3 g NaOH
```

The 7.5 g shortfall means roughly 5% of the oil is never reacted. For water, two times the lye weight gives 286.6 g, a one-third lye solution inside the 25% to 38% [range Dunn found in published recipes](https://www.soapguild.org/tools-and-resources/resource-center/164/the-water-discount/). The batch is 1000 g oil, 143.3 g lye and 286.6 g water.

Run every recipe through a soap calculator before you make it, including this one. Saponification values differ slightly between references, suppliers swap fats, and arithmetic done late at night is not to be trusted. A calculator catches the error that produces a caustic bar.

## What are the steps for your own recipe?

Working out the lye for your own recipe takes five steps: weigh the oils, multiply each by its saponification value, add the results, apply the superfat, then weigh the water. A soap calculator performs the same steps and should check them every time, because a small arithmetic slip is how caustic bars happen.

1. Weigh each oil in grams.
2. Multiply each weight by that oil's NaOH saponification value.
3. Add the results to get the full lye amount.
4. Multiply that total by one minus the superfat, so 5% becomes 0.95.
5. Weigh water at about twice the lye weight and dissolve the lye in it.

## What superfat percentage should you use?

No source we found measured a best superfat percentage, so none can be called correct. The Guild's own worked example uses 5%, industrial toilet soap research uses far less, and one calculator suggests more for coconut-heavy bars. The table shows where each number appears, and why.

| Superfat | Where it appears | Source |
|---|---|---|
| 1 to 2% | Hard industrial toilet soap, to stop cracking | [Kuntom et al., 1999](https://doi.org/10.1007/s11743-999-0084-5) |
| 5% | The example discount in the Guild's own explanation | [Dunn](https://www.soapguild.org/tools-and-resources/resource-center/166/superfatting-lye-discount/) |
| 8 to 10% | Suggested when coconut oil runs above the usual cap | [SoapCalc](http://www.soapcalc.net/info/soapqualities.asp) |

## Can you superfat with one specific oil?

You cannot reliably choose which oil stays unreacted. Dunn tested the idea: soaps made by discounting the lye, with every oil in at the start, and soaps made by holding one oil back until trace gave [virtually identical recovered oils](https://www.soapguild.org/tools-and-resources/resource-center/166/superfatting-lye-discount/). He calls the results preliminary, from three oil blends. Mix all the oils first.

## Which oils end up as the unsaponified part?

The oils that react slowest end up as the unsaponified part. In Dunn's coconut and olive blend, oleic acid was about 7% of the oil but 22% of the oil recovered from the soap. A 2018 study likewise found [86 to 99% of polyunsaturated fatty acids](https://pmc.ncbi.nlm.nih.gov/articles/PMC6225244/) left unsaponified. The superfat leans toward unsaturated oil.

## Does superfat change lather and hardness?

Superfat makes a hard bar less brittle and, in one industrial test, left lather mostly intact. [Kuntom and colleagues](https://doi.org/10.1007/s11743-999-0084-5) added neutral oils or glycerin to a hard, crack-prone toilet soap to increase plasticity, and reported good lather except with palm kernel olein and canola oil. The study used industrial toilet soap, not handmade bars.

## Does a higher superfat make soap more moisturising?

A higher superfat makes soap feel richer, but not without limit. [Wikipedia](https://en.wikipedia.org/wiki/Soap) calls superfatted soap more skin-friendly than soap without extra fat, though it can leave a greasy feel. We found no measurement of a best percentage for skin. A bar that feels tight is better checked for [cure and oil blend](/blog/how-long-soap-needs-to-cure/) first.

## Does superfat make soap go rancid?

Superfat can go rancid, because unreacted oil can oxidise. [Classic Bells' rancidity guide](https://classicbells.com/soap/rancidity.asp) says fatty acids and superfat oxidise from heat, light, water, bacteria and contaminants, forming smelly ketones and aldehydes. The 2018 study confirms unsaturated acids oxidise during [saponification, curing and storage](https://pmc.ncbi.nlm.nih.gov/articles/PMC6225244/).

