How to Calculate Lye for Cold Process Soap: The Complete Guide

Last updated: January 2026

Get lye calculations wrong and your soap is ruined—or dangerous. Too much lye creates harsh, caustic soap that burns skin. Too little leaves unreacted oils that go rancid within weeks.

Every year, new soap makers waste hundreds of dollars on failed batches because they guessed at lye amounts or used incorrect SAP values. This guide teaches you the exact formulas professional soap makers use, so you can create safe, balanced soap every time.

You will learn:

• The chemistry behind why different oils need different lye amounts
• How to calculate lye by hand using SAP values
• Why 5% superfat is the industry standard (and when to adjust)
• Water calculation methods that prevent trace issues
• The 6 most common calculation mistakes and how to avoid them

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What Is Saponification? (The Chemistry Made Simple)

Cold process soap is created through a chemical reaction called saponification:

Oils/Fats + Lye + Water → Soap + Glycerin

During saponification, lye (sodium hydroxide) breaks apart the triglyceride molecules in oils. The fatty acids then bond with sodium ions to form soap molecules, while glycerin is released as a byproduct.

Key insight: Each oil requires a specific amount of lye to fully saponify. This amount is called the saponification value or SAP value.

Why Different Oils Need Different Lye Amounts

SAP values vary because of differences in:

Factor	Effect on SAP Value
Fatty acid chain length	Shorter chains (coconut) = higher SAP
Molecular weight	Lighter molecules = more per gram = more lye needed
Chemical structure	Saturated vs. unsaturated affects saponification

Practical example: Coconut oil requires about 40% more lye per gram than olive oil. Use olive oil's SAP value for coconut oil, and your soap will be dangerously lye-heavy.

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NaOH vs. KOH: Which Lye Type to Use

Lye Type	Chemical Name	Used For	Result
NaOH	Sodium hydroxide	Bar soap	Hard, solid bars
KOH	Potassium hydroxide	Liquid soap	Soft paste or liquid soap

This guide focuses on NaOH for bar soap. KOH calculations follow the same principles but use different SAP values (approximately 1.4x higher).

Purity Matters

Lye Type	Typical Purity	Adjustment Needed?
NaOH	97-99%	Usually not (close enough to 100%)
KOH	85-90%	Yes—always adjust for purity

Most commercial NaOH is 97%+ pure. The Bramble Berry Lye Calculator assumes 97% purity for NaOH, which is standard for consumer-grade lye.

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The Lye Calculation Formula

For a Single Oil

Lye needed = Oil weight × SAP value × (1 - Superfat%)

For Multiple Oils

Calculate each oil separately, then add them together:

Total lye = [(Oil 1 × SAP 1) + (Oil 2 × SAP 2) + (Oil 3 × SAP 3)...] × (1 - Superfat%)

Example with 5% superfat:

Total lye at 0% superfat = 150g
With 5% superfat: 150g × 0.95 = 142.5g NaOH

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SAP Values for Common Soap Making Oils (NaOH)

The SAP value tells you how many grams of NaOH are needed to saponify one gram of oil.

Oil/Fat	NaOH SAP Value	Notes
Coconut oil (76°)	0.190	High cleansing, can be drying
Palm oil	0.141	Hardness and lather
Olive oil	0.135	Conditioning, slow trace
Castor oil	0.128	Lather booster, use at 5-10%
Shea butter	0.128	Conditioning, creamy lather
Cocoa butter	0.137	Hardness, skin feel
Sweet almond oil	0.136	Conditioning, mild
Avocado oil	0.133	Luxurious, conditioning
Sunflower oil	0.134	Light, conditioning
Rice bran oil	0.128	Antioxidant-rich
Lard	0.141	Traditional, creamy lather
Tallow (beef)	0.143	Hard bars, stable lather
Mango butter	0.137	Similar to cocoa butter
Jojoba oil	0.066	Actually a wax ester
Beeswax	0.069	Hardener, use sparingly

Source: Values compiled from From Nature With Love Saponification Chart and The Soap Kitchen.

