Lye, water, superfat, 202 oils with verified saponification values, and FDA-compliant INCI labels - in one apothecary-quiet calculator.
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There is no universal figure because every fat has its own saponification value, so a pound of coconut oil needs roughly 2.8 oz of sodium hydroxide at 5 percent superfat while a pound of olive oil needs about 2.1 oz. The correct amount is the sum of each oil's weight multiplied by its own NaOH factor, then reduced by your superfat percentage. Any single number quoted online is only an average for one specific blend. Run your actual recipe through a calculator rather than borrowing someone else's total.
A SAP value is the milligrams of potassium hydroxide required to saponify one gram of a fat, measured by a standard titration such as ISO 3657 or ASTM D5558. Charts disagree because natural oils vary by crop, region, harvest and refining, so labs publish ranges rather than fixed constants, and different sources pick different points inside those ranges. Olive oil is commonly listed anywhere from 184 to 196 mg KOH/g. Pick one reputable source, stay with it, and keep a superfat cushion to absorb the variance.
For sodium hydroxide, divide the SAP value by 1402.5 to get grams of NaOH per gram of oil, because SAP is expressed in KOH and the molar mass ratio of NaOH to KOH is about 40.0 to 56.1. For potassium hydroxide, divide the SAP value by 1000 instead. Multiply the resulting factor by the oil weight, repeat for every oil, then multiply the total by one minus your superfat. If your KOH is 90 percent pure, divide the KOH total by 0.90.
A general body bar sits comfortably at 5 to 8 percent, which is the range most hobby and small-batch makers default to. Facial and shaving bars are often taken to 8 percent for extra mildness, while high-coconut recipes and salt bars are pushed to 15 or 20 percent to offset how aggressively coconut oil cleans. Solid shampoo bars are usually kept low, around 2 to 5 percent, because excess free oil leaves hair limp and dull. Higher superfat also shortens shelf life, since unsaponified oil is what goes rancid.
Water as a percent of oils measures water against your oil weight, so 38 percent water on 1000 g of oils means 380 g of water regardless of how much lye the recipe needs. Lye concentration measures the strength of the solution itself, calculated as lye divided by lye plus water. The second is more useful because it describes what actually goes into the pot: a 33 percent solution behaves identically whether the recipe is coconut-heavy or olive-heavy. Most calculators let you enter either and convert between them.
Sodium hydroxide produces hard bar soap and potassium hydroxide produces soft paste that dilutes into liquid soap, because the potassium salts of fatty acids are far more soluble. Some liquid soapers use a small NaOH addition, often 5 to 10 percent of the total alkali, to firm up the paste. Flake and pellet KOH sold for soap making is typically 90 percent pure with the remainder mostly water and carbonate, so divide your calculated KOH by 0.90 unless the supplier certifies higher. Never swap one hydroxide for the other at equal weight.
Most cold process bars need 4 to 6 weeks. Saponification itself finishes within a day or two, but curing is mainly water evaporation, which hardens the bar, improves lather and extends how long it lasts in the shower. Castile and other high-olive recipes are soft and slimy early on and need 6 months to a year to develop a firm bar and a stable lather. The practical endpoint is weight stability: weigh a bar weekly and consider it cured when the weight stops dropping.
The zap test checks for unreacted alkali by touching a wet fingertip to the bar and then to your tongue; free lye gives a distinct sting like a battery terminal. Wait at least 48 hours after unmolding, and skip the test entirely if the surface looks wet, weepy or has visible pockets of liquid. A bar that zaps is not safe to use and will not become safe by curing longer if the recipe was mismeasured. Rebatch it with the missing oil, or discard it, and recheck your lye math.
Sodium and potassium hydroxide are corrosive, so wear splash-proof chemical goggles meeting ANSI Z87.1, chemical-resistant gloves such as nitrile, long sleeves and closed shoes, and mix in a ventilated area since dissolving lye releases irritating vapor. For skin contact, remove contaminated clothing and flush with copious running water for at least 15 to 20 minutes; for eye contact, flush for at least 15 to 30 minutes with the eyelids held open and get emergency medical care. Do not attempt to neutralize lye on skin with vinegar, because the reaction generates heat and worsens the burn. In the United States, Poison Control is reachable at 1-800-222-1222.
In the United States, a product that is genuinely soap, meaning its cleansing action comes from alkali salts of fatty acids and it is sold only as soap, is excluded from the FDA cosmetic definition under 21 CFR 701.20 and is instead regulated by the Consumer Product Safety Commission under the Fair Packaging and Labeling Act. That means net weight, identity and the responsible party's name and address, but no ingredient declaration. The moment you make a cosmetic claim such as moisturizing, anti-aging or deodorant, it becomes a cosmetic and needs a full ingredient list in descending order of predominance using INCI-style names per 21 CFR 701.3. Rules differ elsewhere, and the EU requires a Cosmetic Product Safety Report and CPNP notification for soap regardless of claims, so check your own jurisdiction.