A great cold-process soap is a balancing act between hard, cleansing oils and soft, conditioning ones. This calculator takes your oil percentages and predicts the bar’s hardness, cleansing, conditioning, bubbly and creamy lather, plus the INS and iodine values, so you can fine-tune the blend before you ever reach for the lye.
How it works
Each oil contributes a known fatty-acid-derived value to every quality. The recipe property is the percentage-weighted average across all oils:
property = Σ (oil_value × oil_percent) / Σ (oil_percent)For example, coconut oil contributes high cleansing and bubbly numbers but low conditioning, while olive oil is the opposite. Blending them shifts every property toward a weighted middle. INS combines saponification and iodine into a single hardness index, and the iodine value tracks how soft and prone to oxidation the bar will be.
What the seven properties actually measure
Hardness predicts how firm and durable the bar will be. It is driven by saturated fatty acids — lauric, myristic, palmitic, and stearic. Hard oils like coconut, palm, and lard contribute high hardness; liquid oils like olive and sunflower lower it. A bar with hardness below roughly 29 will be soft and mushy in the shower; above 54 it may be brittle and prone to cracking.
Cleansing measures how aggressively the soap strips oil and grease. High cleansing is ideal for hand and dish soap; for facial bars and sensitive skin, lower is better. Coconut and palm kernel oil drive cleansing up sharply — this is why pure coconut soap is excellent at cleaning but notoriously drying.
Conditioning reflects the moisturising quality of the lather. High-oleic oils like olive, avocado, and high-oleic sunflower push this number up. A conditioning value in the mid-to-upper range produces a bar that leaves skin feeling soft rather than squeaky-clean.
Bubbly predicts the fluffy, airy lather that most people associate with soap working well. Coconut and palm kernel oil are the primary drivers. Castor oil also boosts bubbly dramatically at small percentages.
Creamy predicts the denser, stable lather rather than the quick-burst bubbles. Palm, lard, and tallow contribute here. A bar with good creamy lather feels rich and substantial, which is why many soap makers blend coconut with palm or animal fats.
INS is a composite hardness and latherability index. A blend landing near 160 tends to produce a well-rounded bar. Below 136 the bar is soft and slow to form; above 165 it can be harsh and may crack.
Iodine value tracks the proportion of unsaturated fatty acids. High iodine means more double bonds, which are prone to oxidation — this is the chemical root of soap going rancid (developing DOS, dreaded orange spots). High-iodine oils need longer cure times and benefit from antioxidant additives like rosemary extract.
A balanced recipe in practice
A classic blend — 50% olive, 25% coconut, 20% palm, 5% castor — lands inside the recommended range on most properties. Olive contributes conditioning and a soft initial lather that improves with cure; coconut delivers cleansing power and bubbly lather; palm adds hardness and creamy lather; castor boosts lather stability at a level that does not make the bar sticky.
Pushing coconut above about 35% drives cleansing out of the comfortable range toward a bar that strips skin oils too aggressively. Dropping all hard oils in favour of a pure olive bar (Castile soap) produces an exceptionally conditioning bar but one that stays soft for months and lathers poorly until fully cured — sometimes 6 to 12 months.
Tips and notes
Keep cleansing in the low-to-mid range for face and sensitive-skin bars, and keep castor at 5 percent or below to avoid a sticky bar. Soft, high-iodine oils like sunflower and canola boost conditioning but raise oxidation risk, so use them sparingly and cure the soap longer. This tool is a recipe-property estimator only — always run a dedicated lye calculation with your chosen superfat before mixing any batch.