SAP Values in Soap Making:
The Science of Perfect
Lye Calculation
Master saponification values, lye calculation, superfatting, and water discounts — the complete Australian soapmaker's guide to formulating safe, consistent, beautiful bars from scratch.
Natural soap making is equal parts art and science. While creativity shapes scent, colour, and design, chemistry determines whether your finished bar is safe, effective, and a joy to use. At the very core of that chemistry sits the Saponification Value — universally abbreviated to SAP. Get it right and you'll produce consistent, professional-quality soap every batch. Get it wrong and you risk bars that are harsh, oily, or worse — caustic enough to burn skin. This guide takes you from the fundamentals right through to advanced formulation techniques.
What Is Saponification?
Saponification is the chemical reaction between fats or oils (triglycerides) and a strong alkali (sodium hydroxide for bar soap, potassium hydroxide for liquid soap) that produces soap molecules and glycerine as a by-product. It's an exothermic reaction — it generates heat as it proceeds.
Understanding SAP Values
The SAP value of an oil is defined as the number of milligrams of potassium hydroxide (KOH) required to completely saponify 1 gram of that oil. Because different oils have different fatty acid compositions and molecular weights, each oil requires a different amount of alkali. This is why you cannot use one fixed lye amount for all oil combinations — every recipe needs to be calculated individually.
SAP values vary because fats contain different fatty acid chains of different lengths and saturation levels. Short-chain saturated fats (like those in coconut oil — lauric, myristic acids) have higher SAP values because shorter chains mean more molecules per gram, each requiring one alkali unit. Long-chain unsaturated fats (like oleic acid in olive oil) have lower SAP values. This is why coconut oil (SAP 0.190) requires significantly more lye per gram than olive oil (SAP 0.135).
Complete Oil SAP Value Chart
This expanded chart covers the most common oils and butters used by Australian soapmakers, with NaOH SAP values, KOH SAP values for liquid soap makers, and key properties each oil contributes to a finished bar.
| Oil / Butter | NaOH SAP | KOH SAP | Key Properties in Bar Soap |
|---|---|---|---|
| Olive Oil (extra virgin) | 0.134 | 0.189 | conditioninggentle lathercastile base |
| Olive Oil (pomace) | 0.135 | 0.189 | conditioningfaster trace |
| Coconut Oil (76°C) | 0.190 | 0.267 | hard barbig fluffy bubblescleansing |
| Coconut Oil (92°C / fractionated) | 0.191 | 0.268 | hard barcleansing |
| Palm Oil (RSPO) | 0.142 | 0.199 | hardnessstable lather |
| Castor Oil | 0.128 | 0.180 | creamy latherhumectantsoft/sticky |
| Shea Butter (refined) | 0.128 | 0.179 | moisturisingskin feelmild hardness |
| Cocoa Butter | 0.137 | 0.192 | hardnessconditioningstable lather |
| Mango Butter | 0.137 | 0.192 | hardnessemollient |
| Sweet Almond Oil | 0.136 | 0.190 | conditioningskin-softening |
| Sunflower Oil (high oleic) | 0.134 | 0.188 | conditioningsoft bar if high % |
| Rice Bran Oil | 0.128 | 0.179 | conditioningantioxidant |
| Avocado Oil | 0.133 | 0.187 | rich conditioningskin-nourishing |
| Neem Oil | 0.136 | 0.191 | antibacterialacne/eczema |
| Hemp Seed Oil | 0.135 | 0.190 | conditioningsoft / goes rancid faster |
| Jojoba Oil (wax ester) | 0.069 | 0.097 | liquid wax — not a triglyceride, mostly stays in superfat |
| Lard / Pork Fat | 0.138 | 0.194 | hard barcreamy stable latherskin-conditioning |
| Tallow (beef) | 0.140 | 0.197 | very hard barstable lathertraditional |
| Canola / Rapeseed Oil | 0.132 | 0.185 | conditioningaffordable filler oil |
| Argan Oil | 0.136 | 0.191 | luxury conditioningantioxidant |
| Rosehip Seed Oil | 0.137 | 0.193 | anti-ageinggoes rancid quickly — best in superfat |
| Apricot Kernel Oil | 0.135 | 0.190 | conditioningskin-softening |
How to Calculate Lye Using SAP Values
The calculation is straightforward once you understand the logic: you're simply finding how much NaOH is theoretically needed to fully saponify your oil blend, then applying a discount to leave a small percentage of unsaponified oil (your superfat).
Worked Example: Classic Olive & Coconut Soap (5% Superfat)
700 g total oils · 5% superfat · yields approx. 8–10 bars
| Oil | Weight (g) | NaOH SAP | Theoretical Lye (g) |
|---|---|---|---|
| Australian Olive Oil | 500 | 0.134 | 67.0 |
| Coconut Oil (76°C) | 150 | 0.190 | 28.5 |
| Shea Butter (refined) | 30 | 0.128 | 3.84 |
| Castor Oil | 20 | 0.128 | 2.56 |
| Total oils | 700 | 101.9 g (theoretical) |
Water amount: 700 g oils × 0.38 (38% water ratio) = 266 g distilled water
Always verify using SoapCalc or Soapmaking Friend before making any batch.
Superfatting & Lye Discounts: Choosing the Right Level
Superfatting means deliberately using less lye than theoretically needed, leaving a percentage of unsaponified oils in your finished bar. These "free oils" contribute conditioning and moisturising properties — but too much or too little creates problems.
You cannot choose exactly which oil becomes your superfat — it's random which triglycerides the lye encounters last as the reaction completes. However, you can influence the outcome by adding your most luxurious oils at a late trace rather than in the main oil blend. Oils added after trace are less exposed to the lye and more likely to survive as free oils. This technique — sometimes called "superfatting at trace" — is popular with rosehip, argan, and jojoba, which are expensive and beneficial unsaponified but oxidise quickly if saponified.
Water Ratios & Lye Concentration
Water in cold process soap is not just a solvent for the lye — its amount affects trace speed, gel phase behaviour, final bar hardness, and cure time. Most beginners use the "standard" water amount, but experienced makers often apply a water discount.
💧 Full water (38% of oils)
Longest working time. Slowest trace. More time for swirls and designs. Longer dry-out time before unmoulding. Best for beginners and intricate designs.
⚡ 10–20% water discount
Faster trace and harder bar at unmould. Reduces ash formation. Shorter cure. Preferred for high-coconut recipes or when using slow-tracing oils. Less forgiving of mistakes.
❄ 20–30% water discount
Advanced technique. Very fast trace — difficult to work with for swirls. Produces very firm bars that unmould quickly. Popular for salt bars (which trace extremely fast regardless).
⚠ Milk / alternative liquids
Replacing water with milk (cow, goat, coconut, oat) adds sugars that accelerate trace and create a creamier, skin-loving bar. Always freeze milk before adding lye to prevent scorching and discolouration.
Common SAP & Lye Calculation Problems
Always wear nitrile gloves, safety glasses, and long sleeves when handling NaOH. Work in a well-ventilated space — the initial mixing produces fumes. Always add lye to water, never water to lye — the reverse causes a violent, spattering exothermic reaction. Use stainless steel or heavy-duty HDPE containers (no aluminium — it reacts with NaOH). Keep a clean water source nearby for immediate skin rinsing if contact occurs. Store unused lye in an airtight container away from moisture.
Lye Calculator Tools — Always Verify Your Batch
Even experienced soapmakers never rely solely on manual calculation for a final batch. Always run your recipe through a dedicated lye calculator before weighing any NaOH.