Different oils need different amounts of lye to turn into soap. That amount is the oil's SAP value (short for saponification value). The calculator adds up the SAP values for your oils, takes off your superfat, and tells you how much lye and water to weigh out.

Please double-check. Run every recipe through a second, well-known lye calculator (such as SoapCalc or Soapee) before you make it. Results here are math, not a tested recipe.

How to use it

1. Pick NaOH for bar soap or KOH for liquid soap. For KOH, enter the purity from your label. SoapCalc and Soapee both offer a 90% setting.

2. Choose grams, ounces or pounds.

3. Add your oils by weight, or by percentage plus a total weight.

4. Set your superfat (default 7%) and water (default 38% of oils, SoapCalc's default).

5. Weigh out exactly what the results say. Print or save the result as a PDF for your notes.

Superfat

Superfat is the share of oil you leave unsaponified on purpose. It gives a cushion for small differences between batches of oil, and it gives the bar a gentler feel. - 5–8% is the range The Sage says it uses most often for bar soap. Our calculator starts at 7%, inside that range, and you can change it. - At 9–10%, bars come out softer (The Sage). Soap Queen also notes that a superfat of 5% or more makes orange spots from rancid oil (DOS) more likely. - Liquid (KOH) soap recipes usually call for a lower superfat. Follow the recipe you're working from. - Don't go below 0%. SoapCalc warns that a negative superfat means extra lye, which is only for cleaning soaps.

Water: three ways to say it

All three settings describe the same thing in different ways. Pick one.

Setting

What it means

Common starting point

Water as % of oils

Water weight ÷ oil weight

38%: SoapCalc's default and ours

Lye concentration

Lye ÷ (lye + water)

25–40%, often about 33%

Water : lye ratio

Parts water for each part lye

2:1 (the same as a 33.3% concentration)

More water (lower lye concentration) gives slower trace and more time for swirls, but a longer cure, and soft bars or soda ash are more likely. Less water (a "water discount" or higher lye concentration) unmolds and cures faster, but it traces faster, so you have less time to work. For milk, beer and other liquids, see Lye Water & Other Liquids.

The bar quality numbers

For NaOH recipes, the calculator shows SoapCalc-style scores (hardness, cleansing, conditioning, bubbly, creamy, iodine and INS) along with SoapCalc's usual ranges. They're rough guides from fatty-acid profiles, not rules. A 100% olive oil castile bar falls outside several ranges and is still a classic.

How this calculator was checked

We checked 7 test recipes against three established calculators: SoapCalc, Soapee and The Sage (Majestic Mountain Sage). Lye amounts matched SoapCalc exactly and Soapee within 0.04%. They came within 0.6% of The Sage, which uses its own SAP values. The full table is in the README.

Sources - SAP values and quality scores: SoapCalc oil list and calculator. https://soapcalc.net/oil-list · https://soapcalc.net/calculator - Soapee calculator method (NaOH = KOH SAP ÷ 1.403; KOH ÷ purity). https://github.com/nazar/soapee-ui - The Sage lye calculator (independent SAP values). https://www.thesage.com/calcs/LyeCalc.html - Superfat ranges: The Sage lye calculator results page; Soap Queen, Soapy Mess-Up Quick Guide (DOS). https://soapqueen.com/bath-and-body-tutorials/tips-and-tricks/soapy-mess-quick-guide/ - Water vs. lye concentration: Modern Soapmaking, "How to Better Understand Water Discounts." https://www.modernsoapmaking.com/blog/lye-solution-in-soapmaking · Classic Bells, "Lye concentration vs water:lye ratio." https://classicbells.com/soap/waterRatioConc.asp