Soap and lye calculator

The share of your oils deliberately left unsaponified, achieved by using less lye than the oils could consume. Also called lye discount, since the two name the same adjustment from opposite ends. Most bar recipes sit at 5%. It is easy to assume superfat is what makes a bar gentle, but that job belongs mostly to the cleansing number - superfat is better understood as insurance against scale error and against published SAP values not quite matching the oil in your pot.

Water weight measured against oil weight. It describes the same solution as lye concentration from the other end, but the two do not convert by a fixed number - the lye weight depends on which oils you chose. This is why two calculators that look like they agree on water can hand you different figures.

How much fragrance or essential oil to add, in grams per kilogram of oils. Around 30-60 g/kg is typical for bar soap, and 0 leaves it unscented. Suppliers publish a maximum usage rate for each fragrance and they vary widely - some essential oils are restricted well below the typical range - so check the rate for the one you are actually using rather than trusting a general figure. Fragrance is also the usual culprit when a batch seizes or accelerates unexpectedly.

How much of what you weigh out is actually sodium or potassium hydroxide. Hobby-grade KOH is commonly 90%; NaOH flake usually runs 96-99%. The calculator divides by this figure, so lower purity means more product on the scale to deliver the same alkali. Take it from your supplier’s spec sheet, and note it is not the same thing as a liquid lye solution, which brings its own water.