Cold process · hot process · liquid soap
Lye, water and fragrance for your oils, from published SAP values, with superfat, KOH purity and the usual bar-property numbers. Everything is worked out in your browser.
List your oils first. Type a name and pick it from the suggestions (47 oils, butters and fats are built in), then enter each one's weight in grams, ounces or pounds. If your recipe is written as percentages, switch to Percentages and give the total oil weight; the percentages must add up to 100.
Then choose the lye. NaOH (sodium hydroxide) makes hard bar soap; KOH (potassium hydroxide) makes soft and liquid soap, and Both splits the lye between them. KOH is usually about 90 % pure, so the calculator divides by the purity you enter. Superfat is the share of oil deliberately left without enough lye to turn it into soap: a margin for SAP values that vary and scales that read a little high.
The water can be set in three ways that describe the same thing: lye concentration (lye ÷ lye plus water), water as a percentage of the oils (38 % is SoapCalc's default) or a water : lye ratio. Fragrance is optional, as a percentage of the oils or in ounces per pound of oils (1 ppo = 6.25 %).
Olive oil's SAP value is 190 mg KOH per gram. Multiply by 40.00 ÷ 56.11 (the molar masses of NaOH and KOH) to get 0.1354 g NaOH per gram of oil, so 1000 g of olive oil needs 135.4 g NaOH. With a 5 % superfat you weigh 95 % of that: 128.7 g NaOH. At a 33 % lye concentration the water is 128.7 × 67 ÷ 33 = 261.3 g.
When the numbers look right, print the recipe card (it carries the safety notes), copy or download it, or save it in this browser.
The seven numbers under the recipe were popularised by SoapCalc. Each is simple arithmetic on the fatty acids of your oils, weighted by how much of each oil you use:
The shaded band is the range SoapCalc suggests comparing against: hardness 29–54, cleansing 12–22, conditioning 44–69, bubbly 14–46, creamy 16–48, iodine 41–70 and INS 136–165. SoapCalc itself calls these guidelines, and traditional soaps sit well outside them: a 100 % olive oil (castile) bar scores hardness 17 and cleansing 0. Use the numbers to compare two versions of a recipe, not as a grade. They ignore superfat, water, additives and any fatty acid outside the eight tracked, such as the palmitoleic acid in macadamia oil or the long-chain wax esters in jojoba and beeswax.
A saponification (SAP) value is the number of milligrams of potassium hydroxide that turn one gram of a fat into soap. Lab tables give it in mg KOH/g; soap calculators often write the same figure as a fraction (0.190 for olive oil) and convert it to sodium hydroxide with the ratio of the molar masses, 40.00 ÷ 56.11 ≈ 0.713 (0.135 for olive oil).
The SAP values, fatty-acid profiles, iodine and INS figures here are the typical values in SoapCalc's oil list, which many hobby soap calculators share. They are averages. Real oils vary with plant variety, climate, refining and age: published saponification values run from 184 to 196 mg KOH/g for olive oil, 248 to 265 for coconut oil and 170 to 190 for shea butter, and the olive oil trade standard allows anything from 55 % to 85 % oleic acid. That spread is one reason soapmakers keep a few per cent of superfat. If your supplier gives a SAP value or a certificate of analysis for the batch you bought, add it as a custom oil. The oil properties chart lists every oil with its published range where one exists.
Sodium hydroxide and potassium hydroxide are corrosive. NIOSH lists eye and skin burns among their effects, and poison centres treat a splash in the eye as an emergency. Soap made with the right amount of lye and left to cure is no longer caustic, but the dry lye, the lye solution and fresh soap batter are.
For lotions, bigger or smaller batches, and choosing oils.
Phases and percentages that add up to 100 %, grams for any batch, and your supplier's usage rates checked.
Paste any gram or percentage recipe and scale it to a new batch weight, rounded to what your scale can show.
SAP values, fatty acids, iodine and INS for 47 oils and butters, sortable and searchable.
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Try WebTraceCheckSuperfat is the share of oil left over because the calculator deliberately uses less lye than the oils could take. It is a margin for SAP values that differ from batch to batch and for small weighing errors, and it leaves a little free oil in the bar. SoapCalc's default is 5 %. A much higher superfat gives a softer bar that can go rancid sooner, and 0 % leaves no margin at all.
Any calculator can be given a typo, the wrong unit or the wrong oil, and a lye mistake is the one that matters most. Before you make a new recipe, enter the same oils and settings in a second, independent lye calculator and check that the lye amounts agree closely; small differences come from slightly different SAP tables. If they differ by more than that, find out why before you weigh any lye.
Potassium hydroxide is usually sold about 90 % pure; most of the rest is water and carbonate. SAP values assume pure KOH, so the calculator divides by the purity: at 90 % it weighs out about 11 % more. Use the figure on your supplier's label or certificate. Sodium hydroxide is sold much purer, so most soapmakers leave its purity at 100 %.
They describe the same water in different ways, and the results panel shows all three. A 33 % lye concentration means the lye makes up 33 % of the solution's weight, the same as a water : lye ratio of about 2 : 1. Water as a percentage of the oils ties the water to the oils instead of the lye, so the strength of the solution changes from recipe to recipe; that is why many soapmakers prefer one of the other two.
Yes. Choose KOH, enter your KOH's purity and set the water for the paste stage. Liquid soap is usually cooked as a thick paste and diluted with more water afterwards; that dilution water is not part of this calculation.
The calculator only does the arithmetic; selling is regulated. In Great Britain, cosmetic products, including soap for washing skin, need a safety assessment by a qualified safety assessor and a Responsible Person before they go on sale (GOV.UK guidance), and the EU has similar rules. In the US, the FDA explains that true soap is regulated by the Consumer Product Safety Commission, while soap sold with cosmetic or drug claims falls under the FDA (FDA soap FAQ). Check the rules where you sell.