Olive Oil Soap Recipe: How to Make Castile Soap (Beginner Guide)

Quick Answer

Olive oil soap (also called castile soap) is one of the simplest soaps to make, just olive oil, lye (sodium hydroxide), and water. A basic recipe: 16 oz olive oil, 2.1 oz lye, 4.7 oz water. Mix at room temperature, pour into a mold, and cure for 4–6 weeks minimum. For a harder bar with better lather, blend olive oil with coconut oil and castor oil.

Get your ratios right: Use our free soap cost calculator to calculate ingredient costs per bar, and run your recipe through a lye calculator before every batch.

What Is Castile Soap?

Castile soap is soap made primarily or entirely from olive oil. The name comes from the Castile region of Spain, where soap makers began producing olive oil–based soap as an alternative to animal fat soaps during the Middle Ages. Before that, the city of Aleppo in Syria was producing olive oil and laurel oil soap for centuries, Aleppo soap is considered the ancestor of all hard bar soaps.

Today, "castile soap" loosely refers to any soap made with a high percentage of olive oil (typically 70% or more), though purists insist on 100% olive oil.

Olive oil soap is popular for several reasons: it produces an extremely gentle, moisturizing bar with a creamy (rather than bubbly) lather. It's suitable for sensitive skin, babies, and people with eczema or dermatitis. The trade-off is that pure olive oil soap is soft, slow to cure, and doesn't produce the fluffy lather most people expect from a bar of soap.

Understanding Olive Oil SAP Values

Before making any soap recipe, you need to understand saponification (SAP) values. The SAP value tells you how much lye is needed to fully convert a given oil into soap.

Olive oil SAP values:

This means you need 0.1357 oz of sodium hydroxide (NaOH) to saponify 1 oz of olive oil. For 16 oz of olive oil: 16 × 0.1357 = 2.17 oz lye.

Always run your recipe through a lye calculator before making soap. SAP values vary slightly between oil batches, so most soap makers use a 5% superfat (lye discount) to ensure no unsaponified lye remains in the finished bar.

Recipe 1: Basic 100% Olive Oil Soap (Castile)

This is the simplest cold process soap you can make. One oil, lye, and water.

Ingredients

Equipment Needed

You'll need a digital kitchen scale (measure by weight, never volume), a stick blender, a heat-safe mixing container for lye, a stainless steel or high-density plastic pot for oils, a thermometer, a silicone soap mold, safety goggles, and rubber gloves.

Instructions

Step 1: Prepare Your Workspace

Put on safety goggles and gloves. Lye is caustic and will burn skin on contact. Work in a well-ventilated area. Keep vinegar nearby, it won't neutralize a lye burn, but it's useful for cleaning lye splashes off surfaces.

Step 2: Mix the Lye Solution

Weigh 4.7 oz of distilled water into a heat-safe container (Pyrex or polypropylene). Weigh 2.1 oz of lye separately. Slowly pour the lye into the water, never the other way around. Stir gently until dissolved. The solution will heat up to around 200°F. Set it aside to cool.

Step 3: Prepare the Oil

Weigh 16 oz of olive oil into your soap pot. If the oil is at room temperature (around 70–80°F), it's ready. There's no need to heat olive oil for this recipe.

Step 4: Combine and Blend

When the lye solution has cooled to approximately 80–100°F, slowly pour it into the olive oil. Use a stick blender in short bursts (5 seconds on, 5 seconds off) to mix. Blend until the mixture reaches "light trace", the consistency of thin pudding where a drizzle leaves a faint trail on the surface.

Important: 100% olive oil soap is slow to reach trace. This can take 10–30 minutes of intermittent stick blending. Don't rush it by blending continuously, as this can burn out your stick blender.

Step 5: Pour and Mold

Pour the traced soap into your silicone mold. Tap the mold on the counter a few times to release air bubbles. Cover the top with plastic wrap, then insulate with a towel. Let it sit undisturbed for 48–72 hours.

Step 6: Unmold and Cut

After 48–72 hours, check if the soap is firm enough to unmold. 100% olive oil soap is soft, if it's still too soft, wait another day or two. Once unmolded, cut into bars with a sharp knife or soap cutter.

Step 7: Cure

Place bars on a drying rack with airflow on all sides. Cure for a minimum of 4–6 weeks. Here's the thing about castile soap: it gets dramatically better with extended curing. Many experienced soap makers cure 100% olive oil soap for 6–12 months. At that point, the bar becomes much harder, the lather improves noticeably, and the soap develops a smooth, almost polished feel.

Recipe 2: Improved Olive Oil Soap Blend (Harder Bar, Better Lather)

Pure castile soap has limitations: it's soft, doesn't lather well, and takes months to cure properly. This blended recipe keeps olive oil as the primary oil but adds coconut oil for hardness and lather, plus castor oil for bubbles.

Ingredients

Why This Blend Works

Olive oil provides mildness and moisture but makes a soft bar with weak lather. Coconut oil adds hardness and big, fluffy bubbles — but too much (over 30%) can be drying. Castor oil is a lather booster: it stabilizes bubbles and adds creaminess. At 10%, it enhances lather without making the bar sticky.

This 60/30/10 ratio is widely used among cold process soap makers because it balances gentleness with performance.

Instructions

Follow the same steps as the basic recipe with two changes. First, melt the coconut oil before combining it with the olive oil and castor oil, coconut oil is solid at room temperature. Second, this blend reaches trace much faster than 100% olive oil, usually within 2–5 minutes of stick blending. Cure time is 4–6 weeks.

Optional Additives

Sodium lactate tip: Adding sodium lactate at 1 teaspoon per pound of oils makes a noticeably harder bar and allows you to unmold 12–24 hours sooner. Add it to the cooled lye solution before combining with oils.