Important: SAP values can vary by 1-3% depending on oil source, processing, and crop conditions. Always verify with a reputable soap calculator for final recipes.

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Step-by-Step Lye Calculation Example

Recipe:

• 500g olive oil (50%)
• 300g coconut oil (30%)
• 200g shea butter (20%)
• 5% superfat
• Total oils: 1000g

Step 1: Calculate Lye for Each Oil (at 0% Superfat)

Olive oil:   500g × 0.135 = 67.50g NaOH
Coconut oil: 300g × 0.190 = 57.00g NaOH
Shea butter: 200g × 0.128 = 25.60g NaOH

Step 2: Sum the Total

Total NaOH (0% superfat): 67.50 + 57.00 + 25.60 = 150.10g

Step 3: Apply Superfat Discount

With 5% superfat: 150.10g × 0.95 = 142.60g NaOH

Final Answer: 142.60g NaOH

Pro tip: Use a digital scale accurate to 0.1g for batches under 2 lbs. A 1g error in a small batch can shift your superfat by 1-2%.

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Understanding Superfat (Lye Discount)

Superfat means using less lye than required for complete saponification. This leaves some oils unsaponified in the final bar.

Why Superfat Your Soap?

Reason	Explanation
Safety margin	SAP values are averages; actual values vary batch to batch. Superfatting ensures no excess lye remains.
Skin conditioning	Unsaponified oils moisturize and condition skin
Gentler soap	Reduces potential for irritation
Better skin feel	Soap feels luxurious, not stripping

Recommended Superfat Percentages

Superfat %	Best For	Trade-offs
0%	Never recommended	No safety margin—too risky
3%	Minimum for most recipes	Functional but minimal conditioning
5%	Industry standard	Balanced safety, conditioning, and performance
7-8%	Extra moisturizing recipes	Slightly softer bar, reduced lather
10%+	Very conditioning soap	Softer bars, less lather, faster rancidity
15-20%	100% coconut oil soap only	Required to reduce coconut's harshness

Start at 5% superfat. Adjust only after you understand how it affects your specific recipes.

According to Bramble Berry, most professional soap makers superfat at 5% because it provides luxury without compromising bar hardness or lather.

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Water Calculations: Two Methods

Lye must dissolve in water before mixing with oils. Getting water amounts right affects trace speed, cure time, and bar hardness.

Method 1: Water as Percentage of Oils

Water = Total oil weight × Percentage

Water %	Result	Best For
33-38%	More forgiving, slower trace	Beginners, intricate designs
28-33%	Standard, balanced	Most recipes
25-28%	Less water, faster unmolding	Experienced soap makers

Example: 1000g oils × 33% = 330g water

Method 2: Lye Concentration (Water-to-Lye Ratio)

Water = Lye weight × (100 - Concentration%) ÷ Concentration%

Or use the simple ratio method:

Lye Concentration	Water:Lye Ratio	Notes
25%	3:1	Very dilute, slow trace, beginner-friendly
33%	2:1	Industry standard
40%	1.5:1	Fast trace, experienced makers only
50%	1:1	Maximum concentration, advanced technique

Example at 33% concentration (2:1 ratio):

142.6g lye × 2 = 285.2g water

Warning: A 50% lye solution is the physical limit—lye cannot dissolve in less water than its own weight. At this concentration, the solution is extremely dangerous and can precipitate in cool environments. According to the Handcrafted Soap Guild, most soap makers recommend staying at 40% or below.

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KOH Purity Adjustment (For Liquid Soap)

Commercial KOH is typically only 85-90% pure (the remainder is water and carbonates). You must adjust your calculations:

Actual KOH to weigh = Calculated KOH ÷ Purity%

Example:

Calculator says: 200g KOH needed
Your KOH is 90% pure
Actual amount: 200g ÷ 0.90 = 222.2g KOH

Always check your KOH container for purity percentage. Common purities are 85%, 90%, and 95%. The Classic Bells resource recommends using the minimum stated purity to be safe.

