What Is Superfat in Soap Making?

The Definition: What Superfat Actually Means

When lye (sodium hydroxide for bar soap, potassium hydroxide for liquid soap) mixes with oils, a chemical reaction called saponification converts those oils into soap and glycerin. A superfat of 5% means that 5% of your total oil weight is not given enough lye to react — those oils remain in the bar as free, unsaponified fats.

The term is sometimes used interchangeably with lye discount, which describes the same thing from the lye's perspective: the lye amount is reduced (discounted) so that a portion of oils is left unreacted. Both terms refer to the same calculation.

The Standard Range: What Percentage to Use

Different use cases call for different superfat levels. Here are the evidence-based benchmarks most experienced soap makers use:

• 0% — Technically correct but harsh; lathers well but strips skin. Almost never recommended for body bars.
• 2–4% — Used for shampoo bars and dish soap bars where a grease-cutting lather is more important than moisturising.
• 5% — The most popular all-purpose setting. Produces a conditioning bar with good lather and a shelf life of 12–18 months.
• 6–8% — Richer, creamier feel. Ideal for bars targeting dry or sensitive skin. Slightly softer bar and a shelf life of approximately 9–12 months.
• 10% and above — The bar stays soft longer, may feel greasy or leave reside on skin, and the elevated free-oil content accelerates rancidity (DOS — dreaded orange spots).

How Lye Calculators Incorporate Superfat

When you enter a superfat percentage into a lye calculator, the tool reduces the total theoretical lye amount by that percentage. For example:

• 500 g olive oil has a NaOH SAP value of approximately 0.134, requiring roughly 67 g NaOH at 0% superfat.
• At 5% superfat, the calculator provides only 95% of that lye: 63.65 g NaOH.
• The remaining 5% of olive oil (~25 g) is left with no lye partner, and stays as a free oil in the bar.

This is why adjusting superfat is so simple in a calculator: it is a single multiplier on the lye output, not a change to the oil blend itself. Use our Batch Scaler or Fragrance Calculator alongside your lye calculation to build a complete recipe in minutes.

Which Oils Are Best for the Superfat Portion?

Here is a key nuance that trips up many soap makers: you cannot control which specific oil molecules react with the lye. Saponification does not selectively consume cheap oils and spare the expensive ones. Every oil in the pot is at equal risk of reacting.

The workaround used by experienced makers is to add luxury oils at trace — after the initial emulsion has formed — to give them the best statistical chance of remaining unreacted:

• Shea butter — Rich in stearic and oleic acid; adds a silky, creamy skin feel.
• Avocado oil — High oleic content; intensely moisturising, good for sensitive or mature skin.
• Argan oil — Lightweight; contains vitamin E and squalene; adds a non-greasy luxury feel.
• Mango butter — High stearic acid; hardens the bar while conditioning.
• Sweet almond oil — Mild, medium-weight; good all-around conditioning oil for the superfat reserve.

Superfat, Lather, Skin Feel & Bar Hardness

Superfat level interacts with every key quality metric in soap:

• Lather: Higher superfat slightly reduces lather quantity because more oil remains unsoap-ified (free oils suppress foam). For abundant lather, keep superfat at 5% or below and ensure at least 15–20% coconut or palm kernel oil in the blend.
• Skin feel: Free oils coat and condition the skin after rinsing. This is the primary reason to superfat — it directly counteracts the inherently alkaline nature of soap, which has a pH of roughly 9–10.
• Bar hardness: Free oils soften the bar matrix slightly. A higher superfat combined with a high-oleic oil blend can produce a bar that feels soft at unmold even after full cure. Balancing with hard fats (coconut, palm, lard, or tallow) at 25–35% of the recipe compensates for this.
• Shelf life: Unsaponified oils are still subject to oxidation. Higher superfat = more free oil = faster rancidity risk, especially with polyunsaturated oils like sunflower or hemp seed.

Climate, Use Case, and Skin Type: Dialling In the Right %

There is no single perfect superfat percentage — the ideal number depends on context:

• Humid climates: Bars with high superfat absorb ambient moisture more readily and stay soft in the soap dish. Keep superfat at 5% or lower if you live somewhere humid.
• Dry climates / dry skin: You can push to 7–8% safely. The extra free oils are genuinely beneficial for parched skin and the drier air reduces the rancidity risk.
• Facial bars: 5% is the consensus recommendation — enough conditioning without risking clogged pores from excessive free oils.
• Shampoo bars: 0–2%. Hair cuticles respond poorly to excess free oils; too much superfat leaves hair flat and greasy.
• Shaving bars: 3–5%. A slick, conditioning lather is needed, but too much superfat inhibits the dense, stable foam required for a close shave.
• Baby or sensitive skin bars: 5–7% with a high-oleic oil blend (olive, avocado, sunflower) and no coconut oil, or coconut oil limited to 10–15%.

Frequently Asked Questions

What is superfat in soap making?

Superfat is the percentage of oils in your recipe intentionally left unsaponified — meaning they do not react with lye. For example, a 5% superfat means 5% of your total oil weight remains as free, unreacted oils in the finished bar. These free oils add conditioning and moisturising properties to the soap.

What is the best superfat percentage for cold process soap?

For most cold process body bars, a superfat of 5–8% is the standard range. A 5% superfat is the most common all-purpose choice, balancing a conditioning lather with reasonable shelf life. Facial bars are typically kept at 5%, while shampoo bars use 0–2% to avoid greasy hair.

What happens if superfat is too high or too low?

A superfat of 0% produces a very harsh, drying bar because there are no free oils to condition the skin. A superfat above 10% creates a bar that stays soft longer, has a shorter shelf life due to the higher free-oil content, and can feel greasy or leave a sticky residue. The sweet spot for most recipes is 5–8%.

How does a lye calculator handle superfat?

A lye calculator reduces the calculated lye amount by the superfat percentage you select. If your oil blend would theoretically require 140 g of NaOH at 0% superfat, a 5% superfat setting tells the calculator to use only 95% of that amount (133 g). This ensures 5% of oils are left with no lye to react with.

Which oils are best to use for the superfat portion?

Luxury oils such as shea butter, avocado oil, and argan oil are ideal superfat choices. Because you cannot control which specific oils react with lye during saponification, the practical technique is to add these premium oils at trace — after the bulk saponification has already begun — to maximise the chance they remain as free, conditioning oils in the finished bar.

Put This Into Practice

Ready to calculate your recipe with the right superfat? Our free tools make it easy:

• Lye Calculator — Set your superfat %, enter your oils, and get exact NaOH/KOH amounts.
• Batch Scaler — Scale a proven recipe up or down while keeping the superfat ratio constant.
• Fragrance Calculator — Calculate fragrance oil amounts by usage rate for your batch size.

Related Guides

• Beginner's Guide to Cold Process Soap
• Best Oils for Soap Making
• Lye Safety Guide