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Soap lye calculator

For a blend, the saponification value is the weighted average of the oils in it.

%
%
Sodium hydroxide
145.26g
= Exact Soap recipe
Water
330.0
Or potassium hydroxide
203.79
Blend SAP value
0.152 90
Total oil
1000
Lye concentration
30.56 %
Batch weight
1475
Approximate bars
13.4
Show the working
  1. First oil weight = 300
  2. Its SAP value = 0.19
  3. Second oil weight = 400
  4. Its SAP value = 0.134
  5. Third oil weight = 300
  6. Its SAP value = 0.141
  7. Superfat = 5
  8. Water as a share of oil = 33
  9. Total oil = 300 + 400 + 300 = 1000
  10. full_lye = 300*0.19 + 400*0.134 + 300*0.141 = 152.9
  11. Blend SAP value = 1000 > 0 ? 152.9 / 1000 : 0 = 0.1529
  12. Sodium hydroxide = 152.9 * (1 - 5/100) = 145.255
  13. Water = 1000 * 33/100 = 330
  14. Lye concentration = (330 + 145.255) > 0 ? 145.255/(330+145.255)*100 : 0 = 30.5636
  15. Batch weight = 1000 + 330 + 145.255 = 1475.26
  16. Or potassium hydroxide = 145.255 * 1.403 = 203.793
  17. Approximate bars = 1475.26 / 110 = 13.4114

Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.

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    The formula

    • lye = sum of (oil weight x its SAP value) x (1 - superfat)

    Sodium hydroxide is caustic and generates heat on contact with water. Always add lye to water, never the reverse, and wear eye protection and gloves. Run any recipe through a second calculator before making it.

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