Ingredients & Recipes
What Is Superfat in Soap Making?
Superfat is the percentage of oils left unsaponified in your soap for a gentler, more moisturizing bar. Learn how to set it and why it matters.
Superfat is the percentage of oils in your soap that never react with lye. These free, unsaponified oils stay in the finished bar and make it feel gentler on skin. A standard cold-process soap runs at 5% superfat, meaning 5% of the total oil weight is left over after saponification is complete.
That number sounds small. But it makes a meaningful difference in how the bar feels, how long it lasts, and whether it irritates sensitive skin.
At a Glance
- Superfat is a percentage setting in your lye calculator, not an ingredient you add; it works by using slightly less lye than full saponification requires.
- 5% is the standard default for body soap, facial bars run 6-8%, shampoo bars run 0-3%, and high-coconut or laundry bars need 15-20% to avoid being harsh.
- Superfat and "lye discount" describe the same number from opposite directions and always produce an identical bar.
- Higher superfat softens the bar, shortens shelf life, and raises the risk of DOS (dreaded orange spots), a rancidity mark that shows up weeks or months later.
- Free oils are more exposed to light and oxygen than saponified oils, so curing and storing bars in a cool, dark spot measurably slows rancidity.
- Never eyeball superfat. Run every recipe through a lye calculator, and never add extra oil to a batch after you've already weighed your lye.
What Superfatting Actually Means
Lye turns oils into soap through a chemical reaction called saponification. Every oil has a saponification value (SAP value), the precise amount of lye needed to fully convert it into soap. A lye calculator uses those values to figure out exactly how much lye your recipe needs.
Superfat is the intentional gap you leave between the lye you use and the lye you theoretically could use. If your oils require 100 grams of lye for full saponification and you add only 95 grams, you have a 5% superfat. That 5% of oils stays unreacted, floating free in the bar.
Those free oils do two things: they add a light moisturizing quality to the lather, and they act as a small buffer against any slight measurement error. If your scale is off by a fraction of a gram, superfat gives you room without ending up with lye-heavy soap. This buffer is also part of why a properly superfatted bar qualifies as true soap under FDA rules: the bulk of the nonvolatile matter is still an alkali salt of fatty acids, the small unreacted fraction doesn't change that classification.
What "Free Oils" Feel Like
Unreacted oils in the bar are not the same as rubbing straight oil onto your skin. At 5% superfat, the bar still lathers, rinses clean, and feels like soap. But it tends to feel softer and less stripping than a soap made at 0% superfat. For everyday body soap, that's exactly what you want.
What Superfat Is Not
Superfat is sometimes confused with the oil blend itself, or with adding luxury oils at trace. These are related but separate ideas. Superfat is a percentage setting, not an ingredient. It applies uniformly across your entire oil blend; you cannot reliably control which specific oil stays unsaponified, because saponification runs somewhat randomly across the mix.
Why Superfat Percentage Matters
Getting the number right for your intended bar type matters more than most beginners expect. Superfat affects lather quality, bar hardness, shelf life, and how the soap feels on skin.
Common Superfat Ranges by Bar Type
| Bar Type | Suggested Superfat |
|---|---|
| Standard body soap | 5% |
| Facial bar | 6-8% |
| Shampoo bar | 0-3% |
| High-coconut / laundry bar | 15-20% |
| Baby / sensitive skin bar | 5-7% |
| Shaving soap | 3-5% |
The high-coconut and laundry entries stand out. Coconut oil is a cleansing powerhouse, but it can strip skin if the superfat is too low. Recipes built on 80-100% coconut oil typically run at 15-20% superfat to counteract that drying effect. Shampoo bars use low superfat to avoid weighing hair down or leaving residue.
What Happens If Superfat Is Too High
Increasing superfat softens the bar and shortens its shelf life. Free oils are more vulnerable to oxidation than saponified oils, so a high-superfat soap goes rancid faster. The visual symptom is DOS, dreaded orange spots that appear on the cut surface weeks or months after pouring. DOS is more likely with oils that already have a short shelf life (sunflower, grapeseed) and with superfat settings above 8-10%.
The mechanism isn't just heat exposure, light is the bigger driver. Research on household oil storage found that soybean oil kept in the dark for eight weeks showed a 124% rise in peroxide value (a standard oxidation marker), while identical oil exposed to ordinary fluorescent light over the same period showed a 1,473% rise, roughly twelve times worse. Temperature swings within a normal household range had far less effect than light did. That's a strong practical argument for curing and storing soap somewhere dim rather than on a sunny windowsill, regardless of superfat level, and it matters even more the higher your superfat is set.
A very high superfat can also make the bar feel greasy or slow to lather. For most recipes, staying at 5-8% hits the sweet spot between skin feel and a firm, stable bar.
The Difference Between Superfat and Lye Discount
You will hear both terms used in soap-making communities, sometimes interchangeably. They mean the same thing calculated from different directions.
Superfat counts upward from the oil weight: 5% superfat means 5% of oils are left unreacted.
Lye discount counts downward from the full lye requirement: a 5% lye discount means you use 5% less lye than the theoretical maximum.
The math produces identical results. A 5% superfat and a 5% lye discount give you the same bar. The difference is framing. Most lye calculators let you enter either one and will do the conversion automatically.
