Lye Calculator for Cold Process and Hot Process Soap Making

No. This is the most important fact in soap chemistry. Sodium hydroxide does not exist in finished, properly made cold process soap. During the saponification reaction, every molecule of NaOH that participates bonds chemically with fatty acid molecules from your oils to form soap molecules (sodium salt of a fatty acid) and glycerin. The NaOH is consumed by this reaction. It is transformed, not dissolved or suspended.

After full cure (four to six weeks minimum), a properly calculated bar should test at pH 9 to 10. This mild alkalinity comes from the soap molecules themselves, not from residual lye. The pH of your skin surface is approximately 4.5 to 5.5, so soap at pH 9 to 10 does temporarily raise skin surface pH during use. This is normal and expected with any real soap made through saponification. Your skin’s acid mantle recovers within 30 to 60 minutes of rinsing.

Using excess lye beyond what your oils can saponify creates what is called lye-heavy or lye-excess soap. After cure, unsaponified sodium hydroxide remains in the bar and can cause chemical burns on skin. The severity depends on how much excess lye is present. A small excess (1 to 2%) may only cause mild skin irritation. A larger excess (5% or more) can cause serious chemical burns, especially on sensitive or wet skin.

Lye-heavy soap can sometimes be detected before use by placing a drop of water on the bar’s cut surface and touching lightly with a damp fingertip. If you feel a sharp, immediate sting or zap, that is excess lye. However, this test is not completely reliable because the soap’s surface pH may differ from the interior pH. The only reliable method is an accurate pH measurement on a dissolved sample from the interior of the bar, or a formal titration test. If you suspect lye-heavy soap, discard it.

Superfatting (also called lye discount) means using less lye than would be required to fully saponify all your oils, intentionally leaving a percentage of your oil weight un-saponified in the finished bar. This free oil gives the bar a conditioning, moisturizing feel and creates a safety buffer against accidental lye excess from weighing errors.

Choosing your superfat depends on the intended use. For face bars or dry skin formulas, 5 to 8% is common. For body bars, 5% is the standard starting point. For shampoo bars, 0 to 2% is recommended because high superfat leaves waxy residue in hair. For high-cleansing kitchen soaps, 3 to 5% keeps the cleansing power high. For extremely rich moisture bars, some formulators go up to 15 to 20%, accepting that lather will be reduced and shelf life shortened. The superfat slider in this calculator adjusts the lye amount in real time.

Cold process and hot process soap use the same lye calculation and the same oils. The difference is in how and when saponification completes. In cold process, you combine lye water and oils at a controlled temperature, bring the mixture to light trace, pour into molds, and allow saponification to complete over four to six weeks of curing at room temperature. The soap is not heated after pouring. Cold process allows for a wide range of design techniques including swirls, layering, and embeds.

In hot process, you bring the soap batter to a cook after initial trace, usually in a slow cooker or double boiler. The heat accelerates saponification, completing the reaction in one to three hours rather than weeks. Hot process soap can be used within 24 to 48 hours of unmolding, though most formulators still recommend one to two weeks for water evaporation and bar hardening. Hot process soap has a more rustic texture and does not allow for the same decorative options as cold process, but it is faster to usable product and removes most of the extended cure risk.

No. Lye water reacts with several common materials. Aluminum, in particular, reacts violently with sodium hydroxide to produce hydrogen gas, which is flammable and explosive. Never use aluminum containers, spoons, or tools with lye. Thin plastics like drinking cups or PETE bottles can soften and deform when filled with the hot lye solution and may release harmful chemicals.

Safe materials for lye mixing include stainless steel (304 or 316 food grade), high-density polyethylene (HDPE, labeled with recycling symbol 2), polypropylene (PP, labeled with recycling symbol 5), and borosilicate glass. Most soap suppliers sell HDPE or polypropylene pitchers designed for lye mixing. Many soapers use stainless steel mixing bowls and pitchers from kitchen supply stores. When in doubt, check that your container is rated for caustic or corrosive chemicals at elevated temperatures before using it with lye.

The dissolution of sodium hydroxide in water is a strongly exothermic process, meaning it releases significant heat. A lye solution made at room temperature can reach 180 to 200 degrees Fahrenheit within seconds of the NaOH dissolving. This heat is generated by the energy released when sodium and hydroxide ions dissociate from the crystal lattice and form hydration shells in solution. It is a fundamental property of sodium hydroxide, not a defect in your lye or technique.

