Soap Superfat Percentage Effect Calculator

Soap Superfat (Lye Discount)

Superfat is the excess oil left after saponification: oils that the lye doesn’t convert to soap. This unsaponified oil makes the bar less harsh and more conditioning, but reduces lather and shelf life.

The math: Superfat % = (1 - actual lye / theoretical lye) × 100

Or: a 5% superfat means using 95% of the calculated lye amount, leaving 5% of oils unsaponified.

Standard superfat ranges:

Effects of superfat on bar properties:

Why salt bars need 15-20% SF: Salt destroys lather. To compensate, salt bar recipes use very high superfat. The unsaponified oils provide moisture and soften the bar enough to lather despite the salt content.

Why castile soap uses low SF: 100% olive oil saponifies into a slow, slick soap with naturally mild characteristics. Adding extra superfat would make it slimy and prone to rancidity (olive oil already has high oleic acid).

The “selecting your superfat oil” choice: Some soap makers reserve a luxury oil (jojoba, shea butter, sweet almond) as the LAST oil added, on the theory that this oil ends up as the unsaponified superfat. Chemically that is false. Saponification does not queue: the lye attacks whatever triglyceride it meets, so all oils contribute proportionally to the final superfat regardless of the order they went in. Adding the expensive oil at trace does no harm and it does spare it some heat, so carry on if you like the ritual. Just do not expect the bar to be different because of it.

DOS (dreaded orange spots): DOS are rancidity stains caused by oxidation of unsaturated fats. Higher superfat means more unsaponified oil sitting in the bar waiting to oxidise, so the risk climbs with it. What helps:

• Antioxidants. Rosemary extract (ROE) or vitamin E, added to the oils.
• A blend with more saturated fat overall. Note the wording: you cannot pick which oil ends up as the superfat, for the reason given in the section above, so the lever is the whole recipe rather than the last oil in. Shift a few percent from olive or sunflower into coconut, palm or lard and the unsaponified fraction is less oxidisable because everything in the pot is.
• Fresh oils. An oil already halfway through its own shelf life brings that clock into the bar with it.
• Cool, dark storage, with air able to move around the bars.

Trade-offs and recommendations:

• 5% SF is the gold standard for most bath bars
• Above 7%: lather suffers noticeably
• Below 3%: bar becomes harsh
• Salt bars must be 15-20%: exception, not the rule

Math example:

A 32-oz oil recipe, 30% coconut and 30% palm and 40% olive. Those three saponify at 0.183, 0.141 and 0.134 grams of sodium hydroxide per gram, which averages out at 0.151 across the blend. Rounded to 0.150, which is what this page uses and is inside the accuracy any SAP table can honestly claim:

• 0% SF: 32 × 0.150 = 4.80 oz NaOH (the full theoretical amount)
• 5% SF: 4.80 × 0.95 = 4.56 oz NaOH
• 8% SF: 4.80 × 0.92 = 4.42 oz NaOH

The gap between 0% and 8% is a quarter of an ounce on a 2-pound batch. That is a small number doing a lot of work, which is why weighing lye on a scale that reads to 0.1 g matters more than it looks like it should.

The 0.150 figure is the average for that particular blend. Change the oils and it moves, so run your own recipe through the lye calculator linked below rather than assuming it.