Complete Guide to Superfat: What It Is and Why It Matters

Superfat is the percentage of oil in a soap recipe that's deliberately left unsaponified. You get it by using slightly less lye than the oils could react with, which is why it's also called a lye discount. At 5% superfat, the most common setting, 5% of your oils stay behind as free oil in the finished bar, where they make the soap feel milder and give you a safety margin against a lye-heavy batch. Most body bars sit between 5 and 8%, utility soaps go lower, and a few specialty bars go much higher. This guide covers what superfat does, how the calculator applies it, how to pick a number for the soap you're making, and the mistakes to avoid.

What Is Superfat?

Superfat (also called "lye discount") is the percentage of oils in your recipe that remain unsaponified (not converted into soap). These free oils stay in your final bar, providing extra moisturizing benefits.

Simple explanation: If you use 5% superfat, 5% of your oils don't become soap. They remain as conditioning oils in your bar.

Superfat is different from glycerin, the other thing that makes handmade soap feel gentle. Glycerin is a by-product of saponification itself, made in every batch whatever the superfat setting, and commercial makers usually pull it out to sell separately. Superfat is on top of that: actual oil that never turned into soap, and the one you control with a single number.

Why Does Superfat Matter?

1. Moisturizing Properties

Those unsaponified oils act as emollients on your skin. Instead of all the oils being "used up" by lye, some remain to condition and moisturize. The difference between a 3% and an 8% bar is easy to feel: the higher one leaves skin softer after rinsing, and the lower one feels squeaky.

2. Safety Margin

Even with accurate scales, there's room for measurement error. A superfat provides insurance that you won't accidentally make lye-heavy (caustic) soap.

That margin covers more than a wobbly scale. The saponification values in any calculator are averages, and real oils vary. Olive oil from two harvests won't have exactly the same SAP value, and a jar of coconut oil that's been open for a year has drifted from the number in the database. A 5% superfat absorbs those differences. At 0%, every one of them lands on the wrong side of the line.

3. Gentleness

Higher superfat = gentler soap. This is especially important for facial soaps and sensitive skin formulas. Free oil in the bar also cushions the effect of a high-cleansing recipe, which is why coconut-heavy soaps lean on it so hard.

How Superfat Works (The Math)

Soap calculators reduce the lye amount to leave oils unsaponified.

Example:

• Your oils require 100g of lye for complete saponification
• At 5% superfat, the calculator uses 95g of lye
• Result: 5% of oils remain as free oils

In percentage terms, the calculator works out the lye needed for every gram of oil to saponify, then multiplies that by one minus the superfat. A 5% superfat means 95% of the full lye amount. An 8% superfat means 92%. A 0% superfat means all of it, with no margin at all.

Two details trip people up. First, superfat only changes the lye. The oils stay exactly as you entered them, and the water only moves if your calculator ties it to the lye amount through lye concentration, as Soaply does. Second, superfat is a percentage of the oils, not of the lye, and because different oils need different amounts of lye, the calculator has to work it out per oil. Never get there by shaving a round number off someone else's lye figure.

Our Soaply calculator handles this automatically. Just set your desired superfat percentage!

Choosing Your Superfat Percentage

If you're not sure, start at 5%. It's the default in the calculator because it works for almost every balanced recipe, and it's easier to judge what you want from a bar you've already made than from a table. Once you've used a 5% bar for a couple of weeks, you'll know whether you want it milder or more cleansing, and a move of one or two points in either direction is enough to feel.

The Trade-offs

Lower Superfat (0-3%)

Pros:

• More cleansing power
• Harder bars
• Less prone to rancidity (no free oils to go bad)

Cons:

• Can feel drying
• Less margin for error

Higher Superfat (7-10%)

Pros:

• Very moisturizing
• Gentle on skin
• Creamy feel

Cons:

• May reduce lather
• Softer bars
• Higher risk of DOS (Dreaded Orange Spots) from rancidity
• Shorter shelf life

The lather and rancidity points deserve a closer look because they're the ones that surprise people. Free oil in a bar works against lather the same way a greasy pan does: it breaks bubbles as they form. A recipe that foams beautifully at 5% can feel flat at 10%, with nothing else changed.

Rancidity is about what free oil does over time. Saponified oil is stable. Unsaponified oil oxidizes, and the more of it there is, and the more unsaturated it is, the sooner the bar shows dreaded orange spots and a crayon smell. A high superfat on a recipe built around stable oils like olive, coconut, and tallow holds up fine. The same superfat on a recipe heavy in sunflower, hemp, or grapeseed is asking for trouble within a few months.

Which Oils Get Superfatted?

Here's a common misconception: you can't control which specific oils remain unsaponified in cold process soap.

When you mix oils and lye, saponification happens somewhat randomly across all the fatty acids present. The "superfat" is a mix of whatever oils happened to escape saponification.

That has a practical consequence. Adding a tablespoon of expensive argan or rosehip oil "for the superfat" in a cold process batch doesn't put argan in the superfat. It puts argan in the pot, where most of it turns into soap along with everything else, and the free oil left over is a sample of the whole recipe. If you want a luxury oil in the bar for its own sake, that's a fine reason to use it. Just don't expect it to be the oil that survives.

The Exception: Hot Process

In hot process soap, you can add luxury oils AFTER saponification is complete. These oils definitely remain as your superfat. This is the only way to guarantee specific oils stay unsaponified. The hot process method has other trade-offs, but for a bar built around one special oil it's the right tool.

