Best Oils for Soap Making: Complete Guide to Base Oils

Why Oil Choice Matters

Every oil and fat you put into a soap recipe contributes a distinct set of fatty acids, and those fatty acids determine the finished bar's properties: hardness, lather type (fluffy vs. creamy), longevity, cleansing power, and skin conditioning. There are no universal "best" oils — there's only the right blend for your intended bar.

The fundamental tension in soap formulation is between hard oils (saturated fats that harden bars and boost cleansing) and soft oils (unsaturated fats that condition and moisturize but produce softer bars). Use too many hard oils and your bar is harsh and brittle. Use too many soft oils and it's mushy, short-lived, and slow to lather.

A well-designed soap formula balances: hardness (bar stays solid and lasts long), cleansing (removes dirt and oil), conditioning (doesn't strip skin), lather quality (fluffy bubbles vs. dense, creamy lather), and stability (resists rancidity during the cure and on the shelf). Most good beginner recipes target approximately 30–40% hard oils and 60–70% soft or semi-soft oils.

Understanding the fatty acid composition of each oil — and its saponification value — is what transforms soap making from guesswork into a repeatable craft.

SAP Values: The Numbers Behind Every Oil

Every oil has a saponification value (SAP value) — the number of grams of sodium hydroxide (NaOH) required to fully saponify exactly 1 gram of that oil. These values are determined by the fatty acid composition of each oil. When you use a lye calculator, it's doing this multiplication for you across all your oils and summing the lye requirement.

Never calculate your lye requirement by hand from SAP values alone — lye calculators account for superfat percentage, water discount, and batch weight in a single calculation. SAP values are listed here for understanding the chemistry, not for manual calculation.

Here are NaOH SAP values for the 12 most common soap-making oils and fats (grams NaOH per gram of oil):

• Coconut oil (76°F): 0.190 — highest SAP value of common oils; needs the most lye per gram
• Coconut oil (92°F): 0.190 — fractionated coconut; same SAP value
• Palm oil: 0.141
• Olive oil (pomace): 0.134
• Olive oil (extra virgin): 0.134
• Castor oil: 0.128
• Lard (pork fat): 0.138
• Tallow (beef fat): 0.140
• Shea butter: 0.128
• Cocoa butter: 0.137
• Avocado oil: 0.133
• Sweet almond oil: 0.136
• Sunflower oil: 0.134

Notice that coconut oil at 0.190 requires nearly 50% more lye per gram than castor or shea butter at 0.128. This is why you cannot substitute oils freely in a recipe without recalculating — swapping even a small percentage of one oil for another changes the total lye requirement.

Hard Oils: Hardness and Cleansing Power

Hard oils are saturated fats that are solid at room temperature. They contribute bar hardness, longevity, and cleansing ability to your soap.

Coconut oil is the MVP of soap lather. Its high lauric acid (around 48%) and myristic acid (around 18%) content produces large, fluffy, abundant bubbles that rinse clean easily. It also contributes significant hardness. The downside: those same fatty acids are highly cleansing — almost too cleansing at high percentages. Above 30% coconut oil, many people find soap stripping and drying. The ideal usage range is 15–25% for most formulas, where you get the lather benefits without the dryness. Increasing your superfat percentage (to 8% or more) can offset some of the drying effect if you want to go higher.

Palm oil contributes hardness and a stable, long-lasting lather. Its palmitic acid content (around 44%) is what produces the long-lasting lather quality that doesn't disappear quickly under running water. Palm oil doesn't boost lather volume the way coconut does — it makes the lather more stable and dense. Usage rate: typically 25–35%. Important note: palm oil has significant sustainability concerns due to deforestation. Many soapers choose RSPO-certified palm (Roundtable on Sustainable Palm Oil) or substitute with a blend of lard and another oil. If you avoid palm entirely, lard or tallow can fill a similar role.

Tallow (beef fat) and lard (pork fat) are traditional soap-making fats and still produce excellent results. Both contain high percentages of stearic and palmitic acid, creating hard bars with a creamy, conditioning lather and a skin feel that many soapers prefer over plant-based formulas. Lard and tallow are typically used at 20–40% of the formula. If you're making soap for vegans, substitute with a combination of palm, cocoa butter, and shea butter to approximate the same fatty acid profile.

