You've possibly noticed the word "castile" used on all sorts of natural soap products — some made purely from olive oil, others packed with other vegetable oils like coconut and palm oil. Castile soap has been made and used for centuries, and its simple olive oil formula has become known around the world for both personal care and household cleaning. So what actually makes something a Castile soap?
The short answer: true, traditional Castile soap is made with 100% olive oil as the base oil, and no other oils. Most of what's sold as "castile" today, including some very well-known brands, is more accurately described as a castile-style soap — a multi-oil blend that borrows the name without following the original recipe.
Here's where the name comes from, why the market has become so muddled, and how to tell what you're actually buying.
What is Castile soap and where does the name come from?
Castile soap takes its name from the Castile region of Spain, where soap makers became known for producing soap from the olive oil readily available throughout the Mediterranean. Olive trees have grown across the Iberian Peninsula since antiquity, and using the local, abundant olive oil to make soap — rather than the animal tallow used in soap-making elsewhere in Europe — became a regional speciality and eventually gave the soap its name.
At a time when many European soaps were made using animal fats, this olive oil soap developed a reputation for being particularly mild and high quality.
That single-oil formula is what "Castile soap" originally meant: soap made using olive oil as the sole base oil. The olive oil is turned into soap through saponification, a reaction between the oil and an alkali. Solid Castile soap is traditionally made using sodium hydroxide, while liquid Castile soap uses potassium hydroxide.
Why not all Castile soap is true Castile soap
Over time, "Castile soap" drifted from a specific, place-based recipe into a general marketing term for "natural, plant-based soap" — even when the plants involved have nothing to do with Spain or olives.
Today it's common to find Castile soaps made from blends of coconut, palm, hemp seed and jojoba oil, sometimes with only a small proportion of olive oil, or none at all. These oils are added because they change how the soap behaves: coconut and palm oils create more lather and a harder bar, while pure olive oil soap lathers far less and stays softer for longer.
Coconut oil is also inexpensive and produces lots of bubbles, which helps explain why it has become such a common base in modern castile-style soaps.
These may still be natural soaps and free of synthetic detergents, but by the strict, traditional definition, a soap made with multiple base oils is Castile-style, rather than true Castile soap.
Traditional Castile soap vs modern Castile-style soap
| Traditional Castile soap | Modern "castile-style" soap | |
|---|---|---|
| Main oil | 100% olive oil | Coconut, palm, hemp, jojoba — often with only some or no olive oil |
| Lather | Low, dense, almost lotion-like | Fuller, fluffier, more bubbly |
| Feel | Very gentle, moisturising | Can be more drying, especially with a high coconut oil content |
| Bar hardness | Softer, wears down faster | Harder, longer-lasting bar |
| Origin | Named for the Castile region of Spain, historically olive oil-based | Uses the "castile" name as a general natural-soap descriptor |
Which Castile soap is better to use?
For most everyday cleaning, either type will do the job — both are genuine soaps, not synthetic detergents, and both are biodegradable. Where it tends to matter more is on skin.
Olive oil is naturally moisturising and considered gentle even on dry or reactive skin, which is why traditional Castile soap has a long-standing reputation as one of the mildest soaps around. Coconut oil, by contrast, is more cleansing and can be drying with regular use, particularly for people with sensitive or eczema-prone skin.
So if gentleness on skin is your main reason for choosing Castile soap in the first place, it's worth checking whether olive oil is the sole base oil — or just one of several.
This is partly why the formulation matters: a high-olive-oil soap and a high-coconut-oil soap can both carry the word "castile" on the label, yet feel quite different in use.
Good to know: True Castile soap barely bubbles compared with coconut or palm-oil soaps, but more lather doesn't mean a soap is purer or more effective. That low lather is simply a characteristic of olive oil soap — not a flaw or a sign that it isn't cleaning.
How to tell if Castile soap is really 100% olive oil
The only reliable way to know which type of Castile soap you're holding is to check the ingredients list, not the front label.
✓ True Castile soap: Look for olive oil as the sole base oil. Depending on how the ingredients are declared, you may see saponified olive oil, sodium olivate or potassium olivate, along with water — and perhaps a small amount of essential oil for scent. There should be no other base oils.
✓ Castile-style soap: Look for other base oils such as coconut, palm, hemp seed or jojoba oil, often with olive oil appearing further down the ingredients list.
So if you're buying Castile soap specifically for its gentle, olive-oil reputation, it's worth reading past the label.
Which Castile soap should you choose?
Now you know the difference, here's the simple part: choose 100% olive oil Castile for the traditional, gentle formula, or a castile-style blend if you prefer more bubbles and stronger cleansing power.
For true, 100% olive oil Castile
If you're looking for the traditional Castile soap we've been talking about, these are the ones to choose.
Biome Liquid Castile Soap
Made in Australia from Australian-grown olive oil, with no coconut, palm or hemp oils added. A concentrated, versatile liquid soap for hands, body and DIY cleaning around the home.
Biome True Castile Soap Bar
As simple as it gets: 100% saponified olive oil. Handmade in Australia, unscented and palm oil free, true to the traditional single-oil Castile recipe.
Australian Natural Soap Co. Olive Castile
Another beautifully simple option made from 100% saponified olive oil. Fragrance free and true to the traditional olive-oil Castile recipe.
Four Cow Farm Traditional Castile Hair & Body Wash
Prefer something ready to use in the shower? This gentle Castile wash is made specifically for washing hair and body — an easy option for everyday use.
Prefer more bubbles?
A modern castile-style soap blends different plant oils to create a bubblier, more cleansing wash. It's not traditional 100% olive oil Castile, but it may be exactly what you're after — particularly for cleaning around the home.
Ready to find the right one for you?
Not sure what to do with your Castile soap next? Discover 18 ways to use pure Castile soap, or see how Biome's Castile soap compares with Dr. Bronner's.
Castile soap FAQs
Is Dr. Bronner's a true Castile soap?
By the traditional, olive-oil-only definition, no. Dr. Bronner's Pure-Castile soaps use a blend of oils, primarily coconut and palm, with olive, hemp seed and jojoba oil also in the mix. It's a genuine natural soap and a castile-style product, but not a traditional single-oil Castile soap.
What's the difference between liquid and bar Castile soap?
Both can be true Castile soap when olive oil is the sole base oil. The main difference is how they're made and used: liquid Castile soap is convenient for diluting and using in DIY cleaning recipes, while a Castile bar is a simple option for washing hands and body.
Is all olive oil soap Castile soap?
Not quite the reverse question, but closely related — a soap needs to be entirely, or almost entirely, olive oil-based to be considered true Castile soap. A soap with a small amount of olive oil alongside several other oils wouldn't traditionally qualify, even though it may still be marketed as "castile."
Is Castile soap the same as Marseille or Aleppo soap?
They're closely related but not identical. All three come from long Mediterranean soap-making traditions associated with olive oil. Aleppo soap, from Syria, traditionally adds laurel oil; Marseille soap, from France, has its own regional soap-making tradition and process. Castile soap is the Spanish counterpart, historically distinguished by its simple olive oil base.
Why does true Castile soap barely lather?
Lather comes largely from oils such as coconut and palm, not olive oil. A dense, low-foam wash is actually a sign of a genuinely olive oil-based soap, not a lesser product. Lots of bubbles aren't necessary for soap to clean effectively.
What is true Castile soap made from?
True, traditional Castile soap uses 100% olive oil as its base oil. The olive oil is saponified to create soap. If coconut, palm, hemp, jojoba or other oils are used as part of the soap base, we consider it a modern castile-style soap rather than true traditional Castile.
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