Superfat for Different Skin Types

Oily Skin

• Superfat: 3-5% - Don't want too many extra oils, focus on balanced cleansing

Normal Skin

• Superfat: 5% - The standard that works for most people

Dry Skin

• Superfat: 6-8% - Extra conditioning helps, use high-oleic oils in recipe

Sensitive/Baby Skin

• Superfat: 7-10% - Maximum gentleness, pair with gentle oils like olive, shea

The baby-gentle template in the calculator is a ready-made example of the high-superfat, low-cleansing approach, and a good starting point if you're formulating for eczema-prone or very young skin.

How Does Superfat Interact With Your Recipe?

Superfat isn't a number you choose in isolation. The right setting depends on what the rest of the recipe is doing.

High-coconut recipes need more. Coconut oil makes the most cleansing soap there is, and the free oil from a higher superfat is what keeps a coconut-heavy bar from stripping skin. The extreme case is the salt bar, usually made with 80 to 100% coconut oil and a superfat far above the normal range, because at a normal superfat that recipe would be unusable on skin. A 100% coconut laundry bar goes the other way, with a very low superfat, because in the wash you want all the cleansing and none of the oil.

Gentle recipes need less. A Castile bar or any recipe that's mostly olive oil is already mild, with a cleansing score near the bottom of the range. Pushing the superfat up on a recipe like that mostly buys you a softer bar and more time waiting for it to cure. The standard 5% is plenty.

Superfat and the property numbers are separate. The bar property predictions in the calculator describe the soap that forms, and they don't move when you change the superfat. Two recipes with identical scores can feel quite different at 3% and 8%. Read the properties to get the recipe balanced, then use superfat to fine-tune how it feels.

Superfat and lye concentration are separate too. Superfat sets how much lye you use. Lye concentration sets how much water you dissolve it in. Beginners sometimes reach for one when they mean the other. A soft bar caused by too much water wants a higher lye concentration, not a lower superfat, and a harsh bar wants a higher superfat, not more water.

Common Superfat Mistakes

1. Going Too High

Superfat above 10% often leads to soft, mushy bars, oils going rancid (DOS), and reduced shelf life.

2. Forgetting About Recipe Oils

High superfat with unstable oils (like sunflower or hemp) increases rancidity risk. Use stable oils or add Vitamin E.

3. Not Adjusting for Use

A 8% superfat facial bar makes sense. A 8% superfat laundry bar? Too soft and oily.

4. Treating Superfat as a Fix for a Bad Recipe

Superfat can soften the edges of a slightly aggressive bar. It can't rescue a recipe that's out of balance. If the cleansing score is well over the range, the answer is less coconut, not a 12% superfat that leaves you with a greasy, short-lived bar.

5. Changing the Superfat After the Fact

Superfat is set when you weigh the lye. If a batch turns out lye-heavy because of a scale error, you can't stir oil into the mold later and call it superfat. Rebatch it with more oil, or if it zaps badly, treat it as a lesson and start over.

6. Measuring Oils by Volume

Superfat is a percentage of oil weight. A recipe measured in cups can be off by 10% or more on some oils, which swallows the whole safety margin. Weigh everything.

How Do You Test Whether Your Superfat Is Right?

Make a small batch at 5% and use it for two weeks. Pay attention to how your skin feels an hour after a shower, not in the moment. Tight and dry means go up a point or two. Any sense of a film, or a bar that's turning to mush in the dish, means come down.

Then change one thing at a time. If you move the superfat from 5% to 7% and also swap an oil, you won't know which change you're feeling. Log the setting and the cure date for every batch in the batch journal, because a bar tested at three weeks feels harsher than the same bar at six, and it's easy to blame the superfat for an unfinished cure.

A zap test or pH strip tells you whether a bar is lye-heavy. It doesn't tell you anything about the superfat percentage beyond that, so don't expect it to distinguish a 3% bar from an 8% one. Your skin is the instrument for that.

Quick Reference

Try It Now

Experiment with superfat in our Soaply calculator. Try building the same recipe at 3%, 5%, and 8% superfat - notice how the lye amount changes while oils stay the same. Then pull up the baby-gentle template to see how a high superfat and a low-cleansing oil blend work together.

Finding your perfect superfat is part of developing your signature soap recipe!

💬 Frequently Asked Questions

What does superfat mean in soap making?

Superfat (also called lye discount) is the percentage of oils in your recipe that don't get converted into soap. These free oils remain in the finished bar, providing extra moisturizing and conditioning benefits for your skin.

What superfat percentage should I use?

5% is the standard for most recipes and works well for everyday body bars. Use 2-3% for shampoo bars or utility soap, and 6-8% for facial bars or sensitive skin formulas. Our calculator makes it easy to adjust and see how it affects your lye amount.

Can I choose which oils stay as the superfat?

In cold process soap, you can't control which specific oils remain unsaponified. Saponification happens randomly across all fatty acids. The only way to guarantee a specific oil stays as superfat is to add it after cook in hot process soap.

Does high superfat cause soap to go rancid?

It can. Unsaponified oils are more prone to oxidation than saponified ones. If you use a high superfat (above 8%), stick with stable oils like olive or coconut and consider adding Rosemary Oleoresin Extract as an antioxidant.

Is superfat the same as lye discount?

Yes, they're the same concept described differently. Superfat refers to the extra oils left in the bar. Lye discount describes the reduced lye amount. A 5% superfat means 5% less lye than full saponification requires.