Soap Making Lye Calculator: Safe Ratios Every Time
Lye (sodium hydroxide / NaOH) reacts with oils in a chemical process called saponification to create soap. Every oil has a unique SAP value — the grams of NaOH needed to fully saponify one gram of that oil. Get the math wrong and you end up with soap that's either caustic (too much lye) or greasy (too little).
What Is Superfat?
Superfat is a built-in safety margin. A 5% superfat reduces your calculated lye amount by 5%, leaving extra moisturizing oil in the finished bar. Most cold process recipes use 5–8%. Go higher for conditioning bars, lower for cleansing bars.
Safety First: Always add lye crystals slowly to cold distilled water — never the reverse. The reaction produces heat and caustic fumes. Work in a ventilated area, wear gloves and safety glasses, and keep vinegar nearby to neutralize spills.
Soap Making FAQs
What is superfat in soap making?▼
Superfat (or lye discount) is the percentage of oil left unsaponified in your finished soap. A 5% superfat means 5% of your oils won't react with lye, leaving extra moisturizing oil in the bar. Most recipes use 5–8%.
Why must I use distilled water?▼
Tap water contains minerals and chlorine that can interfere with the saponification reaction, cause discoloration, or prevent proper trace. Always use distilled water for consistent results.
What's the difference between cold process and hot process soap?▼
Cold process soap is mixed and poured raw, then cures 4–6 weeks. Hot process soap is cooked (crockpot or oven) until saponification completes, so it can be used sooner — but the texture is more rustic.
Is this calculator safe to use?▼
Yes — the SAP values used are industry-standard. Always double-check your lye calculation before making soap, wear gloves and eye protection, and add lye to water (never water to lye).