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A Super Simple Castile Soap Recipe and Formulation Guide

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Olive Oil Soap made from a Castile Soap Recipe

What’s the Fuss About Castile Soap?

Castile soap has been one of the most popular soaps of all times (if not THE most popular soap) and has remained so for hundreds of years. Castile soap also stirs up question and confusion in the soapy community. So let’s dive in, answer those questions, resolve the confusion, and even make some castile soap with my castile soap recipe!

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What exactly is Castile soap?

Maybe you’ve read about it in social media soapmaking groups or maybe you’ve seen it in grocery stores, and you’ve wondered what the big fuss is all about. And maybe you’ve even wondered how to pronounce it correctly, too!

For starters, Castile is pronounced “Cass-TEEL”; the word comes from a region of Spain that was known all over Europe for its soap that was mild but highly effective, as well as its perfectly pure, white color. Since the 12th century, the way Castile soap is made has been basically (although not exactly) the same, with the same simple ingredients: olive oil, water, and lye.

Olive Oil - the main ingredient in a castile soap recipe

In more recent years, Castile soap has been redefined by modern soap manufacturers as soap made with vegetable oil only (as opposed to soap that is made with animal fat). But for some purists, a “true” Castile soap recipe is made with olive oil, water and lye only – no other additives can be included.

However, there is no official definition of a Castile soap recipe, and even artisan soapmakers don’t always agree. Originally, a Castile soap recipe consisted of cooking olive oil with plant ashes, producing a similar lye to potassium hydroxide, and using brine to harden up the soap. So it could be argued that not even cold process or sodium hydroxide could be used to make a “true” Castile soap.

Most soapmakers today aren’t crazy about the idea of making their own lye or cooking soap for days at a time, so let’s agree that unless our goal is to make historically accurate, medieval-period Castile soap, we can use sodium hydroxide bought from a store, either hot process or cold process, and use other lovely additives to make a spectacular bar of soap with a modern day castile soap recipe.

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A variation of Castle soap recipes: Bastile soap!

For continuity’s sake, let’s assume that Bastile is pronounced “Bass-TEEL” because that rhymes with Castile. 😊

This is a term that probably came from modern artisan soapmaking culture, Bastile soap being a “bastardized” version of Castile soap: a mostly olive oil soap that has a small percentage of other oils.

Why exactly would a soapmaker want to reduce the percentage of olive oil, since Castile soap is such a great soap? As wonderfully mild and gentle as Castle soap is, it does have some negatives.

1: The lather is dense, creamy, and conditioning… but if you’re a fan of loads of big bubbles, a Castile soap recipe will be a bit disappointing.

Commercially made detergent-based cleansers are always going to be huge in the lather department, since that’s one of the best things that detergents do. Think of it as a trade-off: less lather, but kinder to the skin. Embrace that lotion-y, creamy lather! 

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2: Another problem with Castile soap recipes is that it takes much longer for castile soap to become great soap vs. other types of soap recipes.

Typically, most soap cures within 4 to 6 weeks. While a Castile soap recipe will be safe to use after 6 weeks or so, it won’t be too impressive. It’s likely to turn slimy and gelatinous when only a few weeks old. Castile soap needs more time to age, like a fine wine – typically 9 to 12 months. Somewhere around this 9 to 12 month mark is where the magic happens: low lather becomes rich, dense lather that makes your skin feel like silk and Castile soap becomes your new best friend.

Making Castile Soap

Let’s talk about formulating your own Castile soap recipe, now that you’re itching (pun intended!) to make some:

Cold process or hot process for a Castile Soap Recipe?

One advantage of hot processing Castle soap is that it cuts more easily. If you cold process your Castile soap, cut it as soon as it hardens up. If you wait too long, it can become very hard, and the edges can crumble as you cut it.

One advantage of cold processing Castile soap is using room temperature oil and very hot lye solution can be used. As soon as the lye is completely dissolved, it’s ready to make soap. The heat from the lye solution helps the slow-to-trace olive oil get to trace faster.

Use a water discount when formulating a Castile Soap Recipe

Olive oil will take a long time to reach trace if a default water amount from a lye calculator is used. Trace can be reached more quickly by making a stronger lye solution – less water means the lye is more concentrated.

Use a lye calculator to find the right amount of lye to use, but discount or decrease the water amount by changing the percentage of water to somewhere in the range of 40 to 50% (never more than 50% because the lye won’t dissolve!) or use a water to lye ratio, which is the Easy Math way. Lye needs at least an equal amount of water to completely dissolve, so the lowest ratio you can use is 1:1 (one part lye to one part water).