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The 6 Most Common Lye Calculation Mistakes

Mistake 1: Using KOH Values for NaOH (or Vice Versa)

KOH SAP values are approximately 40% higher than NaOH values. Using the wrong values creates dangerously lye-heavy or lye-light soap.

Prevention: Always verify which lye type the SAP value is for. When in doubt, check SoapCalc.

Mistake 2: Skipping Superfat

Calculating at 0% superfat leaves no safety margin. Since SAP values are averages (actual values vary by supplier and batch), you risk caustic soap.

Prevention: Always use at least 3-5% superfat. The extra safety costs only a few cents per batch.

Mistake 3: Not Adjusting KOH for Purity

NaOH is 97-99% pure; KOH is only 85-90% pure. Using calculated KOH amounts without purity adjustment creates lye-light liquid soap that won't thicken properly.

Prevention: Divide calculated KOH by purity percentage.

Mistake 4: Measuring by Volume Instead of Weight

A "cup" of coconut oil weighs differently than a "cup" of olive oil. Volume measurements are inaccurate for soap making.

Prevention: Use a digital scale accurate to at least 1g (0.1g for small batches). According to Certified Lye, weight-based measurements are essential for safe soap.

Mistake 5: Using Old or Degraded Lye

NaOH absorbs moisture and CO2 from air, converting to sodium carbonate (washing soda). Degraded lye is less potent, throwing off your calculations.

Prevention: Store lye in airtight containers. Replace annually. If lye has clumped or changed texture, it's degraded.

Mistake 6: Aggressive Rounding

Small errors compound. Rounding 142.6g to 140g changes your superfat by almost 2%.

Prevention: Round to nearest 0.5g. Better yet, use exact amounts.

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Quick Reference: Lye Amounts per Pound of Oil

For quick reference, here are lye amounts for common oils at 5% superfat (per 454g/1 lb of oil):

Oil	NaOH (grams)	NaOH (ounces)
Coconut oil	81.9g	2.89 oz
Palm oil	60.8g	2.14 oz
Olive oil	58.2g	2.05 oz
Shea butter	55.2g	1.95 oz
Cocoa butter	59.1g	2.08 oz
Castor oil	55.2g	1.95 oz
Lard	60.8g	2.14 oz
Tallow (beef)	61.7g	2.18 oz

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Complete Recipe Examples with Calculations

Beginner Recipe (Balanced, Forgiving)

A classic recipe for new soap makers with good hardness, lather, and conditioning:

Oil	Amount	% of Recipe	NaOH Needed (0% SF)
Olive oil	400g	40%	54.0g
Coconut oil	300g	30%	57.0g
Palm oil	200g	20%	28.2g
Castor oil	100g	10%	12.8g
Total	1000g	100%	152.0g

Final calculations:

• NaOH at 5% superfat: 152.0g × 0.95 = 144.4g NaOH
• Water (33% of oils): 1000g × 0.33 = 330g water
• Expected cure time: 4-6 weeks

Moisturizing Recipe (Higher Superfat)

For those wanting extra conditioning with a creamier lather:

Oil	Amount	% of Recipe	NaOH Needed (0% SF)
Olive oil	500g	50%	67.5g
Shea butter	200g	20%	25.6g
Coconut oil	200g	20%	38.0g
Castor oil	100g	10%	12.8g
Total	1000g	100%	143.9g

Final calculations:

• NaOH at 8% superfat: 143.9g × 0.92 = 132.4g NaOH
• Water (33% of oils): 1000g × 0.33 = 330g water
• Expected cure time: 6-8 weeks (higher olive content)

100% Coconut Oil (Laundry/Utility Soap)

Pure coconut soap requires high superfat to prevent excessive drying:

Oil	Amount	NaOH Needed (0% SF)
Coconut oil	1000g	190.0g

Final calculations:

• NaOH at 20% superfat: 190.0g × 0.80 = 152.0g NaOH
• Water (33% of oils): 1000g × 0.33 = 330g water
• Cure time: 3-4 weeks

Important: 100% coconut soap can still be drying even at 20% superfat. Many soap makers prefer limiting coconut to 30% of recipes for personal care soap.

