Saponification Value & Soap Lye Calculator

What Is Saponification Value?

Saponification value (SV), sometimes called saponification number, is the amount of alkali needed to fully react with (saponify) the fatty acids in one gram of a fat or oil, expressed in milligrams of potassium hydroxide (mg KOH/g). It is one of the oldest and most useful numbers in fat chemistry, because it tells you, in a single figure, roughly how large the fatty acid molecules in that oil are on average — smaller fatty acid chains have a higher saponification value, and longer chains have a lower one.

Every fat and oil used in soapmaking, food chemistry, and industrial lipid analysis has its own published saponification value, and it is this number, converted into a 'SAP value', that soapmakers rely on to work out exactly how much lye a recipe needs.

What Is a Soap Lye Calculator?

A soap lye calculator takes a recipe made up of one or more oils and butters, along with each oil's SAP value, and works out precisely how much sodium hydroxide (NaOH, for solid bar soap) or potassium hydroxide (KOH, for liquid soap) is needed to saponify the batch — plus how much water to mix that lye into. It is the single most important calculation in cold-process and hot-process soapmaking, because getting the lye amount wrong produces soap that is either too harsh (lye-heavy) or too soft and prone to spoiling (lye-light).

This tool builds a recipe from 16 common soapmaking oils and butters with their SAP values already filled in, lets you set a superfat percentage, choose NaOH or KOH, and pick how you want your water amount sized — then instantly shows the exact lye and water weights, along with a full soap-quality breakdown.

The Lye Calculation Formula, Explained Simply

Every soap recipe, no matter how many oils it contains, comes down to the same core formula:

• Pure lye needed (g) = Sum of (weight of each oil × that oil's SAP value)
• Actual lye used (g) = Pure lye needed × (1 − superfat % ÷ 100)
• SAP value = grams of NaOH (or KOH) required to fully saponify 1 gram of that specific oil

What Is Superfat (and Why Soapmakers Use a Lye Discount)?

Superfatting means using slightly less lye than the amount that would react with every last bit of oil, so that a small percentage of the oil is left unreacted in the finished bar as a moisturizing, skin-conditioning extra. A 5% superfat recipe, for example, only uses 95% of the 'pure' lye amount — the other 5% worth of oil never gets converted to soap and stays behind to make the bar milder and less drying.

Most cold-process soapmakers use a superfat somewhere between 3% and 8%, with 5% being the most common default for a balanced, everyday bar. Higher superfat percentages (10%+) are sometimes used for very gentle facial or baby soaps, while some liquid soap recipes are made with 0% superfat and superfatted separately after saponification is complete.

Worked Example: Calculating Lye for a Simple Recipe

Say a soapmaker is making a 1,000 g batch using 700 g olive oil (NaOH SAP 0.134) and 300 g coconut oil (NaOH SAP 0.190), at a 5% superfat.

• Step 1: Pure lye for olive oil = 700 × 0.134 = 93.8 g
• Step 2: Pure lye for coconut oil = 300 × 0.190 = 57.0 g
• Step 3: Total pure lye = 93.8 + 57.0 = 150.8 g
• Step 4: Apply the 5% superfat discount: 150.8 × (1 − 0.05) = 143.3 g NaOH needed
• Step 5: At 38% water (of oil weight): water = 1,000 × 0.38 = 380 g

NaOH vs KOH — Bar Soap vs Liquid Soap

Sodium hydroxide (NaOH, also called lye or caustic soda) produces a hard, solid soap and is what is used in virtually all bar soap recipes. Potassium hydroxide (KOH, also called caustic potash) produces a much softer, more soluble soap paste, which is why it is the standard alkali for liquid soap, shaving soap, and soft soap recipes instead.

Because KOH has a larger, heavier molecule than NaOH, it takes noticeably more of it — roughly 40% more by weight — to saponify the same amount of oil, which is why every oil's KOH SAP value is always higher than its NaOH SAP value. This calculator switches every value automatically the moment you toggle between NaOH and KOH.

Three Ways to Calculate Your Water Amount

Soapmakers size their water in one of three common ways, and this calculator supports all three:

• Water as % of oils: the classic method — water weight is a straight percentage of total oil weight (commonly 33-40%).
• Lye concentration %: sets the strength of the lye solution itself (lye weight ÷ total lye-solution weight), which is more precise for recipes with very different oil totals — 33% is a widely used soft-water default, while 40%+ gives a firmer, faster-tracing batter.
• Water : lye ratio: expresses water as a multiple of the lye weight, e.g. a 2:1 ratio means two grams of water for every gram of lye.