Choosing the Right Olive Oil for Soap Making

Not all olive oil is the same. The type you choose affects cost, trace time, and final bar quality.

Recommendation for most soap makers: Use pomace olive oil. It's the most economical grade, traces faster than extra virgin (which can take forever), and produces a bar that's virtually indistinguishable from one made with expensive EVOO after curing. Save your extra virgin olive oil for cooking.

Common Problems and Fixes

Problem: The Bar Is Too Soft After Curing

This is the most common complaint with olive oil soap. If the bar dents when you press it after 4–6 weeks of curing, either cure it longer (castile soap needs extended curing) or add a water discount next time. Reducing water to 33% of oil weight instead of 38% produces a firmer bar. You can also add sodium lactate to the recipe.

Problem: Slimy Lather Instead of Bubbles

Pure olive oil soap produces a lotion-like lather rather than fluffy bubbles. This is normal for castile soap, it's a feature, not a defect. If you want more bubbles, switch to the blended recipe (Recipe 2) with coconut and castor oils. Even 20% coconut oil makes a significant difference.

Problem: Dreaded Orange Spots (DOS)

Orange or brown spots that appear on cured soap are called DOS (dreaded orange spots). They indicate rancid oils and are more common with olive oil soap because olive oil has a shorter shelf life than some other soap-making oils. Prevention: use fresh oil (check expiration dates), store curing soap in a cool, dry place away from direct sunlight, and add rosemary oleoresin extract (ROE) at 0.02% of total oil weight as a natural antioxidant.

Problem: Very Slow Trace

100% olive oil is one of the slowest oils to reach trace. If you've been stick blending for 20+ minutes with no thickening, check that your lye measurements are accurate (weigh, don't measure by volume), your lye solution and oils are in the right temperature range (80–100°F works well), and your stick blender is actually working (try a different one if possible). Some batches of EVOO take 30+ minutes to trace. Consider switching to pomace grade.

Problem: Lye-Heavy or Zappy Soap

If the finished soap stings your tongue when you touch it (the "zap test"), there's unsaponified lye. This means your lye was measured incorrectly or the soap didn't fully saponify. Don't use it on skin. You can rebatch it: grate the soap, add a small amount of liquid, melt in a double boiler, and repour. Always use a lye calculator and a digital scale accurate to 0.1 oz.

Tips for Better Olive Oil Soap

Use a water discount. Standard lye calculators default to 38% water (as percentage of oils). Reducing to 33% produces a harder bar that cures faster and unmolds more easily. This is especially helpful for 100% olive oil recipes.

Try a room-temperature process. Olive oil soap can be made at room temperature, no need to heat the oil. Just make sure the lye solution has cooled to below 100°F before combining. This simplifies the process for beginners.

Don't gel phase (unless you want to). Gel phase happens when insulated soap heats up internally, becoming translucent before cooling. With olive oil soap, gel phase darkens the color to a deeper gold. Some makers insulate heavily to force gel phase; others don't. Both approaches produce perfectly good soap, it's purely an aesthetic preference.

Consider a hot process variation. Hot process cooking (in a slow cooker) forces saponification to complete during the cooking process rather than during the cure. The result is a rustic-looking bar that's technically safe to use immediately, though curing for 1–2 weeks still improves the bar. Hot process is useful if you want faster turnaround.

Frequently Asked Questions

Is olive oil soap good for your skin?

Yes. Olive oil soap is one of the mildest soaps you can make. It's rich in oleic acid, which is moisturizing and non-stripping. It's suitable for sensitive skin, babies, and conditions like eczema. The high superfat in most castile recipes leaves extra olive oil in the bar as a skin conditioner.

How long does olive oil soap need to cure?

Minimum 4–6 weeks for a basic cure. However, 100% olive oil soap improves dramatically with age. Most experienced soap makers recommend 6–12 months for peak quality. Blended recipes (olive with coconut, castor) cure in the standard 4–6 weeks.

Can I use extra virgin olive oil for soap?

You can, but it's not recommended for most soap makers. EVOO is expensive, traces very slowly, and can cause slight discoloration due to its chlorophyll content. Pomace or pure olive oil produces a nearly identical bar at a fraction of the cost.

Why is my olive oil soap slimy?

This is normal for high-olive-oil soap, especially during the early weeks of curing. Olive oil soap produces a creamy, lotion-like lather rather than fluffy bubbles. Extended curing (3+ months) reduces sliminess significantly. For bubbly lather, add coconut oil and castor oil to your recipe.

What's the difference between castile soap and regular soap?

Castile soap is made with olive oil as the primary fat (typically 70–100%). Regular commercial soap is usually made with tallow (animal fat), palm oil, or synthetic detergents. Castile soap is milder and more moisturizing but produces less lather and a softer bar than soaps with harder fats.

Can I add fragrance to olive oil soap?

Yes. Add fragrance oil at 3% of total oil weight or essential oil at 2% of total oil weight. Add it at light trace, just before pouring into the mold. Some fragrances accelerate trace in olive oil soap, floral and spice scents are common accelerators. Test with small batches first.

Conclusion

Olive oil soap is the best starting point for new soap makers: one oil, lye, water, and patience. Start with the basic castile recipe to learn the fundamentals, then move to the blended recipe for bars that lather better and cure faster.

The key to great olive oil soap is extended curing. Even a 100% olive oil bar becomes a completely different product after 6 months, hard, smooth, and luxuriously creamy.

PotionHub tracks your recipes, batch notes, and cure timelines in one place, so you can compare results across batches and refine your formula over time.