Reading and Interpreting Soap Quality Metrics

What the Numbers Mean

Every oil and fat used in soap making has a known fatty acid profile — the percentages of lauric, myristic, palmitic, stearic, oleic, linoleic, linolenic, and ricinoleic acids it contains. When you enter a recipe into the SoapIndex calculator, we multiply each oil's contribution by its fatty acid breakdown, sum everything up, and derive eight quality metrics. These numbers give you a snapshot of how the finished bar will perform: how hard it will be, how much lather it will produce, how gentle it will feel on the skin, and how long it will last in the shower.

No single metric tells the whole story — a great bar of soap is a balance across all eight. Think of them as dials on a mixing board: turning one up usually means turning another down, and the art of recipe formulation is finding the sweet spot for your goals.

The 8 Quality Metrics

The table below lists every metric the calculator displays, its ideal range for a well-balanced bar, the fatty acids it derives from, and what it tells you in practice.

What Happens Outside the Ranges

When a metric drifts outside its ideal range, you will notice it in the finished bar — sometimes dramatically.

Hardness too high (above 54): The bar may be brittle, crack during cutting, and feel waxy on the skin. Recipes heavy in palm, tallow, and coconut without enough liquid oils tend to land here.

Hardness too low (below 29): The bar is soft, easily dented, and dissolves quickly in the shower. A recipe with 80%+ olive oil will produce a very soft bar unless cured for many months.

Cleansing too high (above 22): The soap strips natural oils from skin, leaving it tight, dry, and possibly irritated. This happens when coconut oil exceeds 30–35% of the recipe. People with eczema or sensitive skin will feel this immediately.

Cleansing too low (below 12): The bar does not lather well and may leave a residue. Castile-style soaps (100% olive oil) have a cleansing value near zero — they clean via a different mechanism (emulsification) but produce minimal bubble lather.

Conditioning too high (above 69): While moisturizing is good, an extremely high conditioning value usually means the bar is soft, mushy, and may develop dreaded orange spots (DOS) during storage because of excess unsaturated fatty acids oxidizing.

Conditioning too low (below 44): The bar feels drying and harsh. It will strip moisture and leave skin feeling squeaky rather than silky.

Iodine too high (above 70): The bar contains too many unsaturated fats and is at risk of going rancid. It will also be very soft. Reduce high-oleic or high-linoleic oils (sunflower, soybean, hemp).

INS too low (below 136): Generally indicates a soft, potentially unstable bar. INS is a broad-brush indicator — useful as a sanity check but not as precise as the individual metrics.

How to Adjust Your Recipe

Once you understand what each metric means, adjusting a recipe becomes a methodical process rather than guesswork. The table below shows common adjustments.

Castor oil is a special ingredient — ricinoleic acid contributes to both bubbly and creamy lather without significantly affecting hardness. Most soap makers use 3–8% castor oil as a lather booster. Going above 10% can make the bar tacky.

Real Recipe Example

Let us analyze a popular all-purpose recipe and see how its metrics stack up.

Running this through the calculator yields approximately:

Every metric falls within its ideal range, which is why this recipe (or close variations of it) appears in so many beginner guides. From here, you can tweak to your preference: swap some olive for avocado oil for more luxury, bump coconut to 30% for more lather (watching the cleansing number), or add a few percent cocoa butter for a harder, creamier bar.