How to Make Cold Process Bar Soap
A Complete Guide
Making your own cold process soap is a rewarding craft that combines chemistry with creativity. This guide will walk you through the entire process, from selecting your ingredients to cutting your finished bars. We'll be using the SoapMath calculator to ensure your recipe is perfectly balanced and safe.
Safety First: Essential Precautions
Before you begin, understand that soapmaking involves working with lye (sodium hydroxide or potassium hydroxide), a caustic substance that requires careful handling.
Safety Equipment You Need:
- Safety goggles or face shield
- Rubber or nitrile gloves
- Long sleeves and closed-toe shoes
- Well-ventilated workspace
- Vinegar nearby (neutralizes lye spills)
Safety Rules:
- Always add lye to water, never water to lye
- Work in a well-ventilated area
- Keep children and pets away
- Have a clear workspace free of distractions
- Never touch raw soap batter with bare hands
Equipment You'll Need
For Mixing:
Heat-safe containers - ALWAYS use stainless steel, silicone, or plastic #5 (PP) or #2 (HDPE)
- NEVER use aluminum - lye reacts with aluminum and can release toxic fumes
- Avoid glass - it can crack or shatter from the heat of the lye solution
- Look for the recycling symbol on plastic containers to verify it's #2 or #5
- Digital scale accurate to 0.1g
- Stick blender (immersion blender)
- Thermometer
- Silicone spatulas
- Soap mold (silicone, wood, or plastic)
For Cleanup:
- Paper towels
- Dish soap
- Dedicated soap-making utensils (don't use for food afterward)
Understanding the Soapmaking Process
Cold process soapmaking works through a chemical reaction called saponification. When oils (fats) combine with lye dissolved in water, they transform into soap and glycerin. The process generates heat naturally, which is why it's called "cold process" - you don't need to apply external heat during mixing.
Key Terms:
- Lye: The alkaline substance (NaOH for bar soap, KOH for liquid soap) that reacts with oils
- Saponification: The chemical reaction that creates soap
- Trace: The point when oils and lye water emulsify into soap batter (looks like thin pudding)
- Cure Time: The 4-6 week waiting period that allows water to evaporate and soap to harden
Step 1: Formulate Your Recipe with SoapMath Calculator
Navigate to the SoapMath calculator and follow these steps:
1.1 Select Your Lye Type (Step 1 in Calculator)
Choose between:
- Sodium Hydroxide (NaOH): For solid bar soap (most common)
- Potassium Hydroxide (KOH): For liquid soap
- Dual Lye (NaOH + KOH): For cream bars and shaving soaps - see our Dual Lye Soap Guide
For your first batch, select Sodium Hydroxide.
NaOH Purity:
The calculator defaults to 100% NaOH purity. If your NaOH is lower purity (usually 97-100%, check the label), adjust this setting accordingly. The calculator will compensate by increasing the lye amount.
1.2 Calculate Your Water Amount
The calculator offers three methods - note that lye concentration and water:lye ratio express the same information in different ways:
Lye Concentration (Recommended):
This method expresses your lye solution as a percentage. A 33% lye concentration means your solution is 33% lye and 67% water. This is the most intuitive method for most soapmakers.
Water:Lye Ratio:
This expresses the exact same relationship as a ratio. A 33% lye concentration equals a 2:1 water:lye ratio (2 parts water to 1 part lye). A 40% lye concentration equals a 1.5:1 ratio. These are the same number expressed differently - the calculator converts between them automatically.
Important safety constraint:
Lye requires at least a 1:1 solution (50% lye concentration) to dissolve properly. The calculator won't let you go beyond this safe limit. Most soapmakers work between 30-38% lye concentration (3.3:1 to 1.6:1 water:lye ratio).
Practical guidelines:
- 33% lye concentration (2:1 ratio): Standard starting point - balanced working time and cure time
- 30-31% concentration (2.3:1 ratio): More water = slower trace, more working time for designs, longer cure
- 35-38% concentration (1.85:1 to 1.6:1 ratio): Less water = faster trace, quicker to unmold, shorter cure time
- 40% concentration (1.5:1 ratio): Water discount for experienced soapmakers, very fast trace
- 50% concentration (1:1 ratio): Maximum concentration - lye barely dissolves, only for advanced techniques
Water as % of Oils: An older method where water equals a percentage of your total oil weight (typically 38%). This works but doesn't account for different lye amounts needed by different oils, so it can create inconsistent water levels.
For your first batch, use 33% lye concentration - it's reliable, safe, and gives you good working time.