Understanding Soap Quality Traits (Hardness, Cleansing, Conditioning & More)

Beyond the raw lye and water amounts, this calculator also estimates seven soap quality traits by weighting each oil's individual fatty-acid-profile contribution by how much of it is in the recipe. These figures — hardness, cleansing, conditioning, bubbly lather, creamy lather, iodine value, and INS — are the same style of quality indicators used across the soapmaking community to sanity-check a recipe before mixing a single drop of lye.

Hardness reflects how firm and long-lasting the finished bar will be. Cleansing measures how strongly the soap strips oils (too high and it can feel drying). Conditioning reflects how moisturizing and skin-friendly the bar feels. Bubbly and creamy describe the two different textures of lather a soap can produce. Iodine value estimates how much unsaturated fat is present (higher values tend to mean a softer bar with a shorter shelf life), and INS is a rough overall hardness/quality index. Each trait includes a commonly used target range, and the bar next to each one turns amber if the recipe sits below that range, or rose if it sits above it.

The 16 Built-In Oils and Butters

This calculator ships with SAP values and quality-trait data for the most commonly used soapmaking fats: olive oil, coconut oil, palm oil, palm kernel oil, castor oil, shea butter, cocoa butter, sweet almond oil, sunflower oil, canola oil, soybean oil, rice bran oil, avocado oil, lard, tallow, and jojoba oil. You can also switch any row to 'Custom oil' and enter your own NaOH and KOH SAP values for anything not on the list — useful for less common oils or when you have a lab-measured SAP value from your own supplier.

Because real-world SAP values can vary by a few percent depending on the crop, growing region, and refining process, always treat the built-in figures as a solid, industry-standard starting point rather than a certified lab result, and re-check against your specific oil supplier's data sheet for critical or commercial batches.

How to Measure Saponification Value in the Lab (Titration Method)

Rather than looking a SAP value up, chemists and quality-control labs can measure it directly using the standard AOCS Cd 3-25 / ISO 3657 titration method. A known mass of fat is refluxed with a measured excess of alcoholic KOH, fully saponifying it. The leftover, unreacted KOH is then back-titrated with a standardized HCl solution, alongside a blank sample containing no fat, run under identical conditions.

• SV (mg KOH/g) = 56.1 × N × (V(blank) − V(sample)) ÷ W
• 56.1 = molar mass of KOH (g/mol)
• N = normality of the HCl solution used for back-titration (mol/L)
• V(blank), V(sample) = mL of HCl used to titrate the blank and the oil sample
• W = mass of the oil/fat sample used (g)

Worked Example: Saponification Value From Titration Data

Suppose a 2.0 g oil sample is refluxed with excess KOH, and back-titrating the leftover KOH with 0.5 N HCl takes 21.3 mL, while a blank (no oil) takes 45.9 mL of the same HCl.

• Step 1: Find the difference in titration volume: 45.9 − 21.3 = 24.6 mL
• Step 2: Multiply: 56.1 × 0.5 × 24.6 = 690.0
• Step 3: Divide by the sample mass: 690.0 ÷ 2.0 = 345 mg KOH/g
• Step 4: Convert to soap-lye SAP values if needed: 345 ÷ 1000 × (40.0 ÷ 56.1) ≈ 0.246 g NaOH per g oil, or 345 ÷ 1000 ≈ 0.345 g KOH per g oil.

Saponification Value vs Acid Value vs Ester Value

Saponification value measures the total alkali needed to react with every fatty acid in the fat, whether that fatty acid is still bound up as a triglyceride ester or floating free as a 'free fatty acid'. Acid value (AV) measures only the free fatty acid portion — the KOH needed to neutralize acids that are not bound to glycerol. Subtracting the acid value from the saponification value gives the ester value, which represents only the alkali used to break down the actual triglyceride esters.

A fresh, high-quality oil typically has a very low acid value relative to its saponification value, since almost all of its fatty acids are still bound as triglycerides. A high acid value relative to SV is a sign of a rancid, degraded, or heavily processed oil.

Why Use This Saponification Value & Soap Lye Calculator

This tool covers both sides of soap chemistry in one place: a full lye and water calculator with 16 built-in oils, superfat, three water-sizing methods, NaOH/KOH switching, and a soap-quality breakdown for recipe formulation — and a lab-grade saponification value calculator for anyone measuring SAP values directly from titration data. Every result updates instantly as you change an input, with no sign-up, spreadsheet, or install required.

Whether you are a home soapmaker dialing in your first cold-process batch, a small-batch soap business scaling a recipe, or a chemistry student running an AOCS titration in the lab, this calculator is built to give you an accurate, transparent answer with the full working shown.