Some older recipes written in grams specify a lye amount directly, with no explicit superfat label. You can reverse-engineer the superfat by running those numbers through a lye calculator and checking what percentage it works out to.
How to Set Superfat in a Lye Calculator
Every reputable lye calculator (SoapCalc, Brambleberry's calculator, or the one at Soap Barn) has a superfat field, usually labeled "superfat %" or "lye discount %." The process is straightforward:
- Enter your oil weights in grams (or ounces).
- Set your superfat percentage. For a first recipe, 5% is a safe, proven default.
- The calculator outputs the exact lye weight and water weight to use.
- Measure those amounts precisely, digital scales accurate to 1 gram minimum.
Do not adjust superfat by eye or estimate it in your head. The lye calculator exists to handle these numbers so you do not have to, and the math is unforgiving if you guess wrong in the lye-heavy direction. The Handcrafted Soap and Cosmetic Guild, the main trade group for artisan soap makers, treats correct lye calculation as foundational technique for exactly this reason: an error toward too little lye is a spoiled batch, but an error toward too much lye is a genuinely caustic, unsafe bar.
If you are building your first recipe from scratch, the guide on the best oils for soap making and what each one does covers how different oils behave before you combine them, which helps you pick a blend worth superfatting thoughtfully. And once you have your percentages in mind, a simple beginner cold-process soap recipe walks through the full pour with real measurements.
Does the Type of Oil Affect Superfat?
Technically, yes, though the effect is subtle at 5%. Some soapers add what they call a "luxury oil at trace", a small amount of expensive oil (rosehip, tamanu, sea buckthorn) stirred in after the main oils and lye have emulsified. The idea is that this oil, added late, has less opportunity to saponify and more of it stays free in the bar.
Whether this reliably preserves the luxury oil's properties is debated. What is clear is that these additions are separate from your superfat setting and do not change the lye calculation you already ran. Never add extra oil without recalculating your lye amount first.
Water Ratio and Its Relationship to Superfat
Water does not directly affect superfat percentage, but it influences how quickly the bar hardens and how much shrinkage you see during cure. High-water recipes (38-42% of oil weight) give you more working time at trace but take longer to unmold. Lower water (28-33%) produces a firmer bar sooner.
If you are still sorting out your water ratio, how much water to use in cold-process soap goes into the specifics. The short version: most beginners start around 33-38% and adjust from there.
Worked Example: Superfat in Grams
Numbers make the concept concrete faster than percentages alone. Take a 1,000 g oil recipe with a calculated full-saponification lye demand of 140 g:
| Superfat Setting | Lye Used | Unreacted (Free) Oil |
|---|---|---|
| 0% | 140 g | 0 g |
| 5% (standard) | 133 g | 50 g |
| 8% (facial bar) | 128.8 g | 80 g |
| 20% (high-coconut) | 112 g | 200 g |
The jump from 5% to 20% looks small as a percentage, but it's the difference between 50 grams and 200 grams of free oil sitting in the bar, four times the amount available to oxidize and go rancid. That's the practical reason high-superfat recipes need a shorter shelf-life expectation and more careful storage.
Frequently Asked Questions
Is 5% superfat safe for all soap recipes?
For most body soap recipes based on a balanced oil blend (olive, coconut, palm or a palm substitute), 5% is a solid, safe default. It leaves enough buffer against measurement variation without making the bar soft. The main exception is high-coconut recipes, which need 15-20% superfat to avoid being harsh on skin.
Can I change the superfat after I have already calculated my lye?
No. The superfat is baked into the lye weight the calculator gives you. If you decide to change it, rerun the calculation from scratch with the new percentage and use the new lye amount. Never add extra oil to a batch after you have already weighed your lye.
Does a higher superfat make soap more moisturizing?
Up to a point. Free oils do add a softer skin feel, but there are diminishing returns past about 8%, and the trade-off is a softer bar with a shorter shelf life. Most people find the difference between 5% and 10% modest. A well-formulated oil blend does more for skin feel than chasing a very high superfat.
What causes DOS, and does superfat make it worse?
DOS (dreaded orange spots) is caused by rancidity in unsaponified oils. Higher superfat means more free oil exposed to air and light, so yes, it does increase the risk, especially with oils already prone to going rancid quickly (sunflower, grapeseed, rice bran). Light exposure specifically has been shown to accelerate oil oxidation far more than modest temperature changes, so cure your soap in a cool, dark spot, not a sunny windowsill, and use it within a year to minimize DOS risk.
How is superfat different from adding oil at trace?
Superfat is a setting in your lye calculator that reduces the lye used, leaving a percentage of your total oil blend unsaponified. Adding oil at trace is a technique where you stir a small extra amount of oil into the soap batter late in the process, after the lye has largely reacted. These are two different approaches, and the oil-at-trace amount is not accounted for in the standard lye calculation, so if you use this technique, add it as a separate, small addition beyond your calculated batch.
Can superfat be negative, and would that ever make sense?
Effectively yes, this is what soapers mean by a "0% superfat" or "full-water discount" recipe when they deliberately use slightly more lye than the oils need, sometimes called soaping "lye-heavy." It is not recommended for beginners. A lye-heavy bar can remain genuinely caustic and irritating to skin, and the small margin of error that superfat normally provides disappears. Shampoo bar recipes sometimes run close to 0% for specific formulation reasons, but going below that requires real experience and precise measuring, not guesswork.