If you’re making cold process or hot process soap, a lye calculator is one of the most important tools you’ll ever use. It ensures your recipe has the correct amount of sodium hydroxide (NaOH) or potassium hydroxide (KOH), keeps your soap safe, and helps you control hardness, lather, and conditioning.

Whether you’re brand new or looking to refine your recipes, here is a simple step-by-step guide on how to use a lye calculator the right way.

What Is a Lye Calculator?

A lye calculator is a tool that determines exactly how much lye you need based on:

The types of oils or butters you use
The amount of each oil
Your desired superfat level
Whether you’re using NaOH (bar soap) or KOH (liquid soap)

Every oil has a unique SAP value (saponification value), which tells the calculator how much lye is needed to turn that oil into soap.

Step 1: Select Your Lye Calculator

There are many free and reliable options:

SoapCalc
The Sage Lye Calculator
Bramble Berry Calculator
Soapee
Majestic Mountain Sage

Choose any one you like—they all do the same job.

Step 2: Choose Your Type of Soap

You’ll usually need to select:

NaOH for cold or hot process bar soap
KOH for liquid soap

Most beginners choose NaOH for bar soaps.

Step 3: Enter Your Oils and Butters

Now add the oils you want to use in your recipe. Most calculators have a long list of oils.

For each oil, enter:

Type of oil (like olive oil, coconut oil, shea butter)
Amount of oil (grams or ounces)

Tip: Stick to one measurement system—don’t mix grams and ounces in the same recipe.

Step 4: Set Your Superfat Level

Superfat is the amount of extra oil left unsaponified in the soap. This adds moisturizing properties and prevents lye-heavy soaps.

Typical superfat levels:

5% – the most common
3% – for very cleansing bars
8–10% – for luxury bars with more oils

Choose your superfat before calculating.

Step 5: Select Water Amount or Water Discount

Most calculators give you two options:

Standard water amount

Good for beginners. Easy to mix and slower to trace.

Water discount (less water)

Good for experienced soap makers. Faster to trace and cures quicker.

If unsure, use the default water amount the calculator provides.

Step 6: Click “Calculate”

After filling in oils, weights, lye type, and superfat, click the Calculate button.

The calculator will show:

Exact lye amount needed
Exact water amount needed
Total batch weight
Oil properties (hardness, conditioning, cleansing, bubbly, creamy, etc.)

This is the most important part—copy or print the results so you can refer to them when making your batch.

Step 7: Review Oil Properties (Optional but Helpful)

Most calculators show predicted soap qualities:

Hardness — how firm your bar will be
Cleansing — how much it removes oils
Conditioning — how moisturizing it is
Bubbly lather — big bubbles
Creamy lather — lotion-like feel
INS value — overall balance (ideal range 130–160)

These values help you tweak your recipe before making the soap.

Step 8: Weigh Your Ingredients Exactly

Once you have your numbers:

Weigh oils with a digital scale
Weigh your lye (never eyeball it!)
Measure liquid for lye solution

Safety reminder:
Always add lye to water, never water to lye.
Stir slowly and wear gloves, goggles, and long sleeves.

Step 9: Save Your Recipe

Always record:

Oils used
Amounts
Water used
Lye amount
Superfat
Date made

This helps you repeat perfect batches later.

Common Mistakes to Avoid

❌ Guessing lye amounts
❌ Mixing ounces and grams
❌ Using wrong lye type (NaOH vs. KOH)
❌ Forgetting to double-check superfat
❌ Not measuring precisely
❌ Adding water to lye (dangerous!)

Final Thoughts

A lye calculator is your best friend in soapmaking. Once you learn how to use it, you’ll be able to create your own recipes safely, consistently, and confidently.