Soft Oils: Conditioning and Skin Feel

Soft oils are liquid at room temperature and are high in unsaturated fatty acids. They're the conditioning, skin-nourishing half of your formula — but too much produces soft, short-lived bars.

Olive oil is the most widely used soft oil in soap making. High in oleic acid (around 70%), it produces a gentle, mildly conditioning bar with a stable, creamy lather. Olive doesn't boost lather volume — it produces the dense, slippery lather associated with Mediterranean soaps. The challenge with olive oil is cure time: high-olive recipes take longer to harden and cure. Castile soap (100% olive oil) is the extreme example — many soapers cure it for 6 months to a year before use to achieve a hard, long-lasting bar. In balanced recipes, olive is typically used at 30–50%.

Castor oil is the unique wild card of soap formulation. It doesn't fit neatly into "hard" or "soft" — it's a conditioning, humectant oil (draws moisture to the skin) that also boosts and stabilizes lather produced by other oils in the recipe. Even 2–3% castor oil noticeably improves lather in most formulas. The sweet spot is 1–5% PPO (per pound of oil). Go above 5–7% and bars become sticky and draggy on skin, and softer than desired. Think of castor oil as a seasoning: a small amount enhances everything, a large amount overwhelms.

Sweet almond, sunflower, and avocado oil are lightweight conditioning oils that add skin benefits without being as heavy as shea or cocoa butter. Sweet almond is high in oleic acid and is particularly gentle for sensitive skin. Sunflower oil is high in linoleic acid, making it good for oily or acne-prone skin types. Avocado is rich in oleic acid plus vitamins A, D, and E. All three should be used at 10–20% maximum in a formula — higher percentages make bars very soft and increase the risk of rancidity (dreaded orange spots, or DOS).

Shea butter adds luxurious creaminess and a soft, emollient skin feel. High in stearic acid (making it technically a harder fat) but its unique "unsaponifiables" fraction — the portion that doesn't react with lye — stays in the bar and contributes significant skin conditioning. Use at 5–10% to add richness without making the formula too slow to cure or too expensive. Cocoa butter is similar in role and can be used at comparable percentages, but it adds a very slight chocolate scent.

The 3-Oil Beginner Formula

If you're new to soap making, don't over-engineer your first recipe. The classic 3-oil formula has been proven by thousands of soapers and produces a genuinely excellent bar:

• 30% Coconut oil — hardness + fluffy lather
• 30% Palm oil — hardness + stable lather
• 40% Olive oil — conditioning + mildness

This formula produces a hard, long-lasting bar with good lather, solid cleansing power, and skin conditioning. It's beginner-friendly because all three oils behave predictably, are widely available at grocery stores and soap suppliers, and work well with most fragrance oils.

For a standard 2-pound oil batch, the weights are:

• Coconut oil: 273g (30% of 910g)
• Palm oil: 273g (30% of 910g)
• Olive oil: 364g (40% of 910g)
• Total oil: 910g

Run these numbers through a lye calculator with a 5% superfat to get your exact NaOH and distilled water amounts. At 5% superfat, roughly 5% of the oils remain unsaponified in the bar, adding conditioning. For sensitive skin, increase superfat to 8%.

You can enhance this basic formula by adding 2–3% castor oil (reduce olive oil accordingly) and 5% shea butter (reduce palm accordingly) for even better lather and conditioning. But master the basic 3-oil formula first — it's a reliable foundation to learn on.

Matching Oils to Skin Type

Different skin types benefit from different oil balances. Here's how to adjust your formula for the intended user:

Oily or acne-prone skin: Increase coconut oil to 25–30% for higher cleansing power. Use sunflower oil (high linoleic acid) at 15–20% — linoleic acid is known to help balance oily skin. Add 1 teaspoon of kaolin clay per pound of oil — it gently clarifies without over-drying. Set superfat at 5% or below. Avoid shea butter and cocoa butter in these formulas.

Dry or sensitive skin: Reduce coconut oil to 10–15% maximum. Increase olive oil and/or shea butter to 40–50% of the formula. Avocado oil at 10–15% adds vitamins. Increase superfat to 8% to leave more conditioning oils unreacted in the bar. Omit palm if possible; substitute lard or tallow for similar hardness without as high a cleansing rating.

