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While I’ve long enjoyed making homemade lotion, lip balm, toothpaste, face serum, hair serum, and all other awesome homemade personal care products, I was initially a bit daunted by the idea of making Castile soap. I first tried making a Castile soap bar, which turned out wonderfully — the process was easier than expected, though not without a few lessons learned. Eventually, I decided to experiment with liquid Castile soap, and to my pleasant surprise, it’s also quite straightforward!

In this post, I’ll share everything you need to know to make your own organic, all-natural, and super healthy liquid Castile soap at home.

Ingredients You’ll Need (and the Science Behind Soap-making)

To make liquid Castile soap, you’ll need Potassium Hydroxide (KOH) — not to be confused with Sodium Hydroxide (NaOH), which is used for solid soap bars. KOH is what makes soap stay liquid. It’s typically sold at around 90% purity, so you’ll need to account for that in your measurements.

Next, you’ll need your oils. My favorite combination is sweet almond oil, olive oil, and coconut oil — each brings unique qualities to the soap.

homemade liquid Castile Soap Ingredients

The Science: Saponification and SAP Values

Soapmaking relies on a chemical reaction called saponification, where the alkali (KOH) reacts with oils to form soap and glycerin. The amount of KOH required depends on each oil’s saponification value (SAP value) — the ratio of oil to alkali needed for complete saponification.

Each oil has a slightly different SAP value, so accuracy matters.

Oil	SAP	Weight (oz)	KOH Needed (oz)
Olive oil	0.190	10	1.9
Sweet almond oil	0.190	10	1.9
Coconut oil	0.257	5	1.29
Total (100% KOH)			5.09 oz

Since commercial KOH is usually 90% pure, divide by 0.9 to get the true amount needed:
5.09 ÷ 0.9 = 5.65 oz, which you can round down slightly to 5.5 oz for a gentle (superfatted) soap.

For water, use a 3:1 water-to-KOH ratio, or about 15–16 oz of distilled water.

Tools You’ll Need

3-quart slow cooker – perfect for this recipe quantity
4-cup glass measuring cup – for heating and melting oils
Silicone spatulas – for storing and scraping
Immersion blender – essential for blending oils and lye solution smoothly

How to Make Homemade Liquid Castile Soap

Yield: Liquid Castile Soap

Prep Time: 5 minutes

Active Time: 25 minutes

Additional Time: 5 hours

Total Time: 5 hours 30 minutes

Difficulty: medium

Homemade Liquid Castile Soap is healthy and straightforward to make, as long as you know a few "know-hows".

Materials

Potassium Hydroxide (KOH), 5 oz if 100%, 5.5 oz if it is 90%
Distilled water, 16 oz
Sweet almond oil, organic cold pressed preferred, 10 oz
Olive oil, 10 oz
Coconut oil, 5 oz
Essential oils - optional, a few drops, such as lemon, orange, etc.

Tools

3-quart slow cooker
4-cup glass measuring cup
Silicon spatulas
kitchen scale

Instructions

Making the Liquid Castile Soap Concentration

Step 1 - Dissolve the KOH. Measure and add 16 oz distilled water in the ceramic pot of the slow cooker. Slowly add KOH to distilled water (never the other way around), stirring until fully dissolved before adding more. Be cautious - this process generates significant heat.
Step 2 - heat the oils. Melt and combine the oils in a heat-safe galss measuring cup. I use a 4-cup glass measuring cup. Put it on a steaming rack in a deep frying pan with boiling water. Stir gently until the coconut oil is fully melted and blended.
Step 3 - Combine the oil and lye solution. Gradually add the warm oil mixture to the dissolved KOH solution while blending gently.
Step 4 - Bend until uniform. Use the immersion blender to mix thoroughly until the mixture reaches a consistency, thick texture - similar to pudding. It worth mentioning that it is OK if the consistency is not there yet after blending for a while. Let it rest for a while and then continue. The residual heat in the mixture helps to bring together the texture consistency.
Step 5 - Cook the soap paste. Cover and slow cook on low for about 5 hours, stirring or folding the mixture with a silicone spatula every hour. As the cooking goes on, the soap mixture will get airy/fluffy and gradually turn into a translucent, gel-like paste once saponified.
Step 6 - Store the soap concentration. Turn off the slow cooker. If you want to add some essential oils, add them now, blend in. Let it cool to room temperature before add to wide mouth glass jars to store.

Dilute/Dissolve the Liquid Castile Soap Concentration

Add one part castile soap concentration and 3 parts of distilled water to the 4-cup glass measuring cup. Let the soap concentration slowly dissolve into the water. This process may take a few days. Do not rush, just let it do its own thing. Once it is done, you will get this clear beautiful golden looking liquid Castile soap.

Using for a Foaming Soap Dispenser

I primarily use the liquid Castile soap in a foaming soap dispenser. Add half liquid Castile soap and half distilled water to the dispenser, shake to mix and use.