Superfat is one of the most misunderstood settings in soap making. It is not simply "extra oil" — it is a calculated reduction in lye that leaves a defined percentage of oils unsaponified. Getting it right affects skin feel, shelf life, bar hardness, and safety all at once.
What superfat actually means
When you set a 5% superfat, the calculator reduces the lye so that only 95% of your oil weight is saponified. The remaining 5% stays in the soap as free fatty acids — unsaponified oils that contribute conditioning and moisturising properties.
The term "superfat" is sometimes used interchangeably with "lye discount", which describes the same thing from the lye side: you are discounting (reducing) the lye by 5% from what would be needed for full saponification.
All oils fully saponified. Maximum hardness and longest shelf life. Can feel stripping on skin. Rarely recommended for rinse-off products.
The standard range for most bar soaps. Good balance of conditioning, hardness, and safety margin. Most beginner recipes target 5%.
More conditioning. Good for sensitive skin or facial bars. Slightly softer bar, shorter shelf life. Watch for rancidity in high-unsaturated-fat recipes.
Very conditioning but significantly softer and prone to rancidity (DOS). Requires a stable, saturated-fat-heavy oil base.
What superfat affects
Skin feel
Higher superfat means more free fatty acids in the finished bar. Depending on which oils dominate your recipe, this translates to a more conditioning wash. Castor oil and olive oil superfat feel noticeably different on skin than coconut oil superfat. The calculator cannot predict which oils become the free fatty acids — saponification is not selective.
Shelf life
Unsaponified oils are more vulnerable to rancidity than soap salts. A 10% superfat soap made with high-linoleic oils (hemp, sunflower) may show DOS (dreaded orange spots) within months. Keep high-superfat soaps away from humidity and use a stable antioxidant — rosemary oleoresin extract (ROE) at 0.1% is the most effective natural option.
Bar hardness
Higher superfat produces a slightly softer bar because there is less soap and more free oil. The effect is modest at typical levels (3–8%) but becomes noticeable above 10%. Oil choice matters more than superfat for hardness — coconut and palm produce hard bars regardless of superfat.
Safety margin
This is the most important function of superfat for new soap makers. SAP values are averages — real oil values vary slightly by crop and supplier. A 5% superfat gives you a buffer: even if your oil's true SAP value is slightly higher than the database value, the soap will not be lye-heavy.
Touch a tiny piece of cured soap to the tip of your tongue. A sharp "zap" (like licking a 9V battery) indicates excess lye. A properly superfatted soap will not zap. Never skip the zap test on a first batch from a new recipe.
Choosing your superfat percentage
| Soap type | Recommended superfat | Reason |
|---|---|---|
| Standard bar soap | 5% | Best all-round balance |
| Facial / sensitive skin bar | 6–8% | More conditioning, gentler |
| Shampoo bar | 2–3% | Lower SF reduces scalp greasiness |
| Shaving soap | 3–5% | Good glide without residue |
| Liquid soap | 3–5% | Higher SF can cause cloudiness |
| Laundry soap | 0–2% | Cleaning-focused; skin feel irrelevant |
| 100% coconut oil recipe | 15–20% | Coconut is very cleansing — high SF balances it |
The 100% coconut oil exception
Recipes made with 100% coconut oil are so cleansing that they strip skin aggressively at a normal 5% superfat. Most formulators run these at 15–20% superfat to compensate. SoapCalc.pro's property chart will flag a very high cleansing score when you load a coconut-heavy recipe — this is the signal to increase your superfat.
Set your superfat and calculate in seconds
Adjust superfat with a single input in SoapCalc.pro. The property chart updates instantly so you can see the effect on your recipe before you make it.