Superfatting Cold-Process Soap — How Much, Which Oils, and When It Backfires

Superfat is one of the first knobs every cold-process soaper learns to turn, and one of the easiest to get wrong. Crank it too low and the bar feels stripping and dry. Crank it too high and the bar goes soft, lathers poorly, and rancidifies on the shelf within months. The defaults baked into most online lye calculators (usually 5%) are a fine starting point — but they’re a starting point, not an answer.

This post walks through what superfat actually does in a soap bar, the percentage ranges that suit different oil combinations, which oils are worth “reserving” for the superfat, and the trade-offs that don’t always show up in beginner tutorials.

What Superfat Actually Is

Superfat (sometimes called “lye discount”) is the percentage of oil in a soap recipe that is intentionally left unreacted with sodium hydroxide. If a recipe contains 500 grams of oils and you set the superfat to 5%, the lye calculator gives you only enough sodium hydroxide to saponify 475 grams. The remaining 25 grams of fat stays in the bar as free oil.

There are two equivalent ways to express the same idea:

• Superfat % — the percentage of oil left unsaponified. 5% superfat means 5% of the fat is unreacted.
• Lye discount % — the percentage of lye removed from the theoretical full-saponification amount. 5% lye discount means you’re using 5% less sodium hydroxide than would be needed for a zero-superfat bar.

Both phrases describe the same outcome. Most lye calculators present the field as “superfat” or “superfat %”, with a default somewhere between 5% and 8%.

The two practical effects of superfat are simple: the bar contains a small reserve of conditioning oil that wasn’t turned into soap, and the recipe carries a built-in safety buffer against a slight error in the lye measurement.

Why Superfat Exists

There are three reasons soapers superfat, and they pull in slightly different directions.

The conditioning argument. Soap is, chemically, the sodium salt of a fatty acid. The whole purpose of a surfactant like soap is to grab oil and rinse it away. If a bar is 100% saponified, it will rinse cleanly — which is exactly what you want in a kitchen degreaser and exactly what you don’t want on dry winter skin. Leaving 3–8% of the fat unreacted gives the bar a residual oil layer that softens the rinse and reduces the post-wash tight feeling.

The safety argument. Sodium hydroxide is sold to industrial purity, but the actual purity sitting in your container can drift — exposure to humid air drops effective NaOH percentage, and scale accuracy on cheap kitchen scales is typically ±1g. A 0% superfat bar with a 1g lye error becomes a lye-heavy bar (caustic, harsh, sometimes painful). A 5% superfat bar with the same error is still well within safe territory. The superfat absorbs the slop.

The cosmetic-feel argument. Some oils — shea butter, mango butter, jojoba, hemp seed — have a recognisable skin-feel even at low percentages. A small unsaponified reserve of those oils carries through to the finished bar. This is the reasoning behind the “reserved superfat” trick covered later.

Typical Superfat Ranges

There is no single correct number. The right superfat depends on the oil profile of the recipe, the intended use of the bar, and how long you expect it to sit before use.

0–2% — castile and salt bars. A bar made entirely from olive oil (a true castile) is naturally mild and very stable. It also rinses slightly draggy if you push the superfat higher, and benefits from cure-time hardness that low superfat preserves. Salt bars (where 50–80% of the oil weight is matched by added sea salt) are typically formulated at 15–20% superfat because the salt aggressively cuts lather and the high superfat compensates — but if you’re working with a non-salt recipe, this range is the wrong choice.

3–5% — workhorse bars for kitchen, laundry, and shampoo. Bars that need to clean effectively benefit from a lean superfat. Kitchen soap with 8% superfat will leave a film on dishes. Hand soap meant for greasy hands runs better at 3–4%. Cold-process shampoo bars (a niche application — most modern shampoo bars are syndet, not soap) are usually set around 3–5% so they don’t weigh hair down.

5–8% — the standard “skin bar” range. Most general body bars, facial bars, and gift soaps live here. This is the default range every lye calculator suggests for a reason: it produces a bar that cleans well, doesn’t feel stripping, and has enough buffer for measurement error. If you don’t have a specific reason to deviate, start here.

8–12% — heavy moisturizing bars. Bars intended for very dry skin, dry winters, or eczema-prone use can run hotter. The trade-off is softer bars, faster wear, and shorter shelf life. Be selective about the unsaponified oils — high-linoleic oils at 12% will rancidify long before they’re used up.

15–20% — salt bars. As mentioned above. Outside salt bars, this range produces an unstable, soft bar that won’t last.

Reserved Superfat (the “save the good oil” technique)

In a normal recipe, every oil in the pot has an equal probability of being the unsaponified one. Sodium hydroxide doesn’t read labels — it saponifies whichever triglyceride it encounters first. So if your recipe is 50% coconut, 30% olive, 15% palm, and 5% shea, your finished bar’s 5% superfat is mostly coconut/olive/palm, with a tiny pro-rata sliver of shea.

Reserved superfat — sometimes called superfatting at trace or hot-process superfatting — is a workaround. You hold back the oil you want preserved and add it after the bulk of the saponification is finished. The mechanism is simple: by the time the reserved oil is stirred in, most of the lye has already reacted with the bulk oils, so the reserved oil has a higher chance of surviving as free fat in the cured bar.

In cold-process, the technique is approximate. Saponification is still in progress at trace, so you can’t guarantee that 100% of the reserved oil stays unreacted. But it does meaningfully bias the outcome.