Normal or combination skin: The standard 3-oil formula (30/30/40) works well with a 5% superfat. Add 2–3% castor oil for enhanced lather and a small boost of moisturization. This formula is the reliable daily-use bar for most skin types.

Baby or very gentle skin: Use a very high olive oil percentage (50–70%), with under 10% coconut oil or none at all. Shea butter at 10% and sweet almond oil at 15% round out the formula. Set superfat at 8–10%. These bars will be soft and require a longer cure (6–8 weeks). Omit all fragrance, or use skin-safe essential oils at very low usage rates (0.5–1% PPO).

Common Oil Mistakes to Avoid

These mistakes are responsible for the majority of failed batches among beginners and even experienced soapers:

Using too much coconut oil: Bars that are 40–50% coconut oil are harsh and drying for most people, even at 5% superfat. Stick to 25% or below unless you're making a specialty cleaning bar (not a body soap). If you see reports of itchy, tight skin after using your soap, excess coconut is often the culprit.

Skipping castor oil entirely: Even 1–2% castor oil makes a measurable difference in lather quality. It costs very little and doesn't meaningfully change the bar's hardness at low percentages. If your bars feel like they don't lather enough, adding 2–3% castor oil to your formula is the first adjustment to make.

Using expired or rancid oils: Oils with high polyunsaturated fat content (sunflower, flaxseed, hemp) go rancid relatively quickly once opened. Using rancid oils introduces oxidation into the bar, leading to DOS — Dreaded Orange Spots — which are patches of orange or brown rancidity that appear on cured bars. Always smell your oils before using them. Rancid oil smells like old crayons or motor oil. Adding 0.5% rosemary oleoresin extract (ROE) to your oil mix helps delay rancidity.

Measuring oils by volume instead of weight: A cup of coconut oil weighs differently depending on whether it's liquid or solid, fresh from the fridge or at room temperature. Volume measurements introduce inconsistency into recipes that depend on precise ratios. Always weigh on a digital scale.

Using expensive specialty carrier oils as primary bases: Argan oil, rosehip oil, and marula oil are beautiful for skincare but poor choices as primary soap oils. They're expensive, high in easily oxidized polyunsaturated fats (leading to DOS), and they produce soft bars. Use these at 2–5% maximum as "superfatting" oils added after trace, where they remain largely unreacted and deliver their skin benefits directly. Save them for your lotion and serum formulations.

Frequently Asked Questions

What is the best oil for making hard soap bars?

Coconut oil and palm oil are the primary hardening oils in cold process soap. Coconut oil adds hardness and big fluffy bubbles (best at 15–25% to avoid dryness), while palm oil contributes long-term hardness and stable lather. Tallow and lard are also excellent hardening oils at 20–40% and produce very hard, long-lasting bars. Using all soft oils like olive alone produces a soft bar that takes a year or more to harden fully.

Can I use vegetable oil to make soap?

Yes — generic vegetable oil (usually a soy or canola blend) can be used in soap, and it has a SAP value of approximately 0.136. However, it produces a soft, slow-curing bar with mediocre lather on its own. It's better used as a small percentage in a balanced recipe. Always run your specific oil through a lye calculator since SAP values vary by oil composition.

What does coconut oil do in soap?

Coconut oil is the primary lather-boosting oil in cold process soap, producing large, fluffy bubbles due to its high lauric and myristic acid content. It also contributes significant hardness to the bar. The trade-off is that coconut oil is highly cleansing and can be drying if used above 30%. The recommended usage rate is 15–25% for most skin-care formulas.

How much castor oil should I use in soap?

The recommended usage rate for castor oil is 1–5% of total oil weight. At this level it boosts and stabilizes lather while acting as a humectant. Going above 5–7% causes bars to become sticky, draggy on skin, and softer than desired. Think of castor oil as a seasoning: a small amount enhances everything, a large amount overwhelms.

What oils make the most lather in soap?

Coconut oil produces the most abundant, fluffy lather due to its high lauric acid content. Castor oil boosts and stabilizes the lather produced by other oils — it's a lather enhancer, not a standalone producer. Palm kernel oil (similar profile to coconut) also contributes big bubbles. High-oleic oils like olive, sunflower, and avocado produce a creamy, stable lather rather than large fluffy bubbles.