How to Use a Lye Calculator (and Why You Need One for Soapmaking)

Altering a soap recipe isn’t like adjusting a food recipe. Learn how to safely make changes using a lye calculator so your soap turns out right every time.

If you’ve ever tried changing a soap recipe and ended up with something too soft, too harsh, or just not right, you’re not alone.

Soapmaking isn’t as forgiving as cooking. Even small changes can affect the final result.

That’s where a lye calculator comes in.

What is a lye calculator?

A lye calculator is a tool that figures out exactly how much lye you need based on the oils in your recipe.

Each oil requires a different amount of lye to turn into soap. A calculator does that math for you so you don’t have to worry about getting it wrong.

That’s based on each oil’s saponification value, which is essentially the formula the calculator uses behind the scenes.

Why you need a lye calculator

Over the years, I’ve gotten a lot of questions from people wanting to modify one of my soap recipes.

Sometimes they want to swap one oil for another. Other times, they’ve already made a change and can’t figure out why their soap didn’t turn out as expected.

In most cases, the issue comes down to balance.

Each oil needs a specific amount of lye to fully turn into soap. Even small changes can affect how much lye is needed. If that balance is off, the soap can end up too soft, too oily, or too harsh.

That’s why using a lye calculator is so important. It helps you keep your recipe properly balanced so your soap turns out the way you expect.

If you don’t want to deal with the math, you don’t have to.

I’ve created a simple lye calculator on the blog to help you quickly figure out how much lye to use for your recipe.

Use my lye calculator here

Why different oils change your soap

The amount of lye isn’t the only thing that matters. The oils you choose also affect how your soap behaves.

Some oils make hard, long-lasting bars. Others make softer soaps. Some create lots of lather, while others make a more conditioning, low-lather soap.

This means that even if you calculate the lye correctly, changing the oils can still give you a very different result.

I’ve written more about this in my guide on the best oils for soapmaking if you want to explore how different oils affect your soap.

What happens when you change oils

I occasionally see comments like this:

“I made the recipe exactly as written, except I switched out the coconut oil for canola oil because I had a lot I wanted to use up. Mine didn’t set up well. What’s wrong with the recipe?”

The problem is that the recipe wasn’t actually followed as written.

Even if you calculate the lye correctly, the new combination of oils may produce a soap that is softer, doesn’t lather well, or doesn’t last as long.

In this example, canola oil behaves very differently from coconut oil in soap. So even with the correct lye amount, the result would be much softer, wouldn’t lather well, and probably wouldn’t last very long.

How to substitute oils safely

If you want to modify a soap recipe, it’s best to replace oils with ones that behave similarly.

For example:

• Animal fats can usually be swapped for other animal fats
• Liquid oils can often be swapped for similar liquid oils

If a recipe only uses a small amount of an oil, you may be able to substitute it without drastically changing the final result.

That said, it’s still not a good idea to replace a solid oil with a liquid oil if you can avoid it.

Even when choosing similar oils, always run your updated recipe through a lye calculator to keep everything balanced.

Superfatting (using slightly less lye on purpose)

When using a lye calculator, one of the settings you’ll usually choose is the superfat level.

Superfatting means intentionally using slightly less lye than needed to react with all of the oils. As a result, a small amount of oil remains unreacted in the final soap.

Those leftover oils help make the soap more conditioning and less drying on the skin, which is why most body soaps are superfatted.

That said, more isn’t always better. If you use too much extra oil, the soap can become too soft, may not lather well, and can spoil more quickly over time.

A small superfat is usually enough to make a soap feel gentle without causing those issues.

Lye excess (using too much lye)

On the other end of the spectrum is lye excess, which means using more lye than needed for the oils in your recipe.

If you do this unintentionally, you’ll end up with a lye-heavy soap.

Soap already has a naturally high pH (generally over 9). When there’s excess lye, that pH goes even higher, which can make the soap irritating to the skin. For something like laundry soap, that might not be a big deal. But for anything you’re using on your body, it’s definitely something you want to avoid.

In some liquid soap methods, people use a slight lye excess and then neutralize it later to help keep the soap clearer. That’s not something I usually do. I prefer to calculate my recipes to use the exact amount of lye needed, and I’ve been able to make clear liquid soaps without taking that extra step.

How to use a lye calculator

Using a lye calculator might look complicated at first, but the basic process is the same across most calculators.

You:

• Choose the type of lye (NaOH for bar soap, KOH for liquid soap, or a combination for certain recipes)
• Enter the oils you want to use
• Add the amount of each oil
• Choose your superfat level
• Let the calculator determine how much lye you need

Most calculators also include additional settings, like water amount or lye concentration.

These settings affect how your soap behaves while you’re making it, like how quickly it traces or how easy it is to work with, but they don’t change how much lye is needed to turn the oils into soap.