Many soapers mix lye water outdoors, in a garage, or under a range hood to manage the brief caustic fumes that rise as the lye dissolves. Some soapers start with refrigerator-cold water or even use frozen water cubes to reduce peak temperature. Others use room temperature water and allow the solution to cool to 90 to 110 degrees Fahrenheit before combining with their oils. The specific temperature at which you combine lye water and oils depends on your formula and personal technique. Most beginners work in the 90 to 120 degree range for both lye water and melted oils.

The hardness of finished soap depends on the counterion paired with the fatty acid soap molecules. Sodium (from NaOH) produces sodium fatty acid salts that are solid at room temperature, which is why bar soap uses NaOH. Potassium (from KOH) produces potassium fatty acid salts that are softer and more water-soluble, which is why liquid soap, soft paste soap, and shaving soap use KOH. This is not a formulation choice. It is a fundamental consequence of the chemistry of sodium versus potassium soaps.

Commercial liquid hand soap uses KOH-saponified oils diluted with large amounts of water to reach pourable consistency. Shaving cream uses KOH soap with specific high-stearic oil blends (like tallow or stearic acid) that produce a rich, dense, stable foam. All the same lye calculation principles apply to KOH soap as to NaOH soap, with the adjustment for KOH’s higher molecular weight and 90% typical purity. This calculator handles both alkali types with the same accuracy.

Time, pH testing, and the physical characteristics of the bar are the three indicators. After four weeks minimum cure, a correctly formulated cold process bar should be hard, dry to the touch, and have a mild soap scent. It should not feel slippery or leave a greasy residue on your hand when wetted. A pH test using narrow-range pH strips (pH 8 to 11 range) should read 9 to 10.

Some soapers use the zap test as an informal check: touch the tip of your tongue lightly to the bar surface. If you feel a sharp, immediate electrical zap or chemical sting, significant residual lye is present. If you feel nothing unusual, the bar is safe. The zap test works because your tongue is sensitive to caustic pH. However, this test only checks the surface of the bar, not the interior. A cut surface from the interior of the bar is a more reliable test point. When in doubt, extend the cure another two weeks and test again.

The iodine value (IV) of an oil is a measure of how many double bonds exist in its fatty acid chains. A double bond is a site of potential oxidation, where atmospheric oxygen can attack and initiate the rancidity process (DOS, short for dreaded orange spots, or rancidity). Oils with high iodine values are rich in polyunsaturated fatty acids and oxidize more quickly when exposed to air and light.

For soap formulation, this means high-IV oils like flaxseed (IV 178), rosehip (IV 181), and hemp seed (IV 165) should be used sparingly, typically no more than 10 to 15% of your total formula, to avoid premature rancidity in your finished bar. Lower-IV oils like coconut (IV 10), tallow (IV 40), and cocoa butter (IV 35) are highly stable and improve bar longevity. The reference table in this calculator includes iodine values for all 44 oils to help you make informed decisions about formula shelf life.

Trace is the point in the soap making process where the oil and lye water mixture has emulsified sufficiently that it begins to thicken and hold a surface impression. When you lift a spatula or stick blender from the mixture and drizzle a small amount back onto the surface, it leaves a visible trace or trail before sinking. This indicates that saponification has begun and the oils and lye are no longer separated into two phases.

Trace has multiple stages. Light trace is a thin, barely pourable batter similar to room-temperature cream. This is the best stage for intricate swirl designs and most decorative techniques. Medium trace is noticeably thicker, similar to a thin pudding. Heavy trace is thick, like mashed potatoes, and pours with difficulty. The speed at which your batter reaches trace depends on your oil combination, lye concentration, temperatures, and whether you use a stick blender. High-coconut recipes with high lye concentration tend to trace very quickly (sometimes within seconds of combining). High-olive recipes can take 30 to 60 minutes to reach light trace without a stick blender.

Under the Modernization of Cosmetics Regulation Act (MoCRA 2022), any person or business that manufactures or processes cosmetics for commercial sale in the United States must register their facility with the FDA and list their products. This applies to soap sold commercially, including at craft fairs, farmers markets, Etsy, and local shops.

The FDA has established a small business exemption for businesses averaging under $1 million in annual cosmetic sales across the three most recent years. Small businesses under this threshold have reduced requirements and extended compliance timelines, but facility registration and product listing are still required. The FDA’s cosmetics registration portal is accessible at FDA.gov. Soap made purely for personal use or given as gifts without commercial exchange does not trigger MoCRA requirements. If you are unsure whether your activity qualifies as commercial, the FDA’s guidance documents available at FDA.gov/cosmetics provide the most current interpretation.