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Why You Should Still Use a Lye Calculator

Hand calculations work, but they're:

Issue	Risk
Time-consuming	Complex recipes take 10+ minutes
Error-prone	One typo = ruined or dangerous batch
Tedious for variations	Testing changes requires full recalculation
Limited information	No data on soap qualities

What Calculators Provide

Online calculators like SoapCalc, Bramble Berry, and The Sage offer:

• Instant calculation for any oil combination
• Soap quality predictions (hardness, cleansing, conditioning, lather)
• Easy batch resizing
• Automatic superfat application
• Error checking

But Understanding the Math Matters

Even with calculators, understanding lye calculations helps you:

• Verify results: Catch calculator errors or typos
• Troubleshoot problems: Understand why a batch failed
• Make informed adjustments: Know what changing superfat actually does
• Teach others: Explain the "why" behind soap making

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Frequently Asked Questions

How accurate does my scale need to be?

For batches under 2 lbs, use a scale accurate to 0.1g. For larger batches, 1g accuracy is sufficient. Avoid kitchen scales that only measure to 5g—the margin of error is too high for safe soap making.

What happens if I add too much lye?

Your soap will be "lye heavy" or caustic. Signs include:

• Soap "zaps" when touched to tongue (feels like touching a battery)
• Causes skin irritation or burning
• May crumble or crack

If this happens: The batch cannot be fixed. Discard it safely.

What happens if I add too little lye?

Your soap will be "lye light" or superfatted beyond intention. Signs include:

• Very soft bars that don't harden
• Oily or greasy feel
• May go rancid quickly (orange spots)
• Poor lather

Mild cases: Soap is usable but won't last. Severe cases: Oils separate or soap never solidifies.

Can I substitute oils without recalculating?

Never. Each oil has a unique SAP value. Swapping coconut for olive without adjusting lye creates dangerously lye-heavy soap. Always recalculate when changing oils.

How long does lye stay good?

Properly stored (airtight, cool, dry), NaOH lasts 1-2 years. Signs of degradation:

• Clumping or hard chunks
• Texture changes
• Won't fully dissolve
• Weaker reaction (soap takes longer to trace)

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Key Takeaways

1. Every oil requires a specific lye amount based on its SAP value
2. 5% superfat is the industry standard—never use 0%
3. NaOH SAP values are different from KOH values (KOH is ~40% higher)
4. Always adjust KOH for purity (typically 85-90%)
5. Measure by weight, never by volume
6. Standard water ratio is 2:1 (water to lye) or 33% of oils
7. Store lye properly and replace annually
8. Verify calculations with a reputable calculator before making soap

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Your Next Steps

Now that you understand lye calculations, put your knowledge to work:

1. Practice the formula on paper before making soap
2. Use a lye calculator to verify your hand calculations
3. Start with a beginner recipe (the balanced recipe above is excellent)
4. Keep detailed batch notes including exact measurements

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Stop Calculating Lye in Spreadsheets

Manual lye calculations work, but they're slow and error-prone. Every batch requires recalculation, and one mistake can ruin expensive ingredients—or create unsafe soap.

PetalMade calculates lye amounts automatically for any recipe, tracks your batches from creation through cure, and monitors IFRA compliance for fragrance safety—all in one place.

What you get:

• Automatic NaOH/KOH calculations with adjustable superfat
• Recipe scaling without manual recalculation
• Batch tracking with cure date alerts
• Full ingredient cost tracking for accurate pricing

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