Why do soap calculators give different numbers?
Usually because the calculators aren't being asked the same question: a different superfat, lye purity or water setting is the most common cause. The rest comes from data: each calculator uses its own SAP value and fatty-acid profile for each oil, and those come from different sources. Small differences in lye, a few tenths of a percent, are normal; big ones mean a setting or an oil doesn't match.
1. Settings that don't match
Check these first. Each one changes the lye or water on its own, worked out here by Soap Ledger's engine:
| Setting | Result | Changed to | Result |
|---|---|---|---|
| Superfat 5% | 138.5 g NaOH | Superfat 0% | 145.8 g NaOH |
| KOH purity 100% (liquid soap, 3% superfat) | 198.4 g KOH | KOH purity 90% | 220.5 g KOH |
| Water: 33% lye concentration | 281.3 g water | Water: 38% of oils | 380.0 g water |
Example recipe: 1000 g of oils (Olive oil 50%, Coconut oil 25%, Shea butter 20%, Castor oil 5%). Some calculators default to 100% purity, some to 90% for KOH; some set water as % of oils and some as a concentration.
2. Different SAP values
Published SAP values for the same oil differ, and every calculator picks its own. Soap Ledger uses 256.5 mg KOH/g for Coconut oil, from Codex Alimentarius CXS 210-1999, Standard for Named Vegetable Oils (Table 2: chemical and physical characteristics); the oil's page lists the other published values we checked. These differences usually move the lye by well under a percent. A large difference often means the two calculators picked different oils (refined versus virgin, or a different cut of palm or tallow).
3. Soap-quality numbers
Hardness, cleansing, conditioning and the other quality numbers add up fatty acids, so they depend on each calculator's fatty-acid profiles, which also come from different sources. Makers regularly notice that the same recipe scores differently in two calculators (Soap Making Forum: SoapCalc and Soapmaking Friend numbers differ). Compare quality numbers within one calculator, not across two.
4. Rounding and units
Some calculators round SAP values or results early, or convert ounces with a rounded factor. Soap Ledger keeps exact decimals and rounds only for display.
How to check
- Put the same oils, weights, superfat, purity and water setting into both.
- Compare the lye before superfat; if that differs, compare each oil's SAP value.
- If one oil is far off, check that it's the same oil (and the same form of it).
- When in doubt, use your supplier's certificate of analysis for the SAP value.
Soap Ledger shows the SAP value and its source for every oil under each result, so you can see where its number comes from.
Lye safety. Verify every weight before you make this, and follow lye safety. Not a safety or compliance certification. Eye protection, gloves and covered skin; ventilation and a clear, uninterrupted workspace.
Try it in the lye calculator. Open the method under any result to see each oil's SAP value and source.
Sources
- Soap Ledger oil reference (sources for every SAP value)
- Soap Making Forum: SoapCalc and Soapmaking Friend numbers differ
- Soap Ledger free lye calculator
Written for Soap Ledger in our own words. Numbers in the examples are calculated by Soap Ledger's engine when the page is built. Last updated .