How to Calculate Soap Lye (SAP Values Explained)

Why Lye Calculations Matter More Than Any Other Step

Getting your lye calculation right is the single most important part of soap formulation. Too much lye and your soap will be caustic and unsafe. Too little and you end up with a soft, oily bar that never fully saponifies. There is no room for guesswork here.

The good news is that lye calculation follows a straightforward formula once you understand the underlying concept: SAP values. Every oil and butter has a known SAP value that tells you exactly how much lye is needed to turn it into soap. Master this, and you can confidently formulate any recipe from scratch.

This guide walks through everything you need to know, from the chemistry behind SAP values to practical step-by-step calculations you can use today.

What Are SAP Values?

SAP value (short for saponification value) is a number that represents how much alkali is required to fully saponify one gram of a specific fat or oil. In practical terms, it tells you the exact amount of lye needed to convert an oil into soap.

Every oil has a unique SAP value because each oil is made up of a different combination of fatty acids. Coconut oil, for example, is rich in lauric acid (a short-chain fatty acid), so it requires more lye per gram than olive oil, which is dominated by oleic acid (a longer-chain fatty acid).

SAP values are determined through laboratory testing and are published in standardized reference tables. You will see them expressed differently depending on whether you are making bar soap or liquid soap.

NaOH vs KOH SAP Values

There are two types of lye used in soap making, and each has its own set of SAP values:

• NaOH (Sodium Hydroxide) is used for bar soap (cold process and hot process). NaOH SAP values are lower numbers because sodium hydroxide is more concentrated by weight.
• KOH (Potassium Hydroxide) is used for liquid soap and cream soap. KOH SAP values are higher because potassium hydroxide is less concentrated, so you need more of it.

The relationship between them is consistent. You can convert a KOH SAP value to NaOH by multiplying by 0.7133 (the ratio of their molecular weights). Conversely, divide an NaOH SAP value by 0.7133 to get the KOH equivalent.

For the rest of this guide, we will focus on NaOH SAP values since bar soap is the most common starting point for formulators.

Common Oil SAP Values

Here are the NaOH SAP values for the most widely used soap-making oils and butters. These values represent grams of NaOH needed per gram of oil.

Oil / Butter	NaOH SAP Value	KOH SAP Value
Coconut Oil (76 deg)	0.178	0.2495
Olive Oil	0.134	0.1879
Palm Oil	0.141	0.1977
Shea Butter	0.128	0.1795
Cocoa Butter	0.137	0.1921
Castor Oil	0.128	0.1795
Avocado Oil	0.133	0.1865
Sweet Almond Oil	0.136	0.1907

These values can vary slightly between sources because natural oils are not perfectly uniform. Reputable SAP tables use averages from laboratory analysis. When you see minor differences between calculators, this is why.

Step-by-Step Lye Calculation

Let us work through a complete example. Say you want to make a batch of cold process soap with the following recipe:

• 400g Olive Oil (40%)
• 300g Coconut Oil (30%)
• 200g Palm Oil (20%)
• 100g Shea Butter (10%)

Total oils: 1000g

Step 1: Multiply Each Oil by Its SAP Value

For each oil in your recipe, multiply the weight by its NaOH SAP value:

• Olive Oil: 400g x 0.134 = 53.60g NaOH
• Coconut Oil: 300g x 0.178 = 53.40g NaOH
• Palm Oil: 200g x 0.141 = 28.20g NaOH
• Shea Butter: 100g x 0.128 = 12.80g NaOH

Step 2: Add the Results

Add all the individual lye amounts together:

53.60 + 53.40 + 28.20 + 12.80 = 148.00g NaOH (before superfat)

This is the amount of lye needed to fully saponify 100% of your oils with zero superfat.

Step 3: Apply Your Superfat Discount

You never want to use the full amount of lye (more on this below). Subtract your desired superfat percentage:

At 5% superfat: 148.00 x 0.95 = 140.60g NaOH

That is your final lye amount. Now you need to calculate your water.

Water Calculation Methods

Lye must be dissolved in water before it can be mixed with your oils. There are three common methods for determining how much water to use, and each has its advantages.

Method 1: Water as a Percentage of Oils

This is the simplest method. You use water equal to a percentage of your total oil weight, typically 33% to 38%.

For our 1000g oil batch at 33%:

• Water = 1000g x 0.33 = 330g water

This method is easy to remember but does not account for how much lye you are dissolving. Recipes with higher lye amounts end up with a more dilute solution, while low-lye recipes get a more concentrated one.

Method 2: Lye Concentration

This method focuses on the concentration of your lye solution, typically targeting a 28% to 33% lye concentration. It gives you more consistent results across different recipes.

The formula is: Water = Lye Weight / Concentration - Lye Weight

At 33% lye concentration with 140.60g NaOH:

• Total solution = 140.60 / 0.33 = 426.06g
• Water = 426.06 - 140.60 = 285.46g water

A higher concentration (less water) means faster trace and faster unmolding. A lower concentration gives you more working time.

Method 3: Water-to-Lye Ratio

This method expresses water as a multiple of your lye weight. A common ratio is 2:1 (two parts water to one part lye) or 1.5:1 for experienced makers who want a faster setup.

At 2:1 ratio with 140.60g NaOH:

• Water = 140.60 x 2 = 281.20g water

Many experienced soapmakers prefer this method for its simplicity and the control it offers over trace speed.

Superfat Explained

Superfat (also called a lye discount) means intentionally using less lye than would be needed to saponify all of your oils. The "extra" unsaponified oil remains in the finished soap, contributing to moisturizing and skin-conditioning properties.

