Soap Lye Amount
Calculator
Results
- Lye at full saponification (g)
- 138
- Lye needed (g)
- 131.1
- Water needed (g)
- 301.53
- Total batch weight (g)
- 1,432.629999
Craft results
| Lye at full saponification (g) | 138 |
| Lye needed (g) | 131.1 |
| Water needed (g) | 301.53 |
| Total batch weight (g) | 1,432.629999 |
formula-map diagram
- Lye at full saponification (g)
- 138
- Lye needed (g)
- 131.1
- Water needed (g)
- 301.53
- Total batch weight (g)
- 1,432.629999
Formula breakdown
Formula
NaOH = oil weight × SAP × (1 − superfat)= 138
Note
Simplified model: these are standard craft estimates based on rectangular layouts, quoted gauge and nominal densities. Real fabric nap, directional prints, shrinkage, pattern matching, yarn lots and supplier SAP values vary, so always make a swatch or test piece and buy a little extra.
More in Crafts and sewing
See all →Frequently asked questions
Why is the lye amount tied to a specific oil, not just the batch size?+
Each oil has its own saponification value (SAP value), the specific amount of lye needed to fully convert one unit of that oil into soap, so the same batch weight of coconut oil needs a different lye amount than the same weight of olive oil.
What does 'superfat' mean and why does it reduce the lye amount?+
Superfatting means adding slightly less lye than the full SAP value calls for, so a small percentage of oil remains unreacted in the finished soap for skin conditioning; a 5% superfat, for example, uses 95% of the calculated full-saponification lye amount.
Why does this calculation matter so much for safety, not just soap quality?+
Too little lye leaves excess unreacted oil (which can go rancid and make a soft, greasy bar), but too much lye leaves free caustic lye in the finished soap, which is a genuine skin hazard; always run the recipe through a lye calculator rather than estimating.
Does using a blend of oils complicate the calculation?+
Yes, each oil in the blend contributes its own SAP value proportional to its weight in the recipe, so the total lye amount is the sum of each oil's weight times its own SAP value, then adjusted for the chosen superfat percentage.
Why does the water amount matter alongside the lye amount?+
Water dissolves the lye and affects trace time and how the soap behaves in the mold, but it doesn't change the required lye weight itself, since that's determined purely by oil weight, SAP value, and superfat percentage.