Formuley Bench Notes
Good soap, fewer lye surprises.
Get useful checks for calculations, ingredients and batches.
Next noteThe 3 numbers to lock before you make a batch
Free Soap Lye Calculator
Calculate precise NaOH or KOH amounts for cold process and liquid soap. Select your oils, adjust superfat and water percentage, and get instant lye amounts with soap quality predictions.
Oils & Fats
g
g
g
Lye Type
5%
0%15%
%
NaOH
80.4g
Water
209.0g
Total
839g
Soap Quality Metrics(tap for details)
How long your bar lasts. Higher = harder bar that lasts longer in the shower.
How strongly the soap removes oils. Even 0 still cleans - higher just strips more oils from skin.
How moisturizing the soap feels. Higher = less drying, more skin-loving.
Size and fluffiness of lather. Higher = bigger, airier bubbles.
Density and stability of lather. Higher = thicker, longer-lasting foam.
Shelf life indicator. Lower = more stable, less prone to rancidity (DOS).
Overall balance score. In the ideal range means a well-balanced, quality bar.
Estimated cure time: 6-9 weeks
- Higher water content extends cure time
- Moderate oleic content - longer cure improves mildness
SAP values sourced from industry databases. Always verify with your supplier's specifications.
How Soap Lye Calculations Work
Understanding Saponification Values (SAP)
Every oil and fat has a unique saponification value, commonly called a SAP value. This number represents the milligrams of potassium hydroxide (KOH) required to fully saponify one gram of that specific oil. Because sodium hydroxide (NaOH) has a different molecular weight than KOH, NaOH SAP values are derived using a conversion factor of approximately 0.713. Coconut oil, for example, has a high SAP value (257 KOH / 183 NaOH) because its short-chain fatty acids react with more lye per gram, while olive oil has a lower SAP value (190 KOH / 135 NaOH) due to its predominantly long-chain oleic acid content.
How the Calculator Determines Lye Amounts
This calculator multiplies each oil's weight by its SAP value (divided by 1000 to convert from mg to g), then sums the results to determine the total lye needed. It then adjusts for your chosen lye purity (97% for NaOH, 90% for KOH, which are standard grades) and subtracts the superfat percentage. The water amount is calculated as a percentage of total oil weight -- the industry standard range is 33-40%. Adjusting any input automatically recalculates the results so you can experiment freely.
Why Accuracy Matters
A difference of even a few grams of lye can mean the difference between a gentle, skin-safe bar and one that causes irritation. Too much lye (a "lye-heavy" recipe) creates a caustic bar that can burn skin. Too little lye leaves excess unreacted oils, which can lead to soft bars that develop rancidity or dreaded orange spots (DOS) over time. Precision is non-negotiable in soap making, and a reliable calculator is the foundation of every good recipe.
Soap Making Tips & Best Practices
Superfat Percentages Explained
Superfat is the percentage of oils in your recipe that remain unsaponified after the lye has fully reacted. A 5% superfat is the most common starting point for cold process soap -- it provides a safety margin against lye-heavy errors while leaving enough free oil for a moisturizing feel. Lower superfats (2-3%) produce harder, more cleansing bars ideal for laundry soap. Higher superfats (7-10%) create richer, more conditioning bars but may reduce lather and shorten shelf life. If you're using luxury oils like argan or avocado, consider adding them at trace so they're more likely to remain as the superfatting oils.
Water Ratios and Their Effects
The water percentage (expressed as a percentage of total oil weight) affects both the working properties and the curing time of your soap. The standard range is 33-38% of oil weight. Higher water percentages (38-40%) give you more working time and a thinner batter, which is useful for intricate swirl designs. Lower water percentages (28-33%), sometimes called a "water discount," produce a thicker batter that traces faster, unmolds sooner, and requires less curing time. However, too little water can cause the batter to seize before you can pour it into molds.
Essential Lye Safety Guidelines
Sodium hydroxide and potassium hydroxide are strong caustics that can cause severe chemical burns. Always wear safety goggles (not just glasses), chemical-resistant gloves, and long sleeves when handling lye. Work in a well-ventilated area or outdoors, because the fumes released when lye dissolves in water are irritating to the lungs. The cardinal rule: always add lye to water, never water to lye -- adding water to lye can cause a violent exothermic reaction. Keep vinegar nearby to neutralize small spills, and always label and store your lye solution away from children and pets. Finally, always verify your calculations with a second independent lye calculator before making a batch.
Understanding Soap Quality Metrics
The quality metrics shown in this calculator (hardness, cleansing, conditioning, bubbly, creamy, iodine, and INS) are derived from the fatty acid profiles of your chosen oils. Hardness and INS predict bar firmness. Cleansing correlates with lauric and myristic acid content -- high-cleansing bars strip more oil from skin. Conditioning reflects the proportion of unsaturated fatty acids like oleic and ricinoleic acid. Bubbly lather comes primarily from lauric and ricinoleic acids, while creamy lather correlates with stearic, palmitic, and ricinoleic acids. Balancing these metrics helps you design a recipe that matches your target skin feel.
Save Your Soap Recipes
Sign up free to save soap calculations, track batches, and see cost-per-bar workflows inside the full platform.
Create a free account for solo work, or book a tailored team demo.