What Is Castile Soap? Ingredients, Uses and Myths

"Castile soap" is printed on everything from baby washes to all-purpose cleaners. The name sounds specific, but today it mostly tells you one thing: the soap is made from plant oils rather than animal fat. Here is where the name comes from, what is actually in a bar or bottle, and a few myths worth ignoring.

Where the name comes from

The name points to Castile, a historic region of Spain. Soap made there from local olive oil was produced on a large scale and traded across Europe.

Its roots go back further. Soapmakers in Aleppo, in what is now Syria, made hard soap from olive oil and bay laurel oil for centuries. Castile soap is usually described as a descendant of that tradition, made with olive oil alone.

What "castile soap" means today

Traditionally, castile meant olive oil soap. Today the word is used much more loosely. Most brands use it for any soap made from vegetable oils instead of animal fats such as tallow, and many popular castile soaps blend olive oil with coconut, palm or other plant oils.

The term is not regulated, so "castile" on a label does not tell you which oils are inside. If it matters to you, for example if you want a bar made only with olive oil, read the ingredient list.

What is actually in it

Castile soap is "true soap." U.S. regulators describe true soap as being made mainly of alkali salts of fatty acids: fats or oils that have been reacted with an alkali. This reaction is called saponification.

Bar soaps are normally made with sodium hydroxide (lye) and liquid soaps with potassium hydroxide. That is why ingredient lists often show oils in their reacted form. For example, "sodium olivate" is olive oil that has been turned into soap with sodium hydroxide.

How people use it

Brands sell castile soap for washing hands and body and, usually diluted with water, for household cleaning. Follow the dilution guidance on the specific product, because concentrations differ between brands.

Castile soap vs other soaps at a glance

• Castile vs regular bar soap: many supermarket bars are synthetic detergent 'beauty bars', or soaps made with tallow and added fragrance. Castile is true soap made from plant oils. See castile soap vs regular soap.
• Castile vs Aleppo soap: Aleppo soap is an olive oil soap with added bay laurel oil, aged for months. It is a close cousin of traditional castile. See what is Aleppo soap.
• Castile vs detergent cleansers: body washes and many 'gentle' bars clean with synthetic surfactants such as SLS. Castile soap cleans with soap molecules alone. See sodium lauryl sulfate vs soap.

Bar vs liquid castile soap

Castile soap comes in two forms, and the difference is the alkali used to make it.

• Bars are made with sodium hydroxide. They are firm, easy to travel with and need no plastic bottle.
• Liquid castile is made with potassium hydroxide, which produces a softer soap that dissolves in water. It is sold as a concentrate, so a small amount goes a long way once diluted.
• Liquid castile is the more versatile of the two for household cleaning. Bars are simpler for hand and body washing.

How to read a castile soap label

Because 'castile' is not a regulated term, the ingredient list is where you learn what you are buying. A few things to look for:

1. The oils, in order. Ingredients are listed from most to least. If olive oil (or sodium olivate) comes first, it is an olive-heavy bar. If coconut comes first, expect more lather and a slightly more cleansing feel.
2. The alkali. You may see sodium hydroxide or potassium hydroxide, or the oils written in their reacted form, such as sodium cocoate or potassium olivate. Both mean the same thing: real soap.
3. Fragrance. 'Fragrance' or 'parfum' usually means a fragrance blend. Essential oils are often listed by name, such as lavender oil. Unscented bars list neither.
4. Extras. Some castile soaps add glycerin, vitamin E, citric acid or plant extracts. These are common and not a problem for most people, but worth checking if you have allergies.
5. Palm oil. If you prefer to avoid it, look for sodium palmate or Elaeis guineensis. Our guide to palm oil in soap explains what to look for.

Who castile soap suits (and who might prefer something else)

• Good fit: people who want a short, plant-based ingredient list; people who want one soap for body, hands and cleaning; and anyone trying to cut down on plastic with bars.
• Maybe not: people with very dry or reactive skin who find alkaline soap tight. A pH-balanced cleanser may feel milder. See soap pH explained.
• For hair: castile soap can clean hair, but many people find it leaves hair feeling coated, especially in hard water. See hard water and soap scum.

Myths and common mistakes

Myth: castile soap is always pure olive oil

Not anymore. Many castile soaps contain several plant oils. Only the ingredient list tells you for sure.

Myth: natural soap suits every skin

Plant-based does not automatically mean gentle for everyone. Any soap, including castile soap, can feel drying or irritating to some people. If you have a skin condition, ask a doctor or dermatologist what to use.

Mistake: mixing castile soap with vinegar

Soap is alkaline and vinegar is an acid, so mixing them in one bottle makes them neutralize each other. The soap breaks down into a cloudy, oily mixture that cleans poorly. If you use both for cleaning, use them separately and rinse in between.

Have a bottle or bar in front of you? Paste its ingredient list into our soap label decoder to see which oils it is made from and whether it contains fragrance or palm.

A simple unscented example

If you want to see what a plain castile-style bar looks like, here is one widely available option. We chose it from its published ingredients and specifications; we have not tested it ourselves.