SAP Values for Soap Making: What They Mean and How to Use Them

A SAP value (saponification value) is the amount of lye needed to turn one gram of a particular oil completely into soap. Soap makers usually write it as grams of sodium hydroxide per gram of oil: olive oil’s SAP value is about 0.135, so 100 g of olive oil needs about 13.5 g of NaOH before superfat. Every oil has its own number, which is why each recipe needs its own lye calculation.

Why each oil has a different SAP value

Oils are mostly triglycerides: three fatty acid chains on a glycerin backbone. Each triglyceride needs the same amount of lye (three hydroxide units) to saponify, no matter how long its chains are.

What differs is how many triglycerides fit into a gram. Coconut oil is rich in short and medium-length fatty acids, so its molecules are small and a gram contains more of them. More molecules means more lye. Olive oil’s chains are longer, so a gram holds fewer molecules and needs less lye.

That’s why coconut’s SAP (about 0.183) is higher than olive’s (about 0.135), and why swapping one for the other without recalculating can leave a bar lye-heavy or soft.

The three ways SAP values are published

This is where most confusion comes from. You’ll find SAP values in three formats:

1. NaOH, grams per gram (e.g. olive 0.135). What most soap calculators and our SAP value chart use for bar soap.
2. KOH, grams per gram (e.g. olive about 0.190). Used for liquid soap, and by some soap charts for all oils.
3. KOH, milligrams per gram (e.g. olive about 190). The chemistry and lab convention, often found on supplier spec sheets as “saponification value”.

The conversions:

• mg KOH/g → g KOH/g: divide by 1,000.
• KOH → NaOH: divide by 1.403 (the ratio of their molecular weights).
• NaOH → KOH: multiply by 1.403.

Oil	NaOH (g/g)	KOH (g/g) = NaOH × 1.403	KOH (mg/g)
Olive oil	0.135	0.189	about 189
Coconut oil (76°)	0.183	0.257	about 257
Shea butter	0.128	0.180	about 180
Castor oil	0.128	0.180	about 180

Those KOH figures are for pure potassium hydroxide. Most KOH sold to soap makers is about 90% pure, so a calculator divides by 0.90 on top of this. See sodium vs potassium hydroxide.

How to use SAP values in a recipe

The formula for lye is:

lye = Σ(oil weight × SAP) × (1 − superfat)

Here’s a four-oil bar worked through by hand: 1,000 g of oils at 5% superfat.

Oil	Weight	SAP (NaOH)	Weight × SAP
Olive oil	400 g	0.135	54.0 g
Coconut oil	300 g	0.183	54.9 g
Shea butter	250 g	0.128	32.0 g
Castor oil	50 g	0.128	6.4 g
Total	1,000 g		147.3 g

At 5% superfat: 147.3 × 0.95 = 139.9 g NaOH.

Notice that coconut oil is only 30% of the oils but needs as much lye as the 40% olive oil. That’s the SAP value at work.

For a liquid soap, you’d use the KOH values, or let the lye calculator convert for you.

Why SAP values are approximate

A SAP value is an average. The same oil varies with where it was grown, the season, how it was processed and how old it is. Published values for olive oil, for example, vary by a few thousandths depending on the source.

That variation is one of the reasons for superfat. A 5% superfat is a buffer that covers normal SAP variation and small scale errors, so even if your oil happens to need slightly less lye than the chart says, the bar won’t end up with free lye.

A few related points:

• Coconut oil types differ. “Coconut oil 76°” (standard refined) and fractionated coconut oil have different fatty acid makeups and different SAP values. Pick the right one.
• Some ingredients have little saponifiable content. Jojoba is technically a liquid wax, not a triglyceride, and has a lower SAP value than most oils. Use the calculator’s value; don’t guess.
• Hydrogenated or blended shortenings vary by brand. Use a value for that specific product if you can find it, and recheck if the brand changes its formula.

Spot-checking a recipe you found

SAP values let you check someone else’s recipe in a minute, which is worth doing before you weigh lye from any source.

Say a recipe calls for 500 g coconut oil and 75 g sodium hydroxide, and doesn’t state a superfat.

1. Full lye for 500 g coconut: 500 × 0.183 = 91.5 g.
2. Compare: 75 ÷ 91.5 ≈ 0.82, so the recipe uses about 82% of the full lye.
3. That’s a superfat of about 18%.

For a 100% coconut oil soap, that’s in line with normal practice: coconut is very cleansing on its own, and 15–20% superfat is common. For an everyday multi-oil bar, 18% would be unusually high. If the recipe used 95 g of lye instead, that’s more than the 91.5 g needed with no superfat at all, and the bar would be lye-heavy. Don’t make it.

Run the recipe through the lye calculator to confirm. The spot-check is a quick way to catch a typo before it becomes a batch.

SAP values vs “INS” and other numbers

Some charts list an iodine value and an INS value next to SAP. Iodine value is a measure of how unsaturated an oil is: high-iodine oils are softer and more prone to going rancid. INS is a soap-maker’s rule of thumb combining SAP and iodine values to predict bar hardness. Neither is needed to calculate lye. Our calculator’s bar quality panel serves a similar comparing role.

Common mistakes

• Mixing formats. Using a KOH value as if it were NaOH gives about 40% too much lye. Always check which column you’re reading.
• Using one SAP value for “vegetable oil”. Check what oil it is. Blends and generic labels can be anything.
• Including fragrance, colorants or clays. They aren’t fats, so they don’t go into the calculation.
• Rounding SAP values too much. 0.183 to 0.18 is a small change; 0.183 to 0.2 is not.
• Assuming the chart is exact. It’s a reliable average, not a measurement of your bottle. Keep a superfat.

Try it

Pull up the SAP value chart, choose a one-oil recipe like castile soap or a 100% coconut oil soap, and calculate the lye by hand. Then check your answer in the lye calculator. The how to use a lye calculator page has a fuller worked example.