Yes, and milk soaps are among the most popular handmade products in the US market. You can substitute all or part of your water with goat milk, coconut milk, almond milk, oat milk, or other milk alternatives. Milk proteins and sugars add conditioning properties to the finished bar and can accelerate trace. However, the sugars in milk also react with lye and generate additional heat, which can scorch the milk proteins and cause the batch to turn orange or brown and develop an unpleasant odor.

The standard technique for milk soaps is to freeze the milk into ice cubes or a solid block before adding lye, slowing the reaction and preventing scorching. Alternatively, some formulators add milk at trace as a liquid rather than mixing it with lye. The lye calculation does not change when substituting milk for water. Your lye and water amounts from this calculator remain the same. You simply use the calculated water weight as your total liquid amount and substitute milk for all or part of that liquid.

Soda ash is a white, powdery or crystalline coating that sometimes forms on the surface of cold process soap during cure. It is a harmless cosmetic defect consisting of sodium carbonate, which forms when unsaponified soap (soap at light trace) contacts carbon dioxide in the air. It does not affect the performance or safety of the soap bar, only its appearance.

Several techniques reduce soda ash formation. Using a higher lye concentration (lower water) means less free water at the surface during cure, reducing the window for ash formation. Pouring at medium to heavy trace rather than light trace reduces surface exposure. Covering the mold with plastic wrap immediately after pouring prevents air contact during the first 24 to 48 hours. Some formulators use a 91% isopropyl alcohol spray on the surface of freshly poured soap, which disrupts the surface tension that allows ash crystals to form. If your bar does develop soda ash, it can usually be removed by buffing with a clean towel, planing the surface with a vegetable peeler, or steaming the bar briefly.

Jojoba is technically not an oil at all. It is a liquid wax ester, composed primarily of long-chain wax esters rather than the triglycerides that make up true vegetable oils. Because wax esters have a fundamentally different molecular structure than triglycerides, they do not saponify in the same way under standard soap making conditions. The very low NaOH SAP value of 0.069 for jojoba reflects this difference. In practical terms, jojoba in a cold process soap recipe contributes very little to the saponification reaction and is present mostly as a free wax in the finished bar.

This is actually one of jojoba’s advantages in soap making. Because it resists saponification, jojoba in your formula is available as a free conditioning agent in the finished bar, contributing its skin-balancing liquid wax properties directly. Most formulators use jojoba at 5 to 10% of the formula for this reason. At 10%, you get meaningful skin benefits without significantly distorting your lye calculation.

Palm-free soap making refers to formulas that exclude palm oil and palm kernel oil due to concerns about sustainability, deforestation, and orangutan habitat destruction linked to conventional palm oil production. The US market for palm-free artisan soap has grown significantly since 2018, with many craft soap buyers actively seeking palm-free certifications.

From a formulation standpoint, palm oil’s primary function in soap is hardness (from palmitic acid) and stable lather. When removing palm from a formula, the usual substitutions are babassu oil (closest functional analog to coconut but palm-free), lard or tallow for the hardness contribution, cocoa butter or mango butter for hardness without cleansing, and increasing the percentage of shea butter for conditioning. Each of these substitutes has its own SAP value, which is why a fresh lye calculation is required whenever you transition to a palm-free formula. The calculator supports all major palm-free alternatives including babassu, murumuru butter, and kokum butter.

The SAP values in this calculator are drawn from published fatty acid composition data from the USDA Agricultural Research Service and cross-referenced against values accepted by the Handcrafted Soap and Cosmetic Guild (HSCG). They represent the best available standardized values for each oil category. However, it is important to understand that natural oils have inherent variability in their fatty acid composition based on crop variety, geographic origin, growing conditions, harvest year, and processing method.

For example, olive oil SAP values in published literature range from 0.130 to 0.137 depending on the source and olive variety. This calculator uses 0.134 for standard olive oil, which is the most widely accepted middle value. In practice, this variability is one reason why a 5% superfat is recommended as a safety buffer. Even if your olive oil’s true SAP value differs from 0.134 by a small margin, the superfat cushion absorbs the difference. For critical commercial production where very high accuracy is required, sending oil samples to a testing laboratory for individual SAP value determination is possible, though rarely necessary for typical craft soap scales.