Superfatting means intentionally leaving extra oils in your soap that don’t get converted into soap during the chemical reaction. Every soap recipe involves a reaction between fats (oils and butters) and lye, and superfatting ensures that some percentage of those fats remain in their original, skin-nourishing form. Most handmade soap uses a superfat of 5% to 10%, meaning that percentage of the total oils stays unreacted in the finished bar.
The Chemistry Behind Superfatting
Soap is made when lye (sodium hydroxide) reacts with fats in a process called saponification. Every oil has its own saponification value, which is the precise amount of lye needed to convert one gram of that oil into one gram of soap. When you use exactly enough lye to convert every molecule of fat, that’s a 0% superfat. Nothing is left over.
Most soap makers deliberately tip this balance so that some oil remains unsaponified. You can do this in two ways: add more fat than the lye can react with, or reduce the amount of lye below what’s needed. The first method is true superfatting. The second is called a lye discount. The end result is the same: unreacted fat stays behind in the finished bar, and that leftover fat is what makes the soap gentler and more moisturizing.
Why It Matters for Your Skin
Your skin is covered by a thin protective layer made of natural oils, sweat, and water. Regular soap strips this layer away, which is why your skin can feel tight and dry after washing. The unreacted oils in a superfatted bar help preserve that protective layer instead of dissolving it completely. This reduces dryness, irritation, and that uncomfortable tightness after you rinse off.
Superfatted soaps also tend to be formulated closer to the skin’s natural pH, which makes them less harsh than conventional soap. For people with sensitive or dry skin, this combination of leftover oils and a milder pH can make a noticeable difference in how their skin feels day to day.
Recommended Percentages by Soap Type
The right superfat percentage depends entirely on what the soap is for. Higher percentages leave more oil behind, which means more moisture but also trade-offs in lather and longevity.
- Body bars: 5% to 10% is the standard range. If you want extra moisture, you can push this to 10% or even 15%.
- Face bars: 10% to 15% works well for facial skin, which is thinner and more prone to drying out.
- Laundry bars: Keep it to 1% to 2%. The goal here is cleaning power, not skin conditioning, and excess oil would leave residue on fabric.
Choosing Oils for Superfatting
Not all oils contribute the same properties. Soft oils high in oleic acid, like olive oil, avocado oil, almond oil, and sunflower oil, are prized for their nourishing feel on skin. “Luxury oils” like argan, jojoba, hemp seed, evening primrose, and meadowfoam are often added at 5% to 10% of a recipe specifically for their conditioning effects. Hard fats like shea butter, cocoa butter, and mango butter add richness and a creamy lather.
Which oils actually end up as the superfat in your bar depends on your soap making method, and this is one of the biggest practical differences between the two main approaches.
Cold Process vs. Hot Process Superfatting
In cold process soap making, saponification happens slowly in the mold over hours and days. Because all the oils are mixed together with lye at once, you can’t control which specific oils end up unreacted. The lye reacts with whichever fat molecules it encounters, so your superfat is a random mix of everything in the recipe.
Hot process soap is different. Saponification happens during cooking, and by the time you’re done, all the lye has been used up. You can then stir in your superfat oils after the cook is finished. This means you can choose exactly which oil becomes the superfat. If you want your unsaponified fat to be pure shea butter or argan oil, hot process gives you that control. This is one of the main reasons soap makers choose hot process despite its rougher texture and less polished appearance.
How to Calculate Superfat
The math is straightforward. Say a recipe calls for 450 grams of oil and 120 grams of lye at a 0% superfat. To superfat by adding extra oil, you’d keep the 120 grams of lye but increase the oil to around 475 grams. Those extra 25 grams won’t have any lye to react with, so they stay in the bar as free oil.
To achieve the same result through a lye discount, you’d keep the original 450 grams of oil but reduce the lye from 120 grams to about 115 grams. Less lye means some fat has no partner to react with, leaving you with the same unsaponified oil in the finished soap. Most online soap calculators handle this automatically. You enter your oils and desired superfat percentage, and the calculator adjusts the lye amount for you.
The Risk of Going Too High
More superfat sounds like it should always be better, but there’s a ceiling. Unreacted oils can go rancid over time, especially soft oils like olive oil that are prone to oxidation. The visible sign of this is called Dreaded Orange Spots, or DOS: small orange or yellowish splotches that appear on the surface of the bar and often come with an unpleasant smell.
Research from the Handcrafted Soap and Cosmetic Guild compared bars made at 0% and 10% lye discount. The 10% discount soap did become more orange over time than the 0% bar, confirming that higher superfat accelerates the problem. That said, even the 0% bar eventually developed some discoloration, so superfatting isn’t the only factor. Bars with high superfat percentages simply have more free oil available to oxidize, which shortens their shelf life. If you’re making soap to sell or store for months, keeping the superfat at 5% to 8% for body bars is a practical compromise between skin feel and longevity.
Storing your finished soap in a cool, dry place away from direct light also helps slow oxidation, regardless of your superfat level.