If you’re just getting started, it’s usually best to leave those settings at their defaults or follow a tested recipe. As you get more comfortable, you can start adjusting them to suit your preferences.

For example, in SoapCalc, you’ll see options like “water as % of oils” or lye concentration, along with other settings grouped into different sections. It can look like a lot at first, but you don’t need to change everything to make a good recipe.

Some calculators also allow you to work with more complex formulations, like using both types of lye for cream soaps.

SoapCalc vs simpler calculators

I’ve used the SoapCalc lye calculator for years when formulating bar soap recipes, and it’s a great tool for understanding how different oils affect things like hardness, lather, and conditioning.

It includes a wide range of oils and detailed options, which can be really helpful once you’re familiar with how everything works.

My lye calculator takes a simpler approach by focusing on the most important parts.

Instead of showing lots of extra numbers or a huge list of oils, it focuses on the ones I use most often and that tend to work well for most recipes. It helps you quickly get the correct lye amount without needing to sort through everything.

I designed it to be especially helpful for beginners or for quickly adjusting simple formulas.

More advanced calculators can be useful when you want more control over your recipe. For example, they may allow you to:

• Work with more complex recipes, like using both NaOH and KOH for cream soaps
• See a more detailed breakdown of fatty acid profiles
• Fine-tune specific properties of your soap with more precision
• Adjust things like water ratios or lye concentration in different ways

Understanding soap properties

Many lye calculators provide values that help you predict how your soap will behave.

Some of the most common ones include:

• Hardness: how firm and long-lasting the bar will be
• Cleansing: how strongly it removes oils and dirt
• Conditioning: how gentle or moisturizing it may feel
• Bubbly lather: the amount of fluffy lather
• Creamy lather: the thickness and stability of the lather

These numbers are helpful, but they make more sense once you connect them to real soaps.

For example, a soap made mostly with coconut oil will usually be harder, very cleansing, and produce lots of bubbly lather. That makes it great for cleaning, but it can feel drying on the skin if it isn’t balanced with other oils or a higher superfat.

On the other hand, a pure Castile soap made with olive oil is much more conditioning. It’s gentle on the skin, but it doesn’t lather much and can feel a bit “slimy” to some people.

Most recipes aim for a balance between these properties. For example, my beginner soap recipe combines coconut oil and olive oil to create a soap that cleans well while still feeling gentle on the skin.

Oils in liquid soap (a bit different)

Liquid soaps don’t show the same extreme differences between oils that you see in bar soaps.

While bar soaps made with coconut oil and olive oil behave very differently, almost like completely different soaps in how they lather, cleanse, and feel on the skin, liquid soaps made with those same oils end up being much more similar in how they perform.

Coconut oil liquid soap is usually lighter in color and a bit more cleansing than my pure liquid Castile soap made with only olive oil, which can make it slightly more drying for some people. The olive oil version tends to be a bit more gentle. That said, both lather well, and I don’t personally notice a huge difference between them in everyday use.

Most soap property numbers (like hardness, cleansing, and lather) are really meant for bar soaps. After all, how can liquid soap be hard? When you try to apply those same numbers to liquid soaps, they don’t translate very well and can make it seem like the differences are bigger than they really are.

Because of that, I didn’t include those soap properties in my lye calculator in the same way. The soap property sliders in my calculator only show up for bar soap recipes and don’t appear for liquid soaps.

Instead, I focused on helping you calculate the right amounts of lye, water, and glycerin for the types of liquid soap recipes I make most often.

A note about liquid soap calculators

For liquid soap, I used to rely on the Summer Bee Meadows Advanced Soap Calculator.

It was widely considered one of the best options, in part because it accounted for the fact that KOH isn’t usually 100% pure.

If a calculator assumes 100% purity, you can end up using too little actual lye, which can throw off the recipe.

That calculator adjusted for that, and my recipes consistently turned out well when using it. I used it for years and really trusted the results.

When it disappeared, I tried running my recipes through other calculators, but I wasn’t getting the same values, especially for things like water and glycerin in my liquid soap recipes.

Why I made my own lye calculator

Part of the reason I created my own lye calculator was because I missed that one.

Since I already had recipes that worked well, I wanted something that would give me similar results so I didn’t have to keep adjusting things or guessing.

I compared the results to formulas I had already tested, making sure it gave similar amounts for things like water and glycerin in the types of liquid soap recipes I make most often.

At the same time, I wanted something simpler to use, without needing to work through a lot of extra settings.

How to choose what to use

There isn’t one “best” lye calculator.

If you want something quick and straightforward, my calculator is designed to make that easy.

If you want more detailed control or to experiment with more complex recipes, a more advanced calculator, like SoapCalc, can be helpful.

What matters most is running your recipe through a calculator anytime you make changes so everything stays balanced.

Originally published on November 3, 2020, this post was completely revised in April 2026 to include the new Oh, The Things We’ll Make! lye calculator and the updated SoapCalc calculator.