Soap INS Predictor

How It Works

The INS value is a composite fatty acid scalar developed by Dr. Robert McDaniel in the 1990s as a shorthand for predicting how a cold process soap will behave from its oil profile alone. The per-oil formula is straightforward:

• INS (per oil) = SAP value − Iodine value
• Blend INS = Σ (oil fraction × INS of that oil)

Saponification value measures the alkali needed per gram of fat, so it tracks with hardness and cleansing. Iodine value measures the degree of unsaturation, so it tracks with softness and conditioning. Subtracting one from the other collapses both dimensions into a single number and weight-averaging across a recipe gives a blend-level scalar.

McDaniel proposed a target range of 136-170 for a balanced bar. Below 136 the bar tends to stay soft, cure slowly, and lather sparsely. Above 170 the bar trends toward brittle and drying. The number is not a physical property — it is a heuristic that correlates with real bar behavior across dozens of common recipes.

The SAP and iodine values in the tool are drawn from McDaniel's original tables, republished in Essentially Soap (Alabu Press, 2000), and cross-checked against the fatty acid tables in Kenna Cote's Modern Soapmaking, Ch. 5: Fatty Acid Profiles. Where sources disagree by more than 2%, the median is used.

How to Use This Calculator

1. Add your oils — select each oil from the built-in database and enter its percentage of the total fat weight. The percentages must sum to 100%.
2. Review the blend INS — the weighted INS updates as you edit. Aim for 136-170 for a general-purpose bar.
3. Check the property scores — hardness, cleansing, conditioning, bubbly lather, and creamy lather each report a numeric score and a rating relative to their recommended ranges.
4. Adjust the ratios — swap a soft oil for a harder oil to raise INS, or the reverse to soften a high-INS blend. Small 5-10% shifts are usually enough to move a single property.
5. Confirm the fatty acid profile — lauric, myristic, palmitic, stearic, oleic, linoleic, and ricinoleic percentages are shown so you can verify the recipe matches your target feel.
6. Export the recipe — save a printable PDF with the oil list, INS, and property scores for your batch record.

Real-World Use Cases

Formulating a castile-plus recipe

A soaper starting from 100% olive oil (INS ~109, very low — bars stay soft and slime-prone) adds coconut and castor to raise the number into range. A trial blend of 75% olive at INS 109, 15% coconut at INS 258, and 10% castor at INS 95 weight-averages to roughly 132 — still short of the target. Nudging coconut to 20% lands the blend inside the 136-170 band without pushing cleansing high enough to strip skin.

Diagnosing a soft bar

A soaper whose bars stay soft weeks past normal cure time checks the recipe and finds the blend INS is 118, well under the 120 practical soft-soap threshold. Swapping 10% of the olive for coconut lifts the INS to about 140 and yields a firmer bar at four-week cure with no other formulation changes.

Balancing a coconut-heavy recipe

A soaper working from a 40% coconut recipe (INS 175, cleansing score 25 — deep into drying territory) blends in 15% olive and 10% avocado. The blend INS drops to about 160 and the cleansing score falls from 25 to 18. The bar still deep-cleans but no longer strips skin on daily use.

Tips & Safety Notes

• INS is a starting point, not a guarantee. Always run a small test batch before committing to a large pour.
• Higher superfat softens a high-INS bar noticeably. Moving from 5% to 8% superfat on a 170-INS recipe pulls the perceived hardness down without changing the INS number.
• Sodium lactate and salt harden low-INS bars without shifting INS. A teaspoon of sodium lactate per pound of oil is the standard hardness additive for olive-heavy recipes.
• Combine INS review with cleansing and conditioning number checks. Two recipes at the same INS can differ significantly in feel because INS collapses cleansing and conditioning into a single scalar.
• INS does not replace safe soap-making practice. Wear gloves and goggles, add lye to water (never water to lye), and always verify the lye number with a dedicated saponification calculator.

Limitations & What This Tool Cannot Do

• INS is a heuristic, not a physical measurement. Two recipes with the same INS can feel different in use because the scalar aggregates fatty acid data and ignores unsaponifiables, additive interactions, and cure duration.
• Historical origin. The INS scale reflects mid-20th-century soap-making norms and target markets. Modern formulators sometimes prefer to weight cleansing and conditioning scores directly rather than rely on the single INS number.
• Does not model additives. INS is derived from the oil profile only. Sodium lactate, sugar, salt, beeswax, and stearic acid additives change bar hardness and lather but do not change INS.

FAQ

What is the INS value in soap making and what does it tell me?

INS is a single number derived from the saponification and iodine values of the oils in a recipe. It predicts how the finished bar will behave in terms of hardness, cleansing, and lather. A higher INS points to a harder, more cleansing bar; a lower INS points to a softer, gentler one.

What is a good target INS range for cold process soap?

McDaniel's original recommendation is 136-170 for a general-purpose bar. Recipes in this band tend to be firm enough to unmold cleanly, lather well, and feel comfortable on skin. Values outside the band can still make excellent soap — 100% olive castile at INS ~109 is a classic mild bar — but require adjustments to cure time or superfat.

How does the INS value relate to hardness, cleansing, and conditioning numbers?

INS correlates positively with hardness and cleansing and negatively with conditioning, but it is not a direct substitute for any of them. Two recipes at the same INS can differ by several points on each individual property score because INS averages information the individual scores keep separate.

Can I have a high-INS soap that still feels gentle on skin?

Yes. Raising the superfat percentage leaves more free fatty acid in the finished bar, which offsets the drying feel of a high-cleansing recipe. A 6-8% superfat on a 165-170 INS bar typically feels closer to a 145-INS bar at the standard 5% superfat.

Why do two recipes with the same INS score feel different in use?

INS aggregates SAP and iodine values into one number, which discards information about the underlying fatty acid mix. A recipe heavy in lauric acid and one heavy in stearic acid can produce the same INS while feeling very different — one will bubble aggressively, the other will feel dense and creamy.

How does INS interact with superfat percentage?

Superfat leaves a percentage of the oils unsaponified in the finished bar. That free fat cushions the skin against the cleansing action, so it effectively lowers the perceived aggression of a high-INS bar without changing the INS number itself. Increasing superfat is the standard fix when a high-INS recipe feels too stripping in a skin trial.

Related Tools

• Lye & Water Calculator — Compute NaOH or KOH weight for the same recipe
• HLB Calculator for Emulsifiers — Take the same oils into emulsion and lotion formulation
• Candle Wax & Fragrance Calculator — Adjacent fat-and-oil chemistry for the same maker

References

• Dr. Robert McDaniel, INS and Iodine Value scale (foundational reference, republished in Essentially Soap, Alabu Press, 2000)
• Kenna Cote, Modern Soapmaking, Ch. 5: Fatty Acid Profiles
• SoapCalc INS reference table (soapcalc.net documentation)

Reviewed by the Craft Calc Lab team on August 1, 2026.