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Learn how to make your own cold process soap recipe from scratch plus information on how to use a lye calculator and other soapmaking notes and tips.

How to Make a Custom Cold Process Soap Recipe Using a Lye Calculator

Learn how to make a custom cold process soap recipe. If you’ve tackled learning how to make cold process soap with lye, then you may be ready to start developing your own custom cold process soap recipes. Part of understanding how to create your own soap recipes, means also understanding how a lye calculator works. Keep reading to learn how to use a lye calculator to formulate your own unique soap recipes using the cold process soap making method.

Learn how to make your own cold process soap recipe from scratch plus information on how to use a lye calculator and other soapmaking notes and tips.

How Do I Make a Custom Cold Process Soap Recipe?

Now that you’ve read my cold process soapmaking tutorial on how to make cold process soap from scratch and turned out a few successful batches, you’re probably ready to explore some of your own creative soap making ideas and learn how to make your own cold process soap recipe from scratch. This does require knowledge of basic chemistry about your soap making ingredients – fats and alkali.

Your fats – or soapmaking oils and butters – will very much determine what properties your soap will have. For example, three of the traditionally popular soapmaking oils, especially for beginners, are olive oil, coconut oil, and sustainable palm oil. Olive oil helps to create a moisturizing bar with a stable lather; coconut oil produces a hard, cleansing bar with a fluffy lather; and palm oil makes for a hard bar with a stable lather. Each of these oils has its own SAP (or saponification) value which determines how much lye should be used in the soap recipe for saponification to occur in such a way that it makes soap. Too much lye and you have an unusable bar of soap. Not enough and you could end up with a really soft soap with excess oil.

A great resource for learning more about the saponification process and the properties of various soapmaking fats & oils is Susan Miller Cavitch’s book, The Soapmaker’s Companion. Her book also contains a great troubleshooting section to help you figure out what might have gone wrong with a soap recipe as well as a nice collection of her own recipes.

This homemade cold process glycerin shaving soap recipe includes extra vegetable glycerin that's added to the regular soapmaking oils during the soapmaking process.

How Do I Use a Lye Calculator to Make a Soap Recipe?

When creating your own homemade soap recipes, there are also a lot of additional free resources to help you with this process. Lye calculators, for example, will automatically calculate the amount of lye you need in a recipe based on the amounts and types of oils you plan to incorporate into your recipe. You can find multiple links to lye calculators by conducting a google search. However, the lye calculator at SoapCalc.net can help you to create a soap recipe that meets your expectations for the properties you’re looking for in your own soaps. It does this by giving recommended ranges for the various soap qualities and fatty acids and tells you where your soap recipe falls in regards to each of these ranges.

Learn how to make your own cold process soap recipe from scratch plus information on how to use a lye calculator and other soapmaking notes and tips.

However, the soap calculator at SoapCalc.net is a bit more complex than some of the other soap calculators available so you’ll need a little more information to get started with this lye calculator. Certainly don’t let that intimidate you though. You can discover some pretty fantastic soap recipes through trial and error.

To use the lye calc at soapcalc.net you’ll need to enter a little bit of information in addition to your ingredients. Since you’ll be making cold process soap you’ll need to choose the radio button for NaOH (sodium hyroxide.) KOH or potassium hydroxide is used to make liquid soap.

Next, choose which measurement you’ll be using to weigh your oils. I recommend using either grams or ounces. I typically weigh out my ingredients in ounces however for smaller recipes you’ll find that grams will always be the most accurate as it’s a smaller unit of measure.

Third choose your water as % of oils or the water discount. I highly encourage you to set this at 33%. Otherwise you’ll likely have a very soft soap to start out with which will not only take longer before it can be unmolded but will also take much, much longer to cure as there’s so much more excess water that needs to evaporate. Occasionally I use less than 33% for soap recipes that are using a lot of oils that are know for creating a softer soap. For example, a 100% olive oil soap – or what is known as a traditional Castile soap – is going to start out as a much softer soap and requires a cure time of 4-6 months rather than 4-6 weeks. In this case I typically use 30.5% as the water discount amount.

