Soap Lye

Calculating Lye and Water for Cold-Process Soap

Cold-process soap making works by mixing oils/fats with lye (sodium hydroxide, NaOH) dissolved in water — the lye reacts with the oils in a process called saponification, turning them into soap. Every oil needs a different amount of lye to fully saponify, because each is made of different fatty acids — this is captured by that oil’s SAP value (saponification value): how many grams of NaOH are needed to saponify exactly one gram of that oil. Enter your oils and their weights, a superfat percentage, and how much water to dissolve the lye into, and this calculator works out exactly how much lye and water your recipe needs.

Superfat (also called a “lye discount”) is the deliberate practice of using slightly less lye than a full saponification needs, so a small percentage of oils are left unreacted in the finished bar — this is what keeps cold-process soap gentle and moisturizing rather than harsh. 5% is a widely-cited standard starting point, though soap makers sometimes use more or less depending on the recipe.

Safety: Handling Lye Safely

Sodium hydroxide is a strong caustic base — it causes serious chemical burns on contact with skin or eyes, and mixing it into water is an exothermic reaction that gets genuinely hot. This calculator computes recipe amounts only; it does not, and cannot, make lye safe to handle carelessly. Before making a batch:

• Always add the lye to the water, never the water to the lye. Adding water to lye can cause a violent, splattering reaction as the lye dissolves.
• Wear chemical-resistant gloves and eye protection (safety goggles, not just glasses) whenever you’re measuring, mixing, or working with lye or raw (unsaponified) soap batter.
• Work in a well-ventilated area. Mixing lye into water releases fumes that can irritate your eyes, nose, and throat — open a window or work outdoors/under a range hood.
• Keep lye, and anything containing it, away from children and pets, and never store it in a container that could be mistaken for food or drink.
• Have plain water on hand in case of accidental skin/eye contact, and know your local emergency/poison-control number before you start.

The Formula

For each oil, the lye it needs (before any superfat discount) is its own weight multiplied by its SAP value:

$$\text{Lye}_{\text{oil}} = \vA{\text{Oil Weight}} \times \vB{\text{SAP Value}}$$

The recipe’s total lye is the sum of every oil’s contribution, reduced by the superfat percentage:

$$\text{Total Lye} = \left(\sum \text{Lye}_{\text{oil}}\right) \times \left(1 - \frac{\vC{\text{Superfat \%}}}{100}\right)$$

Water is a percentage of the total oil weight:

$$\text{Total Water} = \vD{\text{Total Oil Weight}} \times \frac{\vE{\text{Water \% of Oils}}}{100}$$

A reasonable, well-established selection of common cold-process soap-making oils is built in, each with its own widely-published SAP value: Olive Oil (0.134), Coconut Oil (0.183), Palm Oil (0.141), Castor Oil (0.128), Sunflower Oil (0.134), Sweet Almond Oil (0.136), Shea Butter (0.128), and Cocoa Butter (0.137). Different published lye-calculator tools round these slightly differently depending on the exact fatty-acid-profile assumptions they use — always double-check your recipe with a second lye calculator before making a real batch.

Worked Example

500g of Olive Oil, at a 5% superfat and 38% water:

1. Lye before superfat: $\vA{500} \times \vB{0.134} = 67\text{ g}$
2. Total lye (after 5% superfat): $67 \times (1 - 0.05) = 63.65\text{ g}$
3. Total water (38% of 500g oils): $\vD{500} \times \frac{\vE{38}}{100} = 190\text{ g}$
4. Total batch weight: $500 + 63.65 + 190 = 753.65\text{ g}$

Key Factors to Consider

• Each oil contributes different qualities to the finished bar, beyond just its lye requirement. Coconut oil produces big, fluffy lather but can be drying at high percentages; olive oil makes a gentle, conditioning but softer bar; castor oil boosts lather stability in small amounts — soap makers typically blend several oils specifically to balance hardness, lather, and conditioning properties, not just to hit a target lye amount.
• Higher superfat percentages make a gentler, more moisturizing bar, but reduce lather and shorten shelf life. The commonly-cited 5% default is a reasonable general starting point, but a facial or very mild bar might use a higher superfat, while a bar meant purely for cleaning power might use less — this is a real formulation choice, not a fixed universal number.
• Fragrance oils, essential oils, and colorants are separate additions that don’t factor into this lye/water calculation. This calculator computes the base saponification chemistry only — any additional ingredients added at trace (the point when the batter thickens) need their own separate usage-rate research specific to that product.
• SAP values can vary slightly between different published sources, since fatty-acid profiles differ somewhat by an oil’s specific origin and processing. This is exactly why the guidance above recommends double-checking a recipe against a second lye calculator before making a real batch — a small discrepancy between two calculators’ SAP value tables is normal, not necessarily an error in either one.

Common Mistakes

• Adding water to lye instead of lye to water. As the Safety section above emphasizes, doing this in reverse can cause a violent, splattering reaction — always add lye to water, never the other way around.
• Skipping the superfat percentage entirely, or entering 0% by mistake. A 0% superfat recipe fully saponifies every bit of oil, leaving no unreacted fat to keep the bar gentle — most recipes deliberately use a small superfat (commonly 5%) rather than none at all.
• Trusting this calculator’s numbers without double-checking against a second lye calculator. As the Key Factors above note, SAP value tables vary slightly between sources — always verify a real recipe against a second source before mixing an actual batch.

Useful to Know

• Scaling a soap recipe up or down by baker’s percentages instead of a fixed superfat/water ratio? Baker's Percentage Calculator expresses each ingredient as a percentage of a base ingredient’s weight.
• Building a wooden mold or curing rack for your soap batches? Woodworking Board Feet Calculator calculates the lumber you’ll need in board feet.
• Adding fragrance to a candle project alongside your soap making? Candle Wax and Fragrance Calculator works out the right fragrance oil load for a batch of wax.