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Soap Lye Calculator
Calculate precise NaOH or KOH amounts, water ratios, and 7 soap quality metrics for cold process, hot process, and liquid soap recipes. Includes cure time estimates.
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 Calculations Work
Understanding the science behind your soap recipe helps you create better, more consistent results.
SAP Values
Every oil has a saponification (SAP) value that tells us exactly how much lye is needed to convert it into soap. Our calculator uses verified SAP values for each oil to determine the precise lye amount for your recipe.
Superfat & Water
Superfatting reduces the lye below 100% to leave free oils for moisturizing. Water dissolves the lye and controls the consistency of your soap batter. We calculate both based on your chosen percentages.
Quality Metrics
We calculate 7 quality metrics from the weighted fatty acid profile of your oil blend. These predictions help you balance hardness, lather, conditioning, and shelf stability before you make a single batch.
What This Calculator Measures
7 quality metrics calculated from the fatty acid profile of your oil blend.
Hardness
Ideal: 29-54
How long your bar lasts. Controlled by lauric, myristic, palmitic, and stearic acids.
Cleansing
Ideal: 12-22
How effectively the soap strips oils from skin. Driven by lauric and myristic acids.
Conditioning
Ideal: 44-69
How moisturizing and gentle the soap feels. From oleic, linoleic, and ricinoleic acids.
Bubbly
Ideal: 14-46
Size and fluffiness of the lather. Lauric, myristic, and ricinoleic acids contribute.
Creamy
Ideal: 16-48
Richness and stability of the foam. Created by palmitic, stearic, and ricinoleic acids.
Iodine
Ideal: 41-70
Predicts shelf life. Lower values mean less unsaturation and better resistance to rancidity.
INS
Ideal: 136-170
Overall balance score combining hardness and iodine. Values in range indicate a well-rounded bar.
Frequently Asked Questions
What is a SAP value?+
SAP (saponification) value is the number of milligrams of potassium hydroxide (KOH) required to saponify one gram of fat or oil. Each oil has a unique SAP value that determines how much lye is needed to convert it into soap. NaOH SAP values are derived from KOH values by dividing by 1.403.
What does superfat mean in soap making?+
Superfatting means using less lye than needed to convert all oils to soap, leaving some unreacted oils in the final bar. A 5% superfat means 5% of the oils remain as free oils, adding moisturizing properties. Most cold process recipes use 3-8% superfat.
What is the difference between NaOH and KOH in soap making?+
NaOH (sodium hydroxide, or caustic soda) produces solid bar soap. KOH (potassium hydroxide, or caustic potash) produces liquid or cream soap. KOH requires about 1.4x more lye by weight than NaOH for the same amount of oils because of its higher molecular weight.
Why do soap quality metrics matter?+
Soap quality metrics (hardness, cleansing, conditioning, bubbly, creamy, iodine, and INS) help you predict how your soap will perform before making it. They are calculated from the fatty acid profile of your oil blend and help you balance properties like lather, moisturizing ability, and bar longevity.
How long should soap cure?+
Most cold process soap needs 4-6 weeks of cure time. During curing, excess water evaporates, making the bar harder, milder, and longer-lasting. High-oleic recipes like castile soap benefit from 6-12 weeks. Factors like water content, oil hardness, and fatty acid profile all affect cure time.
Can I use this calculator for hot process soap?+
Yes! The lye and water calculations are the same for hot process and cold process soap. The main difference is that hot process soap is cooked until saponification is complete, so it can technically be used sooner, though a 1-2 week cure still improves quality. The quality metrics apply to both methods.
Inside Formuley
This Calculator, Supercharged
The free version is great for one-off recipes. Inside Formuley, it becomes part of your full formulation workflow.
Auto-fill oils from your saved formulas
Save results and link to batches
AI-generated soap recipes with cost analysis
Import recipes from SoapCalc
Cure time tracking with reminders
200+ oils with verified SAP values
Free plan available. Optional Pro trial.
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Save your soap recipes, track costs per batch, import from SoapCalc, and get cure reminders, all in one place.
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