However, this can set up so fast that cutting the soap can be a problem, so a good ratio of water to lye for Castile soap is 1.25:1, or even 1.5:1. To calculate the water, take the amount of lye and multiply that number times 1.25 or 1.5 – it really is that easy!

Can You Include Additives in Castile Soap Recipes?

It’s your soap, your formula, so you decide! Any of the additives used to enhance lather will be perfect to add to Castile soap: goat milk, coconut milk, honey, or other sugars. Veggie and fruit purees, teas and herbs can also be incorporated in Castile soap to add color, texture, and scent. For more details and inspiration, hop over to my blog post here on adding food stuff to handmade soap.

Castile Soap Recipe

If you’ve read the details above, you probably already know what the recipe looks like, but I wanted to include some actual numbers for you anyway 😊

I’ve calculated a basic batch using a standard loaf mold like this one. I’ve used a standard super fat of 5%, and a 1.5:1 water:lye ratio. This will give you a good starting point for formulating your own Castile soap. While you may want to adjust the superfat or water content, or even incorporate some additives, I suggest making this basic Castile recipe first, then adjusting the formula once you’ve tested a few bars.

Ingredients for a Small Batch Castile Soap (1000g):

  • 1000g Olive Oil
  • 128g Sodium Hydroxide
  • 193g Water

Directions to make a Cold Process Castile Soap

Remember, safety first! If you’ve never made soap before I highly suggest this primer from the Handcrafted Soap and Cosmetics Guild. It covers the basics, including safety! Remember, safety first, always!

  1. Measure your oils by weight. Since olive oil is liquid at room temperature, there’s no need to heat your oils. However, warming them will reduce the time needed to reach trace. Try warming the oils up to around 100 degrees (ish) vs. room temperature oils. It’s an eye opening learning experience!
  2. Prep your water and lye. Measure them into separate lye safe containers, then slowly add the lye to your water, stirring all the while.
  3. Add the lye mixture to your oils. Once the lye has fully dissolved, you can add it to your oils. It will be HOT. This heat will help us reach a trace. If you do let your lye solution cool, be ready for a lot of blending.
  4. Bring the batter to a trace. You can opt to bring your batter just past emulsion, but I recommend reaching at least a light trace as it will be easy to misread emulsion with a castile soap. Also, with a trace (this is fully anecdotal, by the way) you’re less likely to experience soda ash on the final soap.
  5. Mold your soap. Once you reach a trace, pour your soap into your mold.
  6. CPOP. While optional, I highly recommend this step. CPOP stands for Cold Process Oven Process and will help ensure a full gel on your soap. To CPOP your soap:         
    1. Warm your oven to its lowest setting.
    1. TURN THE OVEN OFF and place a sign on it warning everyone else to leave the oven off!
    1. Put your soap in the oven and leave it to set up. The warm oven will keep your soap well insulated, ensure a full gel, both of which will help prevent development of soda ash.
  7. Cut, cure, and age your castile soap. Once your soap has set up (is firm enough to remove from the mold), you can cut it into bars. You’ll then need to allow the soap to cure (a process of letting the water evaporate from the soap), and then age to experience the true glories of Castile soap.

Once your soap is cut, start trying it out! I recommend using this soap in stages, taking notes along the way, so you can experience the development of this wonderful (eventually wonderful) lather.

  • Use one bar after the cure. Yes, it will be slimy and underwhelming later. Use it anyway!
  • Then use another bar every 6 weeks or so to see how the lather changes and develops over the course of 9-12 months.

I hope you’ll make a batch of Castile soap soon, and I hope I’ve given you some fresh ideas to try out – if so, please let me know! Comments and pics, please (and pretty please – you know I love seeing your soaps my dear soapy friends!)

4 thoughts on “A Super Simple Castile Soap Recipe and Formulation Guide”

  1. I found this article about Castile soap really informative. As someone who enjoys making my own soap, I appreciated the history and variations of Castile soap recipes explained. The recipe provided is straightforward and I’m excited to give it a try!

  2. Hi. thank you so much for the write up, very informative,
    I ran through the soapcalc before i try to execute this, i found a typo.
    I think, you meant to say 1 lb olive oil (not 1000g) of small batch, 1lb has a corresponding water and lye.
    he he he…

    1. You’re a gem! Yes, there was indeed a mistake, which is now corrected in the recipe above.
      PERFECT example of ALWAYS using a soap calc to double check any recipe, no matter the source =)
      FYI: I originally had a smaller batch written up and changed it last minute after a peer review suggesting that the 1000g batch is better for that common amazon loaf mold.

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