Superfat is the oil left in a finished soap after saponification is done. A lye calculator sets it by reducing the lye: at a 5% superfat it uses 95% of the lye the oils could consume, so some of the oil stays as oil in the bar. The same setting is sometimes called a lye discount.
The reason is a margin for error. Each oil's saponification (SAP) value, the amount of lye it needs to become soap, is a published average, real oils vary from it, and lye is not always fully pure, so a recipe calculated at exactly 100% could end up with lye left over. Holding some lye back is a cushion that makes sure the lye gets used up. Worked example: oils that need 140 g of lye (sodium hydroxide) get 133 g at a 5% superfat.
Five percent is a common starting point for a classic blend of oils. Go much higher and the bar gets softer and lathers less. Any change to the oils, or to their proportions, means running the lye calculation again, superfat included. Lye is caustic: wear gloves and sealed goggles whenever you handle it.
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Related terms
Recipe
A defined set of ingredients, quantities, steps, and equipment needed to produce an item you make — a product you sell, or a finished good or subassembly you stock. Recipes automatically calculate production costs based on current ingredient prices.
Ingredient
A raw material or component purchased from vendors and used in recipes to produce finished goods. Examples: flour, beads, essential oils, fabric.
Batch/Lot Number
A unique identifier assigned to a production run or group of items produced together. Enables traceability from raw ingredients to finished products for quality control and compliance.
Yield
The amount of usable finished product a recipe or production run actually produces, after accounting for trim, waste, and loss.