1.3 Add Superfat and Fragrance
Superfat %:
This is the percentage of oils left unsaponified (not turned into soap). These extra oils make your soap moisturizing rather than stripping.
- Standard superfat: 5%
- For dry skin: 7-8%
- For cleansing bars: 3%
- For salt bars: 15-20%
Fragrance % (of oils):
How much essential oil or fragrance oil to add, calculated as a percentage of your total oil weight.
- Light scent: 2-3%
- Medium scent: 3-5%
- Strong scent: 5-6%
Important fragrance safety guidelines:
- Always follow IFRA limits for Category 9 (soap). Check the IFRA certificate for your fragrance oil - it will list the maximum safe usage level.
- If the IFRA limit is very high (like 30% or more), this doesn't mean you should use that much - it just means the fragrance is safe when diluted to that level. In reality, you'll use 2-6% for a well-scented soap.
- Essential oils also have maximum safe usage rates - research each one before use.
- Some fragrances accelerate trace or cause discoloration - always check reviews or supplier notes.
1.4 Determine Your Batch Size Using the Mold Calculator
Your batch size should be based on your mold capacity. Don't know how much your mold holds? Use the Mold Calculator to determine exactly how big of a batch to make!
Click "Mold Calculator" in the calculator interface.
Select your mold shape:
- Cube/Rectangle (most common - loaf molds, slab molds)
- Cylinder (column molds, PVC pipe molds)
Enter your mold dimensions:
- Choose your Dimension Unit (inches, cm, mm)
- Enter Length, Width, and Height of your mold's interior space
Click "Calculate" and the calculator will tell you exactly how many grams of oils you need to fill your mold perfectly.
Example:
- Mold dimensions: 10 inches (L) × 3 inches (W) × 2.5 inches (H)
- Calculator output: "Use 1150g total oils"
- Enter 1150g in Step 4: Oil Weight
Pro tips:
- Measure the inside dimensions of your mold, not the outside
- For loaf molds, height is how tall you want your soap (usually 2-3 inches)
- Round down slightly (by 50-100g) if you want to avoid overflow
- Save your mold dimensions - you'll use them again!
If you don't have a specific mold yet: Start with 500-1000g of oils for a beginner batch. This creates approximately:
- 500g oils = 3-5 bars
- 1000g oils = 6-8 bars
- 1500g oils = 9-12 bars
1.5 Additives, Colorants & Water Replacements (Step 5 in Calculator - Optional)
After setting your oil weight, Step 5 offers optional enhancements for your soap:
Citric Acid (always visible):
Enter 1-2% of oils to create sodium citrate, which reduces soap scum and protects against DOS (rancidity). The calculator automatically adds extra lye to neutralize the acid.
Water Replacements:
Check "I want to use a water replacement" to substitute part of your water with milk, aloe vera, tea, or other liquids. Select the type and percentage to replace (e.g., 50% goat milk).
Additives:
Check "I want to input additives" to add ingredients like honey, clays, sodium lactate, oatmeal, and more. Input by teaspoon, tablespoon, weight, or percentage of oils.
Colorants:
Check "I want to add colorant" to include micas, oxides, or pigments. The INCI list will include a "May Contain" statement for common colorant ingredients.
For your first batch, you can skip these optional additions and focus on the basic recipe.
1.6 Select Your Oils (Step 6 in Calculator)
Click "Select an oil" and add oils from the dropdown. Build a balanced recipe using these guidelines:
Hard Oils (30-50% of recipe) - Create hardness and longevity:
- Coconut Oil: 15-30% (high cleansing, creates lather, can be drying above 30%)
- Palm Oil: 20-30% (hardness, stable lather)
- Cocoa Butter: 10-15% (hardness, conditioning)
- Shea Butter: 5-30% (conditioning, creamy lather)
Soft Oils (50-70% of recipe) - Create conditioning and moisturizing properties:
- Olive Oil: 30-70% (conditioning, mild, takes longer to cure)
- Avocado Oil: 10-30% (conditioning, creamy lather)
- Sweet Almond Oil: 10-20% (conditioning, mild)
- Sunflower Oil: 10-15% (conditioning, light)
Specialty Oils (5-10% of recipe) - Add unique properties:
- Castor Oil: 5-10% (boosts lather, adds gloss)
- Jojoba Oil: 5-10% (long-lasting conditioning)
Beginner-Friendly Recipe Example:
- Olive Oil: 40%
- Coconut Oil: 25%
- Palm Oil: 25%
- Castor Oil: 10%
Click "Add" after selecting each oil. The calculator will show your recipe's properties including hardness, cleansing, conditioning, lather, and stability.
