How to Calculate Lye Safely for a Soap Recipe

If you are asking “how much lye do I need for this soap recipe,” the safe answer starts with the oils, not the size of the mold. Sodium hydroxide is calculated from the weight of each fat multiplied by that fat’s NaOH SAP value, then reduced by your chosen superfat. Water is a separate design choice. It affects handling, heat, trace, and cure, but it does not decide how much sodium hydroxide the oils require.

The basic bar-soap formula is:

NaOH = sum of each oil weight × that oil’s NaOH SAP value × (1 − superfat).

Use weights, preferably grams. Percentages are fine for designing the recipe, but the lye calculation needs actual oil weights. If a recipe says 50 percent olive oil, 25 percent coconut oil, and 25 percent palm oil in a 1,000 g oil batch, the oil weights are 500 g, 250 g, and 250 g.

SoapCalc lists NaOH SAP values of 0.135 for olive oil, 0.183 for coconut oil at 76 degrees, and 0.142 for palm oil. On that 1,000 g example, the full-saponification sodium hydroxide calculation is:

500 g olive oil × 0.135 = 67.5 g NaOH
250 g coconut oil × 0.183 = 45.75 g NaOH
250 g palm oil × 0.142 = 35.5 g NaOH

That totals 148.75 g NaOH before superfat. A 5 percent superfat means you intentionally use 95 percent of that full amount: 148.75 × 0.95 = 141.31 g NaOH. In practice, round carefully to the resolution of your scale and never round lye upward to “make the numbers neat.”

Superfat is not a safety charm. It is a controlled lye discount. Too little sodium hydroxide leaves excess oils and can make a softer or shorter-lived bar. Too much sodium hydroxide leaves a caustic bar. A normal-looking finished bar can still be wrong if the arithmetic or scale work was wrong, so the calculation deserves the same care as the goggles.

Water comes next. Many calculators let you choose water as a percentage of oils, a water-to-lye ratio, or lye concentration. These are different ways of describing the same solution. With 141.31 g NaOH, a 2:1 water-to-lye ratio uses about 282.63 g water. A 33 percent lye concentration puts NaOH at 33 percent of the total lye solution, so the solution is about 428.22 g and the water portion is about 286.91 g. Those numbers are close, but they are not identical.

Less water makes a stronger lye solution, often a faster-moving batter, and less water to evaporate later. More water gives a more dilute solution and can be more forgiving, though it may lengthen unmolding and drying. Milk, sugar, honey, salt, fragrance, and hot-process methods can change handling. They do not cancel the SAP math.

The part that makes this more than kitchen arithmetic is the material itself. NIOSH identifies sodium hydroxide by the formula NaOH and lists “caustic soda” and “lye” among its names. Its exposure routes include inhalation, ingestion, and skin or eye contact; listed symptoms include irritation and eye and skin burns. NIOSH also notes that solid sodium hydroxide is noncombustible, but contact with water may generate enough heat to ignite combustible materials. That is why the old rule matters: add lye to water, not water to lye, and do it with suitable eye protection, gloves, ventilation, and a container meant for caustic solution.

For a working recipe, I would check three things before mixing. First, each oil has the right identity. Coconut oil, fractionated coconut oil, and palm kernel oil are not interchangeable in a lye calculator. Second, every weight is an oil weight, not a container weight or a volume measure. Third, the lye type is correct. Sodium hydroxide is for bar soap; potassium hydroxide is used for liquid soap. The SAP values are different.

Shadowfetch built Lyestone, listed by Apple as version 2.0.1 with a Jun 26 update, for this exact bench problem: choose fats, enter weights, set superfat and water, and get the NaOH or KOH amount without treating the phone as a substitute for chemical caution. Disclosure: Lyestone is a Shadowfetch app, and this article was AI-assisted under human editorial direction.

The safest lye calculator is not the one that hides the chemistry; it is the one that keeps the caustic number visible until you respect it.