Soap Superfat Calculator

How This Tool Works

This calculator determines the superfat percentage by comparing total oil weight to lye weight used in your soap recipe. Superfat (also called lye discount) represents the percentage of oils that remain unreacted with lye in the finished soap. This excess oil provides moisturizing properties and ensures the soap isn't too harsh or drying. A superfat of 5-8% is standard and provides a good balance between cleansing and moisturizing properties.

The calculation works by determining how much oil would be needed if all the lye reacted, then comparing that to your actual oil amount. The difference is the superfat. For example, if you use 500g of oils and 70g of lye, but the lye would only react with 460g of oils at 0% superfat, you have a 8% superfat (40g excess oil ÷ 500g total oils). Higher superfat percentages create more moisturizing soap but can make the soap softer and reduce shelf life if too high.

Formula

The superfat percentage is calculated using:

Superfat % = ((Total Oils - Lye Used) ÷ Total Oils) × 100

Example: If you use 500g total oils and your lye calculator indicates you need 70g lye for 5% superfat:

If 0% superfat would require 73.7g lye (example):
Excess oil = 73.7 - 70 = 3.7g (unreacted)
Superfat = (3.7 ÷ 500) × 100 = 0.74%
Or more simply: Superfat = ((73.7 - 70) ÷ 500) × 100 ≈ 0.7%

Note: This calculator works backwards from your actual recipe. For forward calculation (determining lye amount), use a lye calculator that accounts for saponification values of your specific oils.

Understanding Superfat

• 0-3% Superfat: Very cleansing, may be drying for some skin types, longer shelf life
• 5-8% Superfat: Standard range - good balance of cleansing and moisturizing, most common
• 10-15% Superfat: Very moisturizing, may be too soft, shorter shelf life, can feel greasy
• Above 15%: Not recommended - soap may be too soft, greasy, and have poor lather
• Purpose: Provides moisturizing properties, ensures soap isn't too harsh, adds skin benefits
• Trade-offs: Higher superfat = more moisturizing but softer soap and potentially shorter shelf life

Soap Making Best Practices

• Use a Lye Calculator: Always use a reliable lye calculator to determine correct lye amounts for your oils
• Safety First: Wear gloves, goggles, and work in well-ventilated area when handling lye
• Measure Accurately: Use a digital scale to measure oils and lye by weight, not volume
• Temperature Control: Monitor temperatures - most soaps work best when oils and lye solution are 38-43°C
• Mix Thoroughly: Ensure complete mixing to trace - incomplete mixing can cause lye pockets
• Cure Properly: Allow soap to cure for 4-6 weeks for best quality and longer shelf life
• Test pH: Finished soap should have pH around 9-10 - test with pH strips to ensure safety
• Keep Records: Document your recipes and results to learn what works best for your soap

FAQ

• What is superfat in soap making?
  Superfat (also called lye discount) is the percentage of oils that remain unreacted with lye in the finished soap. When making soap, you use slightly less lye than would be needed to react with all the oils. This excess oil provides moisturizing properties, ensures the soap isn't too harsh or drying, and adds beneficial properties to the finished soap. A 5-8% superfat is standard and provides a good balance between cleansing and moisturizing. Higher superfat (10%+) creates very moisturizing soap but can make it softer and reduce shelf life.
• What is a good superfat percentage?
  Most soap makers use 5-8% superfat for a good balance of cleansing and moisturizing properties. This range works well for most skin types and provides good lather. Lower superfat (0-3%) creates very cleansing soap but may be too drying for some people. Higher superfat (10-15%) is very moisturizing but can make soap too soft, reduce lather, shorten shelf life, and may feel greasy. For beginners, start with 5% superfat and adjust based on your results. Different oil combinations may work better with different superfat levels.
• How do I calculate superfat when making soap?
  Use a lye calculator (online tool or app) to determine the correct lye amount for your oils at your desired superfat percentage. Enter your oil amounts and desired superfat (e.g., 5%), and the calculator will tell you how much lye to use. The superfat is built into the lye calculation - you use less lye than would be needed for 0% superfat. This calculator works backwards - if you know your total oils and lye amount, it calculates what superfat percentage you have. Always use a reliable lye calculator when formulating recipes.
• Does superfat affect soap hardness?
  Yes, higher superfat percentages can make soap softer because excess oils don't contribute to the soap's structure. However, the type of oils used has a bigger impact on hardness. Hard oils (coconut, palm) create harder soap, while soft oils (olive, canola) create softer soap. A well-balanced recipe with appropriate hard and soft oils can handle 5-8% superfat without becoming too soft. Very high superfat (15%+) will definitely make soap softer and may reduce its ability to hold its shape. Proper curing (4-6 weeks) also helps soap harden.
• Can I adjust superfat after making soap?
  No, you cannot adjust superfat after the soap is made. The superfat is determined by the ratio of oils to lye in your recipe. Once the soap has saponified (turned into soap), the chemical reaction is complete. If your soap is too drying, you'll need to make a new batch with higher superfat. If it's too soft or greasy, make a new batch with lower superfat. This is why it's important to calculate superfat correctly before making your soap. Always test small batches first to find the superfat level that works best for your recipe and skin type.
• What's the difference between superfat and lye discount?
  Superfat and lye discount are the same thing - just different terms for the same concept. Both refer to using less lye than would be needed to react with all the oils, leaving excess oils in the finished soap. "Superfat" emphasizes the excess fat/oil in the soap, while "lye discount" emphasizes that you're discounting (reducing) the lye amount. Some soap makers prefer one term over the other, but they mean exactly the same thing. A 5% superfat is the same as a 5% lye discount.