1.7 Calculate Your Formula
Once all oils are added, click "Calculate Formula." The calculator will display:
- Exact weight of each oil
- Water needed
- Lye needed
- Fragrance amount
- Recipe properties and SAP values
Understanding SAP Values: The saponification value shows how much lye is needed to fully convert each oil to soap. The calculator uses these values to determine your exact lye amount. Different lyes (NaOH vs KOH) have different SAP values for the same oil - this is why you must always recalculate if you change lye type.
Step 2: Prepare Your Workspace
- Clear your workspace - Remove anything unnecessary
- Lay out all equipment - Have everything within reach
- Measure out all oils - Weigh each oil precisely using your digital scale
- Set up your mold - Line wooden molds with parchment paper or use silicone molds as-is
- Put on safety gear - Goggles, gloves, long sleeves
Step 3: Make Your Lye Solution
CRITICAL: Always add lye to water, never water to lye!
- Weigh your water in a heat-safe container (the lye solution will heat to 180-200°F/82-93°C)
- Weigh your lye in a separate container
- Take containers outside or to a well-ventilated area (lye fumes are strong initially)
- Slowly pour lye into water while stirring gently with a silicone spatula
- Stir until completely dissolved (liquid will be clear)
- Set aside to cool to 100-110°F (38-43°C) - this takes 30-45 minutes
What you'll notice:
- Solution becomes very hot immediately
- Strong chemical smell (dissipates quickly)
- Liquid turns cloudy then clear as lye dissolves
Step 4: Prepare Your Oils
- Combine all oils in a large stainless steel pot or heat-safe container
- Gently heat solid oils until just melted (if using palm, coconut, or butters)
- Cool oils to 100-110°F (38-43°C) to match your lye solution temperature
Temperature matching tip: Both lye solution and oils should be within 10°F of each other. Room temperature soapmaking (90-100°F) gives you more working time, while hotter temperatures (120-130°F) speed trace.
Step 5: Mix Oils and Lye
- Double-check temperatures - Both should be 100-110°F
- Pour lye solution into oils (wear goggles and gloves!)
- Start stick blending - Pulse in short bursts (3-5 seconds on, 5 seconds stirring with blender off)
- Watch for trace - The mixture will go from thin and watery to the consistency of thin pudding
Trace stages:
- Light trace: Mixture is emulsified but still quite thin (like cream). Best for intricate swirls.
- Medium trace: Mixture has thickened to pudding consistency. Leaves a faint trail on surface. Good for most designs.
- Thick trace: Very thick, like spackle or thick pudding. Good for textured tops but hard to pour.
For your first batch: Stop at medium trace - you should be able to drizzle the batter and see it sit on top of the surface briefly before sinking in.
Step 6: Add Fragrance and Additives
Once you reach light to medium trace:
- Add your fragrance (essential oils or fragrance oils)
- Stir or briefly pulse with stick blender to incorporate
- Add any colorants (micas, oxides, clays) and mix thoroughly
- Add any other additives (exfoliants, botanicals) and stir in
Warning: Some fragrances accelerate trace (make it thicken faster). For your first batch, consider skipping fragrance or using a well-behaved option like lavender essential oil.
Step 7: Pour into Mold
- Pour soap batter into mold - It should be thick enough to support swirls but thin enough to pour smoothly
- Tap mold on counter - Removes air bubbles
- Create texture or design on top (optional) - Use a spatula, spoon, or leave smooth
- Cover with cardboard or wooden board - Insulates the soap
Step 8: Insulation and Gel Phase
Gel phase is when the center of your soap heats up and becomes translucent. It's optional but creates brighter colors and slightly faster saponification.
To encourage gel phase:
- Cover mold with a towel or blanket
- Place in a warm spot (like an oven with just the light on)
To prevent gel phase:
- Don't insulate
- Place in refrigerator for 24 hours
For beginners: Let it do whatever it wants - both gelled and ungelled soap are perfectly fine.
Step 9: Unmolding
Wait 24-48 hours before unmolding. Your soap should be:
- Firm enough to touch without denting
- Cool to the touch
- Easy to release from the mold
If it's still soft, wait another 12-24 hours. Cold process soap can take up to 72 hours to firm up in some cases.
Unmolding tips:
- Silicone molds: Peel away the mold from the soap
- Wooden molds: Remove liner with soap inside, then peel away paper
- If stuck: Place in freezer for 30 minutes, then try again
Step 10: Cutting Your Soap
Use a soap cutter, knife, or dough scraper to cut bars.