Why Superfat?

There are two important reasons to always include a superfat:

1. Safety margin. Even with careful measuring, slight inaccuracies in scale readings or SAP value variations could result in lye-heavy soap. A superfat provides a buffer.
2. Skin feel. Free oils in the finished bar make it more conditioning and less stripping on the skin.

Typical Superfat Ranges

• 3-4%: Minimum safe margin. Produces a harder, more cleansing bar. Good for laundry soap.
• 5%: The most common default. Balances safety, cleansing, and conditioning.
• 6-8%: More conditioning bars. Popular for facial soaps and recipes with luxurious butters.
• Above 8%: Can lead to a softer bar and may cause the soap to feel greasy or develop DOS (dreaded orange spots) from rancidity of unsaponified oils over time.

For most cold process bar soap, 5% superfat is a reliable starting point. Adjust from there based on your goals and the oils you are using.

Soap Quality Properties

Beyond just calculating lye, experienced formulators evaluate their recipes against a set of soap quality properties. These numbers, derived from the fatty acid profile of your oil blend, predict how the finished bar will perform.

Hardness (29-54)

Measures how hard the bar will be. Higher values mean a longer-lasting bar. Saturated fats like palm oil, coconut oil, and tallow contribute to hardness. Aim for 29 to 54, with most balanced recipes landing around 35 to 45.

Cleansing (12-22)

Indicates how effectively the soap strips oils from skin. This is driven primarily by lauric and myristic acids, which are abundant in coconut oil and palm kernel oil. Too high and the bar will be drying. The recommended range is 12 to 22.

Conditioning (44-69)

Reflects how moisturizing the bar feels. Oleic acid (from olive oil, avocado oil, sweet almond oil) is the primary contributor. Higher conditioning values create a gentler bar. Target 44 to 69.

Bubbly Lather (14-46)

Predicts how much fluffy, airy lather the soap produces. Lauric and ricinoleic acids (coconut oil and castor oil) boost bubbly lather. Range of 14 to 46 is recommended, with castor oil being the secret weapon for lather.

Creamy Lather (16-48)

Measures the thick, lotion-like quality of the lather rather than big bubbles. Palmitic and stearic acids (from palm oil, shea butter, cocoa butter) contribute here. Aim for 16 to 48.

Iodine Value (41-70)

Indicates the degree of unsaturation in your oil blend. Higher values mean more unsaturated (soft) oils. Very high iodine values (above 70) suggest the bar may be too soft and could be prone to rancidity. Keep it in the 41 to 70 range.

INS Value (136-170)

A composite score that combines SAP value and iodine value to give an overall quality prediction. An INS value of 160 is often cited as the ideal for a well-balanced bar, with the acceptable range being 136 to 170.

No single recipe will hit the ideal range for every property. The goal is to balance trade-offs. A bar heavy on coconut oil will have great lather and hardness but poor conditioning. Adding olive oil improves conditioning but reduces hardness. Formulation is the art of finding the blend that matches your goals.

Common Mistakes to Avoid

Even with a solid understanding of SAP values, there are pitfalls that trip up both new and experienced soap makers.

Confusing NaOH and KOH SAP Values

This is the most dangerous mistake. KOH SAP values are roughly 40% higher than NaOH values. If you accidentally use KOH SAP values in a bar soap recipe (which calls for NaOH), you will use far too much lye and produce a caustic, unsafe product. Always confirm which type of SAP value your source is listing.

Not Accounting for Lye Purity

Commercial NaOH is typically sold at 97-99% purity, not 100%. If your lye is 97% pure, you need to divide your calculated lye amount by 0.97 to compensate. Many calculators handle this automatically, but if you are doing math by hand, do not skip this step.

Measuring by Volume Instead of Weight

Lye and oils must always be measured by weight, not volume. Different oils have different densities, and scooping lye by the cup is wildly inaccurate. Invest in a digital scale accurate to at least 1 gram (0.1g for smaller batches).

Using Outdated or Unreliable SAP Tables

SAP values from obscure internet sources may be inaccurate. Cross-reference values across multiple reputable sources, and be especially cautious with exotic or uncommon oils where published data may be limited.

Ignoring Fragrance Oil Behavior

Some fragrance oils accelerate trace dramatically, leaving you no time to pour or swirl. Others cause the batter to seize or rice. While this is not directly a lye calculation issue, it can ruin a perfectly formulated batch. Always research fragrance behavior before adding it to your recipe.

Skipping the Superfat

Running a recipe at 0% superfat leaves no safety margin. Even a tiny measurement error could result in a lye-heavy bar. Always include at least a 3% superfat, and test your finished soap with pH strips or a zap test before sharing or selling.

Scaling Errors

When scaling a recipe up or down, every component must be adjusted proportionally, including lye and water. A common mistake is adjusting the oils but forgetting to recalculate the lye. Always recalculate from scratch when changing batch size.

Putting It All Together

Lye calculation is a skill that becomes second nature with practice. The core formula is straightforward:

1. Multiply each oil weight by its NaOH SAP value.
2. Sum the results.
3. Subtract your superfat percentage.
4. Calculate water using your preferred method.

Once you are comfortable with the math, you can start tweaking your oil blends to target specific soap quality properties, balancing hardness against conditioning, dialing in lather, and building recipes that match your brand's identity.

If you would rather skip the manual math and focus on the creative side, Formuley's built-in soap calculator handles SAP values, lye calculations, water ratios, and soap quality properties automatically. Just enter your oils and percentages, and it generates a complete, accurate recipe with full property analysis, so you can spend less time with a calculator and more time making soap.