Superfat and Lye Discount, Explained With a Worked Example

Superfat is the percentage of oil in a recipe deliberately left unsaponified. Lye discount is the percentage by which the lye is reduced below what the oils could consume. They are the same adjustment viewed from either end. A 5% superfat and a 5% lye discount produce an identical batch, and the number you choose is the single biggest lever on how a bar feels.

Why anyone would use less lye than the recipe needs

Three reasons, in order of importance.

Margin for error. Fats vary. A saponification value is an average for a category, and the actual olive oil in your bottle is not identical to the reference figure. Scales drift. Lye absorbs moisture from the air, so a stored batch is not quite pure NaOH. Building in a deliberate shortfall means small errors land in a safe direction. Without it, an ordinary measurement slip produces a lye-heavy bar.

Feel on skin. Free fatty acids left in the bar are conditioning. A 1% superfat bar and an 8% superfat bar made from the same oils are noticeably different: the first cleans hard and can feel tight, the second feels cushioned.

Choosing what stays. In principle you can superfat with a specific oil added after trace, hoping the expensive one survives. In practice saponification does not respect intent. The alkali reacts with whatever is available, roughly in proportion. Adding shea at the end mostly buys you a marginal shift, not a shea-coated bar.

The arithmetic

Every fat has a saponification value: the mass of NaOH required to fully saponify one gram of it. Take a simple three-oil recipe of 1000 g total.

That 150.8 g is the full saponification value, the amount that would convert every last molecule of oil, leaving nothing spare. Nobody uses it.

Apply a 5% superfat:

150.8 g × (1 − 0.05) = 143.3 g NaOH

You use 143.3 g of lye. The 7.5 g shortfall means roughly 5% of your oils never react and stay in the bar as free fatty acids.

Then the water. A common starting point is a 2:1 water-to-lye ratio:

143.3 g × 2 = 286.6 g water

Which gives the full batch:

• Oils: 1000 g
• Sodium hydroxide: 143.3 g
• Water: 286.6 g

Choosing a number

Two adjustments worth knowing:

• Salt bars are normally made at 15–20% superfat. The high salt content and heavy coconut oil would otherwise be brutally stripping, and salt suppresses lather regardless.
• High-coconut recipes need a higher superfat than the same percentage would imply elsewhere, because lauric acid soap is so cleansing. A 100% coconut bar at 20% superfat is a pleasant bar; at 5% it is a degreaser.

The ceiling nobody mentions

More superfat is not more moisturising past a point, and there is a real cost: unsaponified oil goes rancid. Free fatty acids oxidise, and that is what produces dreaded orange spots: the small rust-coloured freckles that appear on ageing bars and carry a stale, crayon-like smell.

The risk compounds with two other factors:

• Oils high in linoleic acid, such as sunflower, grapeseed and hemp, oxidise fastest. A high superfat of a fragile oil is the worst combination.
• Warmth and humidity accelerate it, so a hot climate shortens every estimate.

For a bar meant to sit on a shelf for a year in a warm place, 5% with stable oils will outlive 10% with fragile ones, and feel better at month eleven.

What to remember

Superfat and lye discount are two names for one number. Five percent is the right default and you need a specific reason to leave it. The safety margin it provides is the point, not a side effect, and it is why finished soap contains no lye at all.