Next choose a superfat amount. A soap with 0% superfat has no extra oils left over in the final soap bars once saponification is complete. To superfat a soap means you have extra fats (or oils) that are left unsaponified in your final bars of soap. These unsaponified oils help to make soap more conditioning. It is standard practice to use at least a 5% superfat unless you are making a laundry soap. This saves your butt in case of small errors in measurements and keeps your soap from stripping too much excess oil from your skin when bathing. For a more conditioning soap you would use a higher superfat of up to 8%. However, in some situations, you may choose to use an even higher superfat amount for personal reasons or simply because it’s “good science.”

For example coconut oil has natural cleansing properties in soap. As such it’s recommended you only use up to 30% coconut oil in your soap recipe to avoid an overly cleansing soap that will strip skin of its natural oils. (I typically only use 20% or less as I prefer soaps that are more conditioning than cleansing.) 100% coconut oil soaps are very cleansing which is great for making laundry soap. However, it’s not so great for skin. You can overcome this in a 100% coconut oil soap however by superfatting your soap recipe at 20%.

There are times however, or “bad science,” that a really high superfat doesn’t make sense. Coconut oil has a very long shelf life and therefore isn’t prone to going rancid. Other oils, however, have a much shorter shelf life. Because of the higher percentage of oils left unsaponified at a superfat percentage above 8%, they can go rancid more quickly than you might like and cause what is commonly referred to as DOS. DOS or dreaded orange spots are basically unsaponified oils that have gone rancid. Should this occur to any of your soaps, and sadly it sometimes does, you can salvage the soap by grating it and using it as a laundry soap.

Next, if you are using a fragrance for your soap recipe choose your fragrance amount. Typically fragrance oils are used at 1 oz. per pound or less and essential oils at half that amount or less. However, this can vary depending on the type of fragrance oil or essential oil you are using. Typically fragrance oils are used at a max of 5-6% of your recipe. Sometimes the maximum usage amount for a cold process soap recipe is lower. In this case you’d need to refer to the manufacturer’s guidelines on the maximum amount of fragrance oil that is safe to use. For essential oils, the usage rate is typically between 1-3%.

Now select your soapmaking oils, fats and waxes for your soap recipe. Click on the first oil, fat or wax you are using then click on the plus sign for the #1 spot on the Recipe Oil List. If you know the specific oil weight you’ll need for your soap recipe – say you have a 3 lb. mold – hit the lb radio button. Enter the amount of the first oil you plan to use.

Learn how to make your own cold process soap recipe from scratch plus information on how to use a lye calculator and other soapmaking notes and tips.

For example, if you are making a 3 lb. batch of soap and you want to use coconut oil at 20% of your recipe, multiply 48 ounces (if you chose the weight of the oils to be in ounces) times .2 for 9.6 ounces. Enter 9.6 in the first box.

Now repeat with all of the remaining oils until you’ve reached the total oil weight of your recipe. Click on Calculate Recipe then View or Print Recipe. Your recipe will open in a second window and give you the amount of lye and water you’ll need based on the data you entered, the amount of fragrance oil to use and information on what the soap bar quality will yield. While it’s not a hard and fast rule that you fall within the recommended ranges of soap bar quality for hardness, cleansing, conditioning, bubbly, creamy, iodine and INS, it’s safer to stay within these ranges if you’re just starting out and learning for more successful results until you learn more through experience and further research.

Learn how to make your own cold process soap recipe from scratch plus information on how to use a lye calculator and other soapmaking notes and tips.

Pictured above is a test recipe I threw together as an example of what your final recipe will look like after inputting your information into SoapCalc. Pretty neat, huh? (This lye calc has actually taught me that both hemp seed oil and sunflower oil are high in iodine.)

