How and Why to Use a Lye Calculator for Soap Making

A bar of caramel-colored handmade soap on a lacy cloth.

Learn why you need to use a lye calculator whenever you try a new soapmaking recipe, tweak one, or develop your own soap recipe. Handmade soap requires a precise amount of sodium hydroxide to saponify the ingredients into soap. Find out how to use a soap calculator here.

How to use a lye calculator

If you've read any of my soapmaking posts, you'll know I'm adamant that you use a lye calculator with any new soap recipe to find out how much sodium hydroxide (lye) you need to use. 

Or if you tweak a recipe you've used before, or if you develop your own recipe.

Soapmaking requires a precise amount of lye in order to saponify the ingredients into a bar of soap. It's a chemical reaction based on science and the ingredient amounts need to be as exact as possible.

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For instance, if you are making soap from a recipe you've used before but you don't have the ten ounces of coconut oil it calls for. You might think, oh, I can just substitute two ounces of another oil instead.

Yes, you can. However, the amount of lye for this batch probably needs to be adjusted. So use a lye calculator to get the correct amount of lye to use. It's simple and easy when you use a lye calculator.

What a lye calculator does

Back in the old days - you know, before the internet - a soap maker had to look up the saponification rate (or "sap rate") for each oil in their soap recipe. 

Then, through some complicated calculations, she'd figure out how much lye would be needed to turn that particular combination and amount of oils and water into soap that would turn out just right.

Each type of oil needs a different amount of lye to make the chemical process known as saponification happen, so different amounts of an oil would need a different amount of lye. 

And then she'd need to figure out the sap rates of all the other oils in her recipe... well, you see what I mean, right?

Thank goodness for online lye calculators

I am so thankful for these websites that perform this complicated math for me! You can still find tables of saponification rates in books, but it's so much easier now to let the calculator do the work.

I realize that there are those among us who are preparing for the possibility of a world without electronics, where online calculators won't be available. 

These people will want to learn how to figure out a recipe's saponification rate by hand. I'm not going to get into that here in this post, but you can find the information in this post from SoapQueen.

When to use a lye calculator for soap making

If you use the same soap making recipe over and over, and have it clearly and legibly written down somewhere, you won't need to use a lye calculator every time you make soap with that recipe. 

You should use it the first time you use a new recipe though. Even if the recipe is from a book or a trusted soap making site online, it's a good idea to use a lye calculator, because typos can happen.

If you're tweaking a recipe, you'll also need to use a lye calculator. Even a small change, such as replacing one ounce of castor oil with corn oil because the bottle of castor oil wasn't as full as you thought it was, could require changing the amount of lye in your recipe.

Using a lye calculator is so easy these days. Please spend the few minutes it will take to check your recipe.

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Where to find a lye calculator

There are several soap making lye calculators online and they are free to use. I also checked the Google Play Store and found dozens of apps available.

Some sites are called "soap calculators" rather than "lye calculators."

Of the many lye calculators online, I've used and Soapmaking Friend. I also like the lye calculator at Savvy Homemade

Although they all include a large selection of soap making oils, there are a few less-common oils that are only available at one site or the other, so use whichever one lists the oils you intend to use in your soap.

I'm going to give you a quick guide to using the lye calculator at in this post. 

Other calculators will work in a similar fashion and all of them will give you the amount of lye and of liquid (in my case, goat milk) that you should use in your recipe.

How to use a lye calculator

To begin, all lye calculators will ask you some basic questions about your recipe.

A graphic: how to use a lye calculator to make soap

Type of soap

Use the drop-down menus to choose what type of soap you're making: solid or liquid. 

Solid soap uses sodium hydroxide and is the type of soap I've discussed in my six-part soapmaking series. Liquid soap uses potassium hydroxide instead.


"Units" refers to whether your recipe uses ounces or grams. 

Superfat level

The "super-fatting level" refers to excess oil in your recipe. 

A soap recipe that uses a 0% superfat will be drying to the skin. Usually laundry soap has a low superfat rate but for a body bar, facial bar or shampoo bar, you'll want a higher superfat rate. A common level is 5% superfat.

I use goat milk as my liquid, which contains some fat of its own. I use the 5% superfat rate in my recipes, but I know that my finished soap will contain a little more than that. 

How much more? Well, that would depend on how much butterfat is in the milk you use, which depends on the goat the milk came from and the time of year she was milked. All I can tell you is that the superfat level I use makes my finished soap positively luxurious.

So I recommend you set the lye calculator to 5% superfat. You can experiment later if you want, but this is a great place to start. Then click on the NEXT button.

If you'd like to learn more about superfatting your soap, you can read this tutorial from Soap Queen.

Enter your oils

The next step is to enter your oils in the calculator. Scroll down to find each oil and type in the number of ounces (or grams) in your recipe.

A graphic: a list of oils used in soapmaking

Add all of the oils you plan to use and click on NEXT.

The calculator will "think" for a few seconds and the next screen will have all the information you'll need to make your soap: how much lye to use, how much liquid, the total amount of oil you are using, and the amount of finished soap your recipe will produce.

A graphic showing the calculator's results - how much lye and liquid you need to make this recipe.

Some extra benefits of the lye calculator

Knowing how much soap the recipe will make is helpful so you'll know if your recipe will fit the mold you're using. 

The recipe I'm using as an example makes 50 ounces (49.9 oz) of soap and my mold holds 40 ounces, so I would use the RESIZE button to scale the recipe down to the amount I need.

Now your recipe, including the amount of sodium hydroxide and liquid, is right there on the screen in a neat, easy-to-read format, such as in the image above. 

Right under this table of information, you can give your recipe a title and print it out.

I love having my recipe printed on a sheet of paper when I make a batch of soap. I like to check off each kind of oil as I measure and add it. I'm always afraid I'll skip one! 

Uh, yes, I have done that. What a disaster!

I write the date I made the batch, and add information about the essential oils or fragrance oils and colorants I've used (if any), then I file the paper in my soap making binder. 

And later, I can add notes such as how the soap turned out, did I like the essential oil blend, did the soap change color over time, did it lather well, and so on. 

I've made a printable soap batch sheet just like the one I use myself,
and I'll send to you for FREE.
Make as many copies as you need for your personal use.
Tell me where to send it in the form below.

Writing it all down is especially helpful when I'm developing a new recipe or tweaking an existing recipe. 

For instance, when I made my first batch of shampoo bars, I thought the bars were really soft. I made plans to add some harder oils to the recipe the next time. 

But before I made a second batch, the bars hardened up nicely after curing and I really liked them. It took a couple extra weeks, but it was worth the wait.

I don't think I'll make much in the way of changes after all - I just need to plan ahead and allow plenty of time for them to cure before using.

So, now you know how to use a soap calculator. Feel free to play around with it so you're familiar with how it works. 

Then read my post on measuring oils for your first batch of goat milk soap, gather the soap making supplies you'll need, and make your first batch of soap.

You'll find all the posts in this six-part soapmaking series here, along with all of my soapmaking and shampoo bar posts.

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A bar of handmade soap

Related Posts:
What you'll need to make your first batch of goat milk soap
How to use a shampoo bar
Soapmaking safety


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