In hot-process, the technique is much cleaner. After the cook (when the lye is fully consumed), any oil added at “cool-down” stays unsaponified. This is the only soap method where reserved superfat is chemically precise.

Oils that benefit from the reserved approach:

• Shea butter, mango butter — distinctive skin-feel worth preserving
• Jojoba — technically a wax, slow to saponify, recognisable mid-bar feel
• Hemp seed, evening primrose, rosehip — fragile high-linoleic oils that lose value when saponified
• Castor oil — actually the opposite case: most soapers prefer to fully saponify castor for lather, so it’s almost never reserved

Oils not worth reserving:

• Coconut, palm, lard, tallow — these are workhorses, no special skin-feel to preserve
• Olive oil — the soap version (sodium olivate) has nicer skin-feel than free olive oil at typical use levels
• Sunflower, safflower, rice bran — generic carrier oils with nothing distinctive to bring across

The Trade-Offs No One Mentions

Superfat % is one of the most over-recommended dials in DIY soaping. “Just bump the superfat” is the standard answer to every complaint about dryness — and it’s often the wrong answer. Three trade-offs deserve attention.

Lather. Every percent of superfat is a percent of bar mass that isn’t soap. Higher superfat means less surfactant per gram, which means slower lather build-up and a smaller bubble crown. A 12% superfat coconut-heavy bar still lathers fine because coconut is so aggressive — but the same 12% superfat on an olive-heavy recipe produces a sluggish, low-foam bar.

Hardness and cure speed. Free oil softens a bar. A 10% superfat olive bar takes weeks longer to harden than a 4% superfat olive bar made on the same day. If the recipe is already soft (high in olive, sunflower, or rice bran), pushing the superfat compounds the softness problem.

Rancidity and Dreaded Orange Spots (DOS). Free unsaponified oil in a bar is vulnerable to oxidation in a way saponified soap is not. The unsaturated fatty acids in oils like sunflower, hemp, grapeseed, rice bran, and rosehip oxidize over time, producing rancid odor and yellow-orange spots on the bar’s surface. The higher the superfat, the more raw oil is available to oxidize, and the more the choice of superfat oil matters.

A 10% superfat shea/olive bar can sit happily for a year. A 10% superfat hemp/sunflower bar may DOS in 4–6 months. Same superfat number, very different outcomes.

If you want a long shelf life and a high superfat, choose stable oils: shea, cocoa butter, olive, coconut, palm, lard, tallow, or jojoba. Add ROE (rosemary oleoresin) at 0.1% of the oil weight to slow oxidation. Avoid sunflower-heavy recipes at 8%+ superfat unless you plan to use the bars quickly.

How to Set Superfat in a Lye Calculator

Any reliable lye calculator — including the DIY Cosmetica soap calculator — has a superfat field. The number you enter affects only the lye amount, not the oil amounts and not the water amount.

The mechanics:

1. Enter your oil weights as normal.
2. Enter your superfat % (between 0 and 20).
3. The calculator reduces the lye amount proportionally.
4. The water amount is unaffected — it’s calculated as a percentage of oils or a fixed ratio to lye, not the saponified mass.

A worked example: 1000g of oils at 5% superfat with a SAP value of 0.139 (a typical olive-heavy recipe). At 0% superfat, you’d need 139g of NaOH. At 5%, you need 139 × 0.95 = 132.05g — about 7g less lye than the zero-superfat version.

If you change superfat after entering the recipe, only the lye amount updates. The water and oil weights remain the same. This is why “I’ll just bump the superfat to 10%” is such an easy decision in the moment, and such an easy way to end up with a bar that’s softer than expected.

Common Superfat Mistakes

Stacking high superfat with fragile oils. A 12% superfat sunflower/hemp/rice bran bar will rancidify within months. Either drop the superfat or swap the oils.

Using superfat to fix bar-feel problems. If the bar feels stripping, the cause is more often the oil profile than the superfat. A high-coconut bar (45%+) feels stripping regardless of whether the superfat is 5% or 12%, because the underlying issue is too much sodium cocoate. Reformulate the oils, don’t paper over with superfat.

Forgetting to recalculate when changing the oil profile. If you swap olive for sunflower, the SAP value changes and the lye amount changes. Just bumping the superfat without re-running the recipe through a calculator is how lye-heavy or oil-heavy bars happen.

Treating “0% superfat” as a hard ceiling. Zero superfat is a fine target for laundry bars and some kitchen soap. It’s not “dangerous” — it just leaves no buffer for measurement error. Use it deliberately, with a calibrated scale, not by accident.

Trusting reserved superfat in cold-process more than it deserves. Reserved superfat at trace biases the outcome but doesn’t guarantee it. If you absolutely need a specific oil to survive as free fat, hot-process is the only method that delivers.

Bottom Line

Start at 5% for a general skin bar. Drop to 3% for kitchen, laundry, and hand soaps where you want lean cleaning. Push to 8% for a winter bar or a sensitive-skin formula. Save 10%+ for very specific moisturizing bars, knowing the trade-off is softer bars, slower lather, and shorter shelf life.

If you want a particular oil’s character to show in the finished bar, hot-process superfat is the only reliable way to deliver it. Cold-process reserved superfat tilts the odds but doesn’t lock them in.

And if a bar feels wrong, look at the oil profile first. Superfat is the easiest dial to turn, but the answer is usually somewhere else.

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Related on DIY Cosmetica:

• Cold-process vs hot-process soap — when each one wins
• Soap calculator
• Definitions: saponification, trace, gel phase, cool-down phase