What is superfatting (lye discount)?

Superfatting means using less lye than needed to fully saponify all the oils. The un-saponified oils remain in the soap, adding moisturising properties. A 5% superfat means 5% of the oils remain as free oil in the finished soap. The terms superfat and lye discount are interchangeable.

What superfat percentage should I use?

For most bar soaps, 5% is the standard starting point. For facial or luxury soaps, 6-8% gives extra mildness. For laundry or cleaning soap, 1-3% gives better cleansing. Above 10% can lead to a soft bar with a shorter shelf life due to unsaponified oils going rancid.

Does lye purity matter?

Yes. Most food-grade sodium hydroxide is 97-99% pure. If your lye is 97% pure, you need slightly more to achieve the same saponification. This calculator adjusts for purity automatically. Always check your supplier label for the purity percentage.

What is the difference between NaOH and KOH?

Sodium hydroxide (NaOH) makes solid bar soap. Potassium hydroxide (KOH) makes liquid soap. KOH requires approximately 1.4 times more lye than NaOH for the same amount of oil because potassium has a higher molecular weight than sodium.

CraftCalcs

Lye Discount Calculator

Calculate superfat percentage and adjusted lye amounts for soap recipes.

Total Oil Weight

Combined weight of all oils in recipe

Combined SAP Value

Weighted average SAP value of oil blend (NaOH)

Superfat / Lye Discount

Percentage of oils to leave unsaponified

Lye TypeType of lye for your soap

How We Calculate This

Full Lye Amount

Full Lye = Oil Weight x SAP Value

Lye Discount

Discounted Lye = Full Lye x (1 - Superfat%)

Purity Adjustment

Adjusted Lye = Discounted Lye / (Purity% / 100)

Frequently Asked Questions

Related Calculators

Lye Calculator

Calculate sodium hydroxide (NaOH) or potassium hydroxide (KOH) amounts for cold process soap.

Oil Ratio Calculator

Balance your soap recipe oils for hardness, lather, conditioning and cleansing.

Fragrance Load Calculator

Work out safe fragrance or essential oil amounts for soap batches.

Mould Volume Calculator

Calculate soap mould volume and the total oil weight needed to fill it.

SAP Value Calculator

Look up and calculate saponification values for common soap-making oils.

Last updated: March 2026

All calculations are estimates. Always verify quantities before purchasing materials.