FYI It is good practice to always double check the amount of lye in a recipe with a lye calculator if you are unsure of its source.

How Do I Resize a Cold Process Soap Recipe?

Not using the lye calculator at SoapCalc? Other lye calculators, like the one at Brambleberry automatically give you a more standard amount of water or liquid needed for your soap recipe. This makes it somewhat simpler to use if you’re just getting started. While it doesn’t offer as many oil choices as SoapCalc and won’t give you an idea of what properties your soap will have, Brambleberry does have a simple and easy tool to resize your soap recipe once you input it into the lye calculator. You can learn how to re-size a cold process soap calculator using Brambleberry’s lye calculator here.

Need a quick and easy way to learn to what properties the soapmaking oils you want to use have? It’s as easy as a google search! Properties and suggested amounts can be found on most soapmaking supplier websites. So typically you can visit the website of your favorite supplier and the product page of the ingredients you are interested in buying will tell you what properties a particular ingredient will lend to the soap as well as the recommended amount. Alternately you can type into your search box phrases like “properties of hemp seed oil in soap” or “recommended amount of hemp seed oil in soap” and you’ll find all kinds of valuable information.

Summer Bee Meadow also has a simple chart for quick reference on their website that provides a collection of commonly used oils in soapmaking along with their fatty acids and resulting soap characteristics. Another website, Lovin’ Soap Studio, has a chart with commonly used oils and the recommended usage rates. You should also be able to determine the shelf life of your ingredients from the manufacturer or soap supply company you purchase your ingredients from.

Learn how to make your own cold process soap recipe from scratch plus information on how to use a lye calculator and other soapmaking notes and tips.

How Do I Figure Out How How Much Soap I Need for My Mold?

Need to resize a soap recipe to fit into a mold but aren’t sure what to scale the recipe to? There’s a simple formula for determining the weight of the soapmaking oils needed for your mold. About.com has a formula and instructions for calculating the amount of soap needed to fill a mold here. They give the basic math equations for determining the correct soap recipe size for standard square or traditional molds, round or tube molds and odd or irregularly shaped molds.

Got it? Great! Be sure to head back over to my article on how to make cold process soap from scratch for reference when making your homemade soaps and links to other great soap making resources.

Don’t forget to follow me on Pinterest for collections of not only my homemade soap recipes and beauty DIY’s but also some of my favorites from around the web.

Or simply keep up with of all my new homemade soap recipes and other DIY creations by following Soap Deli News blog via Blog Lovin’. You can also find me on Facebook, Twitter and Instagram.


  • Deborah

    August 1, 2015 at 9:51 am

    Nothing makes your skin feel as good as homemade soap! It is awesome. And the best all purpose cleaner I have ever tried. I’ve been making my own for several years. Need to make some more real soon. My all time favorite is made with Tallow.

  • Debbie Minter

    August 1, 2015 at 1:22 pm

    Thank you for your posts! You have helped me immensely. Especially with your post about how to make milk soap! You cannot know how much I appreciate it!

  • Anne-Marie Faiola

    August 7, 2015 at 6:40 pm

    What a fantastic, information packed tutorial. Thanks for sharing how to make your own cold process soap recipe. Customizing your own recipes is always so much fun =)

  • angel

    September 14, 2015 at 7:11 pm

    hi i’m angel from manila, philippines and I just want to say THANK YOU for all the knowledge that is in your website. I have been self teaching myself how to make soap for almost two months now. i’ve read on (entirely) about 20 websites (yours is one of my favorites) and last weekend I made my first batch of soap inspired by your Tomato basil recipe. I have an enormous supply of dried basil in the house and then i used hi quality tomato paste from the grocery. oils i used (and are the only ones available to me here) are olive, palm & coconut. It is curing now I hope it turns out well. I super admire all you soap makers abroad because you all seem to know each other, like you have a community, and we simply don’t have that here. I wish you more power, God bless you for what you do, always.

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