Soap Superfat & Lye Purity Adjuster – Online Cold Process Calc

Superfat (also called lye discount) is the practice of using less lye than theoretically required to saponify all oils. This leaves a portion of oils unsaponified in the final soap, providing extra moisturizing properties, a gentler feel, and a safety margin against lye-heavy soap. Typical superfat ranges from 3% to 8%. A 5% superfat means 5% of the oils remain as free oils in the finished bar.

Commercial lye (NaOH or KOH) is never 100% pure. Food-grade NaOH is typically 97-99% pure, while technical-grade may be 90-96%. KOH is often 85-95% pure. If your lye is 95% pure, you need to use more to compensate for the 5% inert material. Our calculator automatically adjusts the lye weight based on the purity percentage you specify, ensuring your soap turns out correctly.

NaOH (Sodium Hydroxide) produces solid bar soap. It creates a hard, long-lasting bar suitable for body and hand soap. KOH (Potassium Hydroxide) produces liquid or soft paste soap. It's used for liquid soaps, shaving soaps, and cream soaps. The SAP value (saponification value) differs between the two — KOH requires about 1.403× more weight than NaOH for the same amount of oil because KOH has a higher molecular weight.

A common water:lye ratio for cold process NaOH soap is 2:1 to 3:1 (water is 2-3× the lye weight). This corresponds to a lye concentration of about 25-33%. Less water (steeper water discount, like 2:1) means faster trace and shorter cure time, but can be trickier to work with. More water (like 3:1) gives more working time but longer curing. For KOH liquid soap, a 3:1 ratio is typical. Our default is 2.5:1 for NaOH and 3:1 for KOH.

SAP (Saponification Value) is the number of milligrams of KOH required to saponify 1 gram of a specific oil. Each oil has a unique SAP range. For example, coconut oil has a high SAP (~256), meaning it requires more lye, while olive oil has a lower SAP (~192). Accurate SAP values are critical for calculating the correct lye amount. Our calculator uses industry-standard KOH SAP values and converts them for NaOH when needed using the factor 1.403 (KOH MW ÷ NaOH MW = 56.1 ÷ 40.0).

Superfat above 10% can lead to a soft bar that spoils faster (rancidity risk from excess free oils) and reduced lather. For most recipes, 5-8% is the sweet spot for a moisturizing yet firm bar. Laundry or cleaning soaps often use 0-2% superfat to maximize cleaning power. High superfat (8-10%) works well for facial bars or dry-skin formulas but may shorten shelf life. Above 12% is generally not recommended for solid bar soap.

Yes! The lye calculations are identical for cold process and hot process soap. The difference is in the method: hot process uses external heat to complete saponification before molding. With hot process, superfat oils are often added after the cook (after saponification is complete), so you have more control over which oils remain as the superfat. The lye amounts calculated here work for both methods — just adjust your process accordingly.

If you overestimate lye purity (thinking it's 99% when it's actually 90%), you'll end up with lye-heavy soap — harsh, drying, and potentially irritating to skin. If you underestimate purity, you'll have more superfat than intended, resulting in a softer, possibly greasy bar. Always check the purity stated on your lye container. If unsure, you can test purity or use a conservative estimate (e.g., 95% for technical-grade NaOH). Our calculator makes adjusting for purity straightforward.