How to Use the Soap Calculator

What this tool does

The calculator computes lye (NaOH or KOH) and water for a given oil blend, taking superfat and lye purity into account. All inputs are in grams.

Safety first: always wear PPE (gloves, goggles), add lye to water (never water to lye), and verify SAP values for unusual oils.

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Quick start (5 steps)

1. Add oils
   • Pick from “Add common oil” or add your own under “Add custom oil” (enter its NaOH SAP).
2. Enter weights (g) for each oil in the table.
3. Choose Lye type: NaOH (bar soap) or KOH (mostly liquid soap).
4. Set Superfat (%) (common range: 3–8%).
5. Set Lye purity (typical: NaOH 98–100%, KOH 85–95%) and a Water setting:
   • % of oils (e.g., 30–33%) or Lye concentration (e.g., 28–33%).
     The results update instantly: Lye required, Water required, Total batch, plus an Oil breakdown.

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Inputs explained

• Oil – Name of the oil/fat/butter.
• NaOH SAP – Grams NaOH needed per gram oil. For KOH, the calculator converts internally (≈ NaOH SAP × 1.403).
• Weight (g) – Oil mass in grams.
• Lye type – NaOH for bar soap, KOH for liquid/soft soaps.
• Superfat (%) – Extra unsaponified oils for conditioning (reduces lye accordingly).
• Lye purity (%) – Adjusts for pellet purity (enter what’s on your label).
• Water as % of oils – Simple method: water = oils × %.
• Lye concentration (%) – Advanced method: % of lye in the lye solution (L/(L+W)).

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Worked example

Blend: Olive Oil 500 g + Coconut Oil 300 g
Settings: NaOH, Superfat 5%, Lye purity 100%

• Lye (before SF):
  • Olive: 500 g × 0.134 = 67.00 g
  • Coconut: 300 g × 0.183 = 54.90 g
  • Total = 121.90 g
• Lye (after 5% SF): 115.81 g

Water option A — % of oils (33%):

• Water = 800 g × 0.33 = 264.00 g
• Total batch = 800 + 115.81 + 264.00 = 1,179.81 g

Water option B — Lye concentration (30%):

• Water = 115.81 × (1−0.30)/0.30 = 270.21 g
• Total batch = 1,186.02 g

(Rounded to 2 decimals; your screen will round similarly.)

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Interpreting results

• Lye required – Weigh this exact amount of the chosen lye type.
• Water required – Mix with lye to make your lye solution (cool before combining with oils).
• Total batch – Oils + lye + water (pre-cure weight).
• Oil breakdown – Percentage of each oil in the blend (helps compare recipes).

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Practical tips

• Typical ranges
  • Superfat: 3–8% (0–3% for laundry; 8–12% for extra conditioning).
  • Water: 28–33% lye concentration or ~28–35% of oils.
• Trace & hardness
  • More water → slower trace, longer cure. Less water → faster trace, harder bar sooner.
• Additives (fragrance, clays, milks) are not included in the math; add them per supplier guidance.
• Record keeping – Save the oil names, SAPs, and batch numbers for repeatability.
• KOH vs NaOH – Don’t swap them without changing the lye type; the math differs.

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Troubleshooting

• Soft or weepy soap: Too much water, inaccurate scale, or over-superfatting. Reduce water or recheck weights.
• Seizing or instant trace: High solid fats, high temperature, or accelerating fragrance. Lower temps or water discount less.
• Didn’t set: Wrong lye type/purity or wrong SAP value—verify inputs.

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FAQ

• Grams only? Yes. To convert: 1 g = 0.035274 oz (1 oz = 28.3495 g).
• Where do I find SAP values? Reputable soapmaking references or supplier data sheets (values vary slightly by source and oil lot).
• Liquid soap? Choose KOH, set appropriate superfat (often lower), and use a reasonable lye concentration.