lye calculator

Calculate precise Sodium Hydroxide (NaOH) or Potassium Hydroxide (KOH) amounts for soap making.

Ingredient	Weight (g)	Percentage (%)

What is a Lye Calculator?

A Lye Calculator is an essential tool for soap makers, whether they are hobbyists or professionals. It calculates the precise amount of Sodium Hydroxide (NaOH) or Potassium Hydroxide (KOH) required to turn fats and oils into soap through a chemical process called saponification. Without a reliable Lye Calculator, soap makers risk creating "lye-heavy" soap that can burn the skin or "soft" soap that never hardens properly.

Anyone practicing cold process or hot process soap making should use a Lye Calculator. It eliminates the guesswork and ensures that every batch is safe for use. A common misconception is that you can use a single lye amount for any oil; however, every oil has a unique Saponification Value (SAP), meaning they require different amounts of lye to neutralize.

Lye Calculator Formula and Mathematical Explanation

The math behind a Lye Calculator is based on the SAP value of the specific oils used. The SAP value represents the number of milligrams of potassium hydroxide required to saponify one gram of fat.

The core formula used by this Lye Calculator is:

Lye Amount = (Oil Weight × SAP Value) × (1 - Superfat Percentage)

Variable	Meaning	Unit	Typical Range
Oil Weight	Mass of fats/oils used	Grams (g)	100 – 10,000g
SAP Value	Saponification index	Ratio	0.120 – 0.200
Superfat	Excess oil for skin conditioning	Percentage (%)	1% – 10%
Water Ratio	Amount of water per unit of lye	Ratio	1.5:1 – 3:1

Practical Examples (Real-World Use Cases)

Example 1: Pure Olive Oil Soap (Castile)

If you are making a 1000g batch of pure Olive Oil soap using this Lye Calculator with a 5% superfat:

• Inputs: 1000g Olive Oil, 0.135 SAP, 5% Superfat.
• Calculation: 1000 × 0.135 = 135g (Theoretical Lye). 135 × 0.95 = 128.25g Lye.
• Result: You need 128.25g of NaOH.

Example 2: Coconut Oil Cleaning Soap

Coconut oil is highly cleansing. For a 500g batch with 1% superfat:

• Inputs: 500g Coconut Oil, 0.191 SAP, 1% Superfat.
• Calculation: 500 × 0.191 = 95.5g. 95.5 × 0.99 = 94.55g Lye.
• Result: 94.55g of NaOH is required.

How to Use This Lye Calculator

1. Enter Oil Weight: Input the total weight of the oils you plan to use.
2. Select Oil Type: Choose the oil from the dropdown to automatically apply the correct SAP value.
3. Choose Lye Type: Select NaOH for solid bars or KOH for liquid soap.
4. Set Superfat: Adjust the superfat percentage (5% is standard for body soap).
5. Define Water Ratio: Set how much water you want to use relative to the lye.
6. Review Results: The Lye Calculator updates in real-time to show lye, water, and total weight.

Key Factors That Affect Lye Calculator Results

• SAP Value Accuracy: Different batches of natural oils can have slightly varying SAP values.
• Lye Purity: Most NaOH is 99% pure, but KOH is often sold at 90% purity, requiring adjustment.
• Superfatting: Higher superfatting makes soap gentler but reduces lather and shelf life (DOS risk).
• Water Discounting: Using less water (lower ratio) speeds up curing but makes the soap batter thicken faster.
• Temperature: While not in the formula, temperature affects how quickly the lye reacts with the oil.
• Additives: Ingredients like sugar, salt, or silk don't change lye requirements but change soap properties.

Frequently Asked Questions (FAQ)

1. Why does the Lye Calculator show different amounts for NaOH and KOH?

NaOH (Sodium Hydroxide) creates hard bars, while KOH (Potassium Hydroxide) creates liquid soap. KOH molecules are heavier, so you need more of it by weight to saponify the same amount of oil.

2. What is a safe superfat percentage?

For most body soaps, 5% to 8% is ideal. For laundry soap, 0% to 1% is preferred to avoid leaving oil on clothes.

3. Can I use this Lye Calculator for dual-lye recipes?

This version calculates for one lye type at a time. For dual-lye (shaving soap), you would calculate both and split the ratios manually.

4. What happens if I use too much water?

The soap will take much longer to harden and cure. It may also experience "glycerin rivers" or excessive shrinkage.

5. Is the SAP value the same for all brands of Olive Oil?

Generally, yes, but "Pomace" olive oil has a slightly different SAP value than "Extra Virgin." This Lye Calculator uses average values.

6. Why is my soap crumbly?

This often happens if the lye amount was too high or if you used too little water. Always double-check your inputs in the Lye Calculator.

7. Does the Lye Calculator account for fragrance oils?

No, fragrance oils do not saponify and do not require lye. They should be calculated as a percentage of the oil weight separately.

8. Can I swap oils without recalculating?

No! Swapping Coconut oil for Olive oil without using a Lye Calculator is dangerous because their SAP values are significantly different.