Soap Calculator

How the Soap Lye Calculator Works

This soap calculator uses saponification (SAP) values — the amount of lye required to saponify (convert to soap) exactly 1 gram of each oil. To find the total lye needed:

Lye = Σ (oil weight × SAP value) × (1 − superfat %)

Water amount = lye weight × water-to-lye ratio (default 2.5×). Yield is estimated at 75% of total batch weight (oils + lye + water), accounting for approximately 25% water evaporation during the cure period.

SAP Values — What They Mean

Each oil has a unique molecular structure that determines how much NaOH or KOH is needed for saponification. Oils high in lauric acid (coconut, palm kernel) have high SAP values because their fatty acid chains are shorter. Oils high in oleic acid (olive, canola) have lower SAP values.

• Coconut oil — SAP 0.190 (NaOH) / 0.268 (KOH). High lauric acid, hard bar, great lather
• Olive oil — SAP 0.134 / 0.189. High oleic, conditioning, slow to trace
• Castor oil — SAP 0.128 / 0.180. Adds creamy lather, use 5–10% max
• Shea butter — SAP 0.128 / 0.180. Moisturizing, soft bar contributor
• Palm oil — SAP 0.141 / 0.199. Adds hardness and stable lather

Superfat — Finding the Right Percentage

Superfat is the percentage of oils left unreacted (unsaponified) in the finished soap:

• 0–2% — very cleansing, minimal conditioning. Good for laundry soap.
• 3–5% — standard for face and body bars. Good balance of cleansing and moisture.
• 6–8% — moisturizing, good for dry skin. Slightly shorter shelf life.
• 8–12% — very conditioning, used for specialty/luxury bars. Use quickly.

The superfat discount is applied to lye — you use less lye than theoretically needed, leaving the excess oil free in the bar.

Water-to-Lye Ratio Options

The water serves as a carrier for lye and evaporates during cure. Common ratios:

• 2:1 — "water discounted" soap. Sets faster, harder bar. Less working time.
• 2.5:1 — standard. Good working time, typical cure time.
• 3:1 — more working time for complex designs. Longer cure needed.

Cold Process Soap Making — Basic Steps

1. Calculate your recipe using this lye calculator
2. Weigh all oils and melt to ~100–110°F (38–43°C)
3. Wearing PPE, carefully add lye to cold water (never water to lye) — mixture heats to ~200°F
4. Allow lye solution to cool to ~100–110°F, same temperature as oils
5. Combine oils and lye solution, blend with stick blender to "trace"
6. Add fragrance, colorants, and additives; pour into mold
7. Insulate and rest 24–48 hours; unmold and cut
8. Cure in a ventilated space for 4–6 weeks before use

NaOH vs. KOH — Choosing the Right Lye

Use NaOH (sodium hydroxide) for solid bar soap. It is widely available at hardware stores as drain cleaner (100% pure) or from soap supply vendors. Use KOH (potassium hydroxide) for liquid or soft soap — it produces a soluble soap paste that dissolves in water. KOH is commonly sold at 90% purity, so if your supplier's KOH is 90% pure, divide the calculated lye amount by 0.90 to get the actual grams to weigh out.

The saponification chemistry is the same for both — lye reacts with triglycerides (fats/oils) to produce glycerol and fatty acid salts (soap). For chemistry fundamentals like concentration calculations, see our molarity calculator. Soap making shares the same recipe-precision mindset as cooking — if you enjoy other DIY kitchen projects, our turkey calculator applies similar weight-based ratios for roasting times.