Standard bar sizes:
- 1 inch thick (25mm) - Standard full-size bar
- 0.75 inches thick (19mm) - Guest-size bar
Cutting tips:
- Cut on a clean cutting board
- Wipe blade between cuts for clean edges
- If soap crumbles, it may be too dry - wait less time before cutting next time
- If soap is too soft to cut cleanly, wait another 12-24 hours
Step 11: Curing Your Soap
Why cure?
Fresh soap is chemically safe to use after 24-48 hours (saponification is complete), but it needs to cure for optimal performance:
- Water evaporates, making bars harder and longer-lasting
- Lye molecules continue to neutralize fully
- Bars become milder and less likely to irritate skin
How to cure:
- Place bars on a rack with space between each bar for airflow
- Keep in a cool, dry location away from direct sunlight
- Flip bars weekly to ensure even air exposure
- Wait 4-6 weeks minimum
Cure times by recipe:
- Standard recipe: 4-6 weeks
- High olive oil (Castile): 8-12 weeks
- Salt bars: 6-8 weeks minimum
- Coconut oil heavy: 4 weeks
How to tell if soap is cured:
- No longer loses weight (weigh weekly - when weight stabilizes, it's done)
- Feels hard and dry
- pH around 9-10 (test with pH strips)
Troubleshooting Common Issues
| If you see this... | What's happening... | Solution |
|---|---|---|
| Soap won't trace | Not blended enough, temperature too low, or too much liquid oil | Keep mixing longer; warm oils and lye solution slightly; next batch reduce soft oils to 60% or less |
| Soap traced too fast | Fragrance accelerated trace, temperature too high, or over-blended | Research fragrance behavior before using; mix at lower temperatures (90-100°F); use more hand stirring, less stick blending |
| White film on top (soda ash) | Reaction with air during cure | Prevent: spray top with 91% isopropyl alcohol immediately after pouring, cover mold quickly, or use less water. Remove: wash bars with water or steam them |
| Soap is soft and won't unmold | Not enough hard oils, too much water, or needs more time | Wait another 12-24 hours; next batch increase coconut/palm/butters to 35-40% or use higher lye concentration (35-38%) |
| Oil pockets or greasy spots | Not mixed thoroughly, mismeasured ingredients, or false trace | Blend more thoroughly at trace (3-5 minutes minimum); always double-check scale measurements; ensure oils and lye are same temperature before mixing |
| Crumbly or cracked | Too much lye, not enough water, or overheated | Recheck calculator - did you enter everything correctly? Next batch use lower lye concentration (30-33%); don't insulate if soap is overheating |
| Liquid separating out | Lye heavy (too much lye) or recipe imbalance | Test pH - if above 11, DO NOT USE (unsafe); recheck all calculator inputs; ensure superfat is at least 3-5% |
| Volcano/overflow in mold | Overheated from gel phase or reactive fragrance | Don't insulate; place in cool location or refrigerator; avoid oven; research fragrance before use; reduce fragrance amount |
| Streaks or swirls of color when not intended | Partial gel (edges gelled, center didn't) | Cosmetic issue only - safe to use; prevent by either fully insulating (towels + warm spot) or refrigerating to prevent gel entirely |
| Soap is sticky or won't harden | Not enough lye or too much superfat | Recheck calculations; wait 4-6 weeks - sometimes resolves with cure; if still sticky after 6 weeks, likely too much superfat (rebatch or use as laundry soap) |
| White spots inside soap | Undissolved lye or stearic spots from hard oils | If hard and crystalline = likely undissolved lye (don't use); if soft and fatty = stearic spots (safe to use); prevent by stirring lye water until completely clear and mixing at proper temperature |
Testing Your Soap's Safety
Before using your soap, test it:
- Visual inspection - Should be uniform in color and texture
- Zap test - Touch tongue to a small piece (yes, really) - if it "zaps" like a 9V battery, it's still lye heavy. Wait another week and retest.
- pH test - Use pH strips. Soap should be 9-10. Above 11 indicates lye excess.
- Lather test - Does it lather well? How does it feel on skin?
Tips for Success
For your first batch:
- Keep it simple - use an easy recipe without colors or swirls
- Don't overheat - work around 100-110°F
- Don't over-blend - you have more time than you think
- Take notes - write down what you did so you can replicate success
As you gain experience:
- Experiment with different oil combinations
- Try natural colorants (clays, plant powders, micas)
- Learn swirling techniques
- Create custom fragrance blends
- Try milk soaps, beer soaps, or other specialty recipes
Frequently Asked Questions
Tip: Use the Help Me Pick % Tool
Not sure what oil percentages to use? The Help Me Pick % tool in the Soap Calculator analyzes your selected oils and recommends optimal percentages based on your hardness and lather preferences.