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Multiply total oil weight by 0.135 for lye and by 0.33 for water. One thousand grams of blended oils need about 135 grams of lye and 330 grams of water, but exact lye depends on the oil blend, so always verify a real recipe with a full soap calculator that lists your exact oils before making it.
Each oil needs a different amount of lye to saponify, and blended recipes use an average factor. Water at about one third of oil weight gives a workable trace time for cold process soap. This soap batch calculator uses 0.135 grams of lye per gram of blended oils and 0.33 grams of water per gram of oils, so 1000 grams of oils need 135 grams of lye and 330 grams of water, and 500 grams need 67.5 grams of lye and 165 grams of water. Treat these as starting estimates for a blended recipe, not as exact values for a single oil.
The SAP value is the grams of sodium hydroxide needed to saponify one gram of an oil. Common SAP values are olive oil 0.134, coconut oil 0.190, palm oil 0.141, shea butter 0.128, and castor oil 0.128. A recipe with more coconut oil needs more lye than the same weight of olive oil, so a full soap calculator multiplies each oil weight by its own SAP value, sums the results, and then applies the superfat discount. Always verify a recipe with that per oil calculation before making it, because a blended average can be off for a single oil batch.
Superfat is a small lye discount that leaves a portion of oils unsaponified after the reaction. That leftover oil is what keeps a bar gentle instead of drying. Most cold process recipes run a 5 percent superfat, facial bars often run higher, and shaving or laundry soap can run closer to zero. For example, 1000 grams of olive oil at SAP 0.134 needs 134 grams of lye before superfat, and at 5 percent superfat it needs 127.3 grams. A higher superfat makes a softer bar with a shorter shelf life, while a lower superfat makes a harder, more cleansing bar.
Water is usually set as a percentage of oil weight, commonly 33 to 38 percent, or as a lye concentration, commonly 28 to 33 percent lye in the total liquid. More water gives more working time for swirls and designs, but a longer cure. Less water traces faster, unmolds harder, and reduces soda ash, but leaves less room for error. This calculator uses 33 percent water by oil weight, which is a reliable starting point for most cold process recipes. Use distilled water, because tap water minerals can interfere with saponification and lather.
Lye is caustic, so wear safety goggles, chemical resistant gloves, long sleeves, and closed shoes, and work in a well ventilated area. Always add lye to water, never water to dry lye, because pouring water onto dry lye can cause a violent surge. The solution heats quickly to 65 to 90 degrees Celsius and may fume, so do not lean over it, and let it cool in a clearly labelled container before combining with oils. Never use aluminium containers, which react with lye. Keep children and pets away, and follow your supplier safety data sheets.
For homemade soap, measurements must be by weight on a digital scale accurate to 0.1 grams, never by volume, because a gram of lye error shifts the superfat. A good starting batch for cold process bar soap is 500 to 1000 grams of total oil weight, which typically yields 8 to 14 bars at 100 to 120 grams each after cure. Smaller batches of 300 to 500 grams are easier while learning trace, scent acceleration, and unmolding. Size your mold to the total batch weight, which is oils plus lye plus water, not just oils. Cure bar soap on a ventilated rack for 4 to 6 weeks before use or sale: curing completes saponification and evaporates excess water, so cured bars weigh less than fresh bars. To scale a soap recipe, keep the oil percentages fixed and recalculate lye and water from the new total oil weight.
No, it is a blended oil estimate. Different oils need different lye amounts, so verify any recipe with a full soap calculator that lists your exact oils and SAP values before making it.
Adding water to dry lye can cause a violent steam surge and dangerous splashing. Adding lye slowly to water lets heat dissipate gradually as the crystals dissolve.
Five percent is standard for cold process soap. Facial bars often run 8 to 10 percent, while shaving and laundry soap can run closer to zero to 3 percent.
Water is commonly 33 to 38 percent of oil weight, or a lye concentration of 28 to 33 percent. This calculator uses 33 percent water by oil weight, a reliable starting point for most cold process recipes.
NaOH, sodium hydroxide, makes hard bar soap. KOH, potassium hydroxide, makes liquid or paste soap, and KOH is typically sold at 90 percent purity, so it needs a purity correction. Never swap one for the other without recalculating.
Cure bar soap for 4 to 6 weeks on a ventilated rack. Castile soap made from 100 percent olive oil may need 8 to 12 weeks to harden fully and develop creamy lather.
Start with 500 to 1000 grams of total oil weight, which yields about 8 to 14 bars after cure. Smaller batches are easier while learning trace and unmolding, so scale up only once a formula is tested and stable.