What is saponification and why does the SAP value matter?

Saponification is the chemical reaction between a fat (triglyceride) and a strong alkali (lye) that produces soap (salt of a fatty acid) and glycerol. Each oil has a unique fatty acid profile, so a fixed gram of lye saponifies a different quantity of each fat. The SAP (saponification) value is the number of grams of lye required to fully saponify exactly one gram of a given oil. The calculator multiplies each oil weight by its SAP value, sums the results, then applies your superfat discount.

What is superfat and what percentage should I use?

Superfat is a deliberate lye discount — you use less lye than would be needed to saponify every gram of oil, leaving a small fraction of free unsaponified fat in the finished bar. This softens the lather and leaves the skin feeling conditioned. A superfat of 5% is the most common starting point for bar soap. Go lower (2–3%) for shampoo bars or laundry soap where full cleansing is the goal, or slightly higher (6–8%) for a more luxurious feel. Above 8% the soap may feel greasy and will have a shorter shelf life.

What is the difference between NaOH and KOH?

NaOH (sodium hydroxide, caustic soda) is used for hard bar soap because sodium soaps are solid at room temperature. KOH (potassium hydroxide, caustic potash) is used for liquid or paste soap because potassium soaps remain fluid. KOH has a higher molecular weight than NaOH, so its SAP values are roughly 40% higher. Commercial KOH is often only 90% pure; the calculator divides the theoretical KOH SAP value by the purity fraction to give you the correct real-world amount to weigh.

How is the water amount calculated?

The calculator uses lye concentration to determine water. Lye concentration is defined as: lye_conc (%) = lye_weight / (lye_weight + water_weight) × 100. Rearranging gives: water = lye × (100 / lye_conc − 1). At 30% lye concentration, for every 100 g of lye you add 233 g of water. A lower concentration (more water) gives a slower trace and longer curing window; a higher concentration (less water) speeds trace and unmoulding but increases the risk of soda ash on bar soap.

Why must I add lye to water and not water to lye?

When lye dissolves in water it releases a large amount of heat and can briefly reach 80 °C. Adding water to a concentrated lye pile risks a violent steam explosion and violent spattering of caustic liquid. Adding lye granules slowly to a larger volume of cold water disperses the heat safely and gives you control. Always wear heat-resistant gloves and safety glasses, work near an open window or outdoors, and never use aluminium vessels (lye reacts with aluminium releasing hydrogen gas).

Can I use tap water instead of distilled water?

Distilled or deionised water is strongly recommended. Tap water contains minerals, chlorine, and other dissolved solids that can interfere with saponification, cause rancidity, or leave white spots on finished soap. Rainwater, filtered water, or cooled herbal infusions are used by experienced soapers, but start with distilled water until you are confident in the process.

Soap Lye Calculator — Gera Tools

A soap lye calculator that gives you the exact amount of NaOH or KOH needed for any cold-process or liquid soap recipe, along with the water quantity, a full per-oil breakdown, and a step-by-step formula working — all computed instantly in your browser without sending any data anywhere.

How it works

Every oil or fat saponifies at a different rate depending on its fatty acid chain lengths. The SAP value (saponification value) encodes that rate: it is the number of grams of lye needed to convert exactly one gram of a given fat into soap and glycerol. To find the total lye needed for a recipe you multiply each oil weight by its SAP value, sum everything, and then apply the superfat discount:

lye = oil_weight_g × SAP_value × (1 − superfat / 100)

For NaOH the SAP values range from about 0.069 (jojoba, a liquid wax with very short chains) to 0.190 (coconut oil, which is rich in short-chain lauric and myristic acids and therefore needs the most lye per gram). For KOH the same values are multiplied by roughly 1.4 to account for the higher molecular weight of potassium versus sodium. If you are using a commercial KOH that is only 90 % pure, the calculator automatically divides the theoretical KOH SAP value by 0.90, so you weigh out the correct real-world amount.

The water amount is derived from the lye concentration you specify. Concentration is defined as the mass of lye divided by the total mass of the lye solution (lye plus water):

water_g = lye_g × (100 / lye_concentration − 1)

At a standard 30 % concentration, 100 g of lye dissolves in 233 g of water to form a 333 g solution. A lower concentration slows trace and is gentler on acceleration-prone recipes; a higher concentration speeds unmoulding.

Worked example

A classic beginner recipe using 400 g olive oil, 200 g coconut oil (76°F), and 50 g castor oil with 5 % superfat and 30 % lye concentration:

Oil	Weight	NaOH SAP	Raw lye	After 5% SF
Olive oil	400 g	0.134	53.60 g	50.92 g
Coconut oil	200 g	0.190	38.00 g	36.10 g
Castor oil	50 g	0.128	6.40 g	6.08 g
Total	650 g		98.00 g	93.10 g

Water at 30 % concentration: 93.10 × (100/30 − 1) = 93.10 × 2.333 = 217.2 g

Total batch weight: 650 + 93.10 + 217.2 = 960 g (~32 oz, perfect for a 900 g loaf mould with a little headroom).

Formula note

SAP values used here are the widely accepted industry averages sourced from the Kester Lye SAP value database and Kevin Dunn’s Scientific Soapmaking. Actual SAP values can vary slightly by growing region and processing method — for high-precision commercial production, request a Certificate of Analysis SAP value from your oil supplier and enter a custom recipe accordingly. For home-batch cold-process soap the published average values are accurate to within 1–2 %, well inside any reasonable superfat buffer.