The same guide says polyunsaturated fats are more prone to oxidise and offers a rule of thumb to keep linoleic plus linolenic acid at or below 15%, while admitting it has no strong scientific basis. It advises cool, dark storage. In a warm climate, the [soap-care habits](/blog/how-to-make-soap-last-longer/) that keep a bar dry also help.

## Frequently asked questions

### What happens if a soap has no superfat at all?

It has no safety margin. The amount of lye a given oil needs varies from batch to batch, so a recipe at exactly 100% can end up with a little alkali left over. Dunn warns that excess alkali makes a bar harsh and perhaps even dangerous, which is the reason makers discount the lye.

### Do factory soaps use superfat?

Some do. The Indian toilet soap standard IS 2888:2004 deleted its unsaponified matter requirement so that superfatted soaps would be covered, and a 1999 industry study superfatted toilet soaps with 1 to 6% neutral oils or glycerin to stop cracking.

### What is a water discount, and is it the same thing?

No. Water takes no part in the saponification reaction, so there is no single correct amount to discount from. Dunn found published recipes using lye solutions anywhere from 25% to 38%. Using less water mostly shortens the cure.

### Does a bigger lye discount make soap safer?

It makes the bar less likely to contain free alkali, but not better in every way. More superfat means more unsaponified oil, which is the part that can oxidise and turn rancid. The goal is enough margin to cover the uncertainty, not as much as possible.


## Sources

- Dunn, Superfatting and the Lye Discount, Handcrafted Soap & Cosmetic Guild: https://www.soapguild.org/tools-and-resources/resource-center/166/superfatting-lye-discount/ (supports: The two practices defined; a measured palm oil SAP value about 3% below the supplier figure; oils recovered from discounted and superfatted soaps were virtually identical.)
- Dunn, The Water Discount, Handcrafted Soap & Cosmetic Guild: https://www.soapguild.org/tools-and-resources/resource-center/164/the-water-discount/ (supports: Published recipes use lye solutions of 25% to 38%; water does not take part in the reaction and has no single correct amount.)
- Vidal et al., The Effects of Cold Saponification on the Unsaponified Fatty Acid Composition and Sensory Perception of Commercial Natural Herbal Soaps, Molecules, 2018: https://pmc.ncbi.nlm.nih.gov/articles/PMC6225244/ (supports: 86 to 99% of polyunsaturated fatty acids stayed unsaponified in cold process soaps; unsaturated fatty acids oxidise during saponification, curing and storage.)
- Kuntom et al., Effects of superfatting agents on cracking phenomena in toilet soap, Journal of Surfactants and Detergents, 1999: https://doi.org/10.1007/s11743-999-0084-5 (supports: Superfatting hard toilet soaps with 1 to 2% neutral oils or glycerin gave crack-free bars (abstract checked via Crossref).)
- IS 2888:2004 Toilet Soap: Specification, Bureau of Indian Standards (public copy on Internet Archive): https://archive.org/details/gov.in.is.2888.2004 (supports: The standard's foreword says the unsaponified matter requirement was deleted to cover superfatted soaps.)
- SoapCalc oil list: SAP values: https://soapcalc.net/oil-list (supports: The saponification values used in the worked calculation.)
- SoapCalc: soap qualities chart: http://www.soapcalc.net/info/soapqualities.asp (supports: Suggested 8 to 10% superfat when coconut oil is above the usual cap.)
- Classic Bells: Rancidity and DOS: https://classicbells.com/soap/rancidity.asp (supports: Superfat and fatty acids oxidise from heat, light, water and contaminants; polyunsaturated fats are more prone; the 15% linoleic plus linolenic rule of thumb has no strong scientific basis.)
- Wikipedia: Soap: https://en.wikipedia.org/wiki/Soap (supports: Superfatted soap is described as more skin-friendly than soap with no extra fat, though it can feel greasy.)