How to Make Cold Process Liquid Soap

Complete Guide to Making Soap Paste and Diluting to Liquid Soap

Liquid soap making uses potassium hydroxide (KOH) instead of sodium hydroxide (NaOH) to create a soap that remains liquid rather than solid. This guide covers the paste method of cold process liquid soap - making a concentrated soap paste that you dilute into liquid soap.

Liquid Soap vs. Bar Soap

Property	Bar Soap (NaOH)	Liquid Soap (KOH)
Lye Type	Sodium hydroxide (NaOH)	Potassium hydroxide (KOH)
Final State	Hard bars	Liquid/paste
Superfat	Higher (3-8%)	Lower (0-3%)
Cure Time	4-6 weeks	Dilute immediately or after rest
Usage	Direct use after cure	Requires dilution step

Why make liquid soap?

Create custom hand soaps and body washes
Control ingredients and avoid harsh detergents
More economical than buying bottles
Customize scent and properties
Natural alternative to commercial options

Safety First

KOH is more caustic than NaOH

Extra care is essential when working with potassium hydroxide!

Mandatory Safety Equipment:

Safety goggles or face shield
Heavy-duty rubber gloves (KOH can penetrate latex)
Long sleeves and long pants
Closed-toe shoes
Well-ventilated area
Vinegar nearby for neutralizing spills

Safety Rules

Always add KOH to water, NEVER water to KOH
Work in well-ventilated space (KOH fumes are strong)
Keep pets and children away
Never touch raw soap paste
KOH heats water significantly more than NaOH does

Equipment You'll Need

For Making Paste:

Material Compatibility

ONLY use stainless steel, silicone, or plastic #5 (PP) or #2 (HDPE).

•NEVER use aluminum - KOH reacts violently with it.
•Avoid glass - extreme heat can cause it to shatter.

Digital scale accurate to 0.1g
Slow cooker (recommended for consistent heat)
Stick blender (immersion blender)
Thermometer
Silicone spatulas

For Dilution:

Large pot or container
Distilled water
Strainer or cheesecloth (for clarity)
Storage bottles (pump or squeeze)

Liquid Soap Formulation

The SoapMath calculator shows five quality scores for your KOH recipe. These are purpose-built for liquid soap formulation.

SoapMath Exclusive

These five liquid soap quality scores were developed by SoapMath. You won't find them in standard soap-making references.

Cleansing — Target 10–25

Measures how effectively the soap removes oils. Too high is drying; too low leads to poor lather. Coconut and babassu are primary sources.

Lather — Target 15–35

Total lather quality. Castor oil boosts this without raising cleansing strength, adding creamy stability.

Mildness — Target 50–75

How skin-friendly the soap is. Olive, sunflower, canola, and castor oil score high here.

Clarity Risk — Aim for <15

Driven by solid fats like palmitic and stearic acid. High scores mean cloudiness and separation. Clear soap targets <8.

Dilution Ease — Aim for 40+

Predicts how easily the paste dissolves. Low scores require heat and extended stirring to dissolve.

Best Oils (High Clarity)

Coconut Oil: 20–50% (lather + cleansing)
Castor Oil: 5–10% (boosts lather, improves clarity)
Olive Oil: 40–75% (mildness, high clarity)
Sunflower/Canola Oil: 10–40% (economical mildness)

Oils to Use Sparingly

Palm Oil: max 10–15% (causes cloudiness)
Shea Butter: max 5–10% (can cause gel lumps)
Cocoa Butter: max 5% (difficult to dilute)

Oils to Avoid

Tallow/Lard: High clarity risk, separates easily
Beeswax/Mineral Oil: Won't saponify correctly

Superfat Strategy:

0% SF: Clearest soap, highest cleansing
1-2% SF: Recommended balance of clarity and mildness
3% SF: Mild but prone to clouding
Above 3%: Likely to cloud and separate

Pro Tip

Not sure which oils to pick? Check out our guided tools:

→ Help Me Pick Oils: Liquid Soap (KOH)
→ Help Me Pick %: Liquid Soap (KOH)

Step 1: Formulate Your Recipe

Select Potassium Hydroxide (KOH)

Ensure Step 1 in the SoapMath calculator is set to Potassium Hydroxide (KOH). NaOH will create a solid bar that won't dilute.

Water Calculation

For liquid soap paste, use 25-33% lye concentration (3:1 to 2:1 water:lye ratio).

Recommended: 25-30%. The extra water makes the paste much easier to blend and cook.

Batch Size

Start with 500-1000g of oils. This yield creates about 3-12kg of finished liquid soap after dilution!

Step 2: Make the Soap Paste

Prepare the Lye Solution

Weigh distilled water into a heat-safe plastic or stainless steel container. Weigh KOH separately. Slowly add KOH to the water while stirring.

Fumes and Heat

KOH generates intense heat and strong fumes. Always work in a ventilated area.

Prepare Oils

Weigh your oils into your slow cooker or large stainless steel pot. Heat until melted (approx 140°F / 60°C).

The Mix

Pour the lye solution into the oils. Use your stick blender to mix to thick trace.

Liquid soap paste takes longer to trace than bar soap. It may look like it's separating at first - keep blending!

The Cook

Cover and cook on low heat. The paste will go through several stages:

• Applesauce stage (chunky/separated)
• Mashed potato stage (thick/opaque)
• Taffy stage (very thick/translucent)

Cook for 1–4 hours until it reaches a translucent, vaseline-like appearance.

The Test

Perform a "clarity test": Dissolve 10g of paste in 20g of hot distilled water. If it's clear (not milky), it's done.

Step 3: Dilution

Once the paste is cooked, you must dilute it to make liquid soap.

Calculate Water

Weigh your total soap paste. A standard starting ratio is 1:2 (1 part paste to 2 parts water).

Combine and Wait

Add hot distilled water to the paste. Cover and let it sit. Do not stir vigorously, as this creates excess foam.

Pro Tip

Patience is best. Letting it sit overnight is the easiest way to dilute without bubbles.

Strain and Scent

Strain the soap through cheesecloth to remove any undissolved bits. Add fragrance (0.5-2%) and preservatives if desired.

Formula Inspiration

Body Wash (creamy, mild):

Dilute 1:2 or 1:3 (paste:water)
Add 2-3% glycerin
Add 1% jojoba or sweet almond oil (after dilution)

Dish Soap (high cleansing):

Use high coconut oil recipe (60-70%)
Dilute 1:4 (paste:water)
Add 1-2% washing soda for grease cutting

Troubleshooting

If…	Then…	Solution
Soap is cloudy	Excess superfat or hard water used.	Reduce superfat in next batch; always use distilled water.
Soap is too thin	Too much water added during dilution.	Add more paste or use a thickener like salt (carefully) or guar gum.
Two layers in bottle	Separation due to high superfat or heavy oils.	Shake before use; reduce superfat to 1-2% in future recipes.
Paste won't trace	Temperature too low or not enough blending.	Increase heat; stick blend in 30-second bursts until it thickens.

Tip: Calculate your liquid soap paste

Use SoapMath to calculate exact KOH and water amounts for your soap paste. Remember to set the lye type to Potassium Hydroxide!

Open SoapMath →