Classic Castile Soap (100% Olive Oil)
By The Soap Brain Team — Cold-process soap makers & formulators
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About this recipe
A true castile bar is 100% olive oil, and it is the most forgiving recipe to learn saponification on: one oil, one clear lye figure, no blending to balance. What you trade for that simplicity is time. Olive oil is almost entirely oleic acid, which makes a soft bar that takes weeks to firm up and is best cured for a full six months — the reward is an exceptionally mild, low-lather, conditioning soap that suits sensitive skin.
Because there is nothing to hide behind, a castile is also the clearest way to see why weighing matters. Every gram of olive oil needs a precise, matching amount of lye; the calculator below works out that exact NaOH weight for your batch size and superfat. Expect a slow trace, a soft loaf for the first few days, and a translucent, glassy bar months later. Patience is the whole recipe.
The oils
Percentages for a 1,000 g batch of oils. Change the batch size in the calculator and every weight — and the lye — recomputes.
| Oil | Percentage | Weight |
|---|---|---|
| Olive oil | 100% | 1,000 g |
| Total oils | 100% | 1,000 g |
Method
- Suit up: goggles, gloves, long sleeves, good ventilation. Weigh everything on a scale — never by volume.
- Weigh the olive oil into your soaping pot and warm gently to about 40°C.
- Weigh the water, then weigh the lye (NaOH) using the figure from the calculator below.
- Add the lye TO the water (never water to lye), stir until clear, and let it cool to about 40°C.
- Pour the lye solution into the oil and blend to a light trace — a castile can take a while.
- Pour into the mould, cover, and leave 2–3 days before unmoulding.
- Cut, then cure on a rack for up to six months; the longer the cure, the harder and milder the bar.
Calculate the lye for this recipe
The calculator below is pre-loaded with this recipe. Resize it to your batch or mould and it works out the exact NaOH (lye) weight, water and quality numbers. Always weigh lye, oils and water — never measure by volume, wear gloves and eye protection, and add lye to water (never the reverse).
Loading the calculator…
Where these numbers come from
The lye weight and quality numbers here come from the same cited oil values you'll find on every oil page. Here's the sourcing for each oil in this recipe:
SAP data last updated · 51 oils covered.
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- SoapCalc oil list — SAP, iodine
- From Nature With Love — Saponification Chart — SAP range
Recipe FAQ
- Do I have to use the exact weights shown for Classic Castile Soap (100% Olive Oil)?
- No. This recipe is defined by percentages, so you can scale it to any batch size. Enter your total oil weight (or size it to your mould) in the calculator and it recomputes the exact lye, water and quality numbers for you.
- What superfat and water amount does this recipe use?
- It uses a 5% superfat with a 33% lye concentration. You can change either in the calculator below and watch the lye weight update instantly.
- Can I swap an oil or leave one out?
- You can, but every oil has its own saponification (SAP) value, so the lye weight changes with it. Never reuse this recipe's lye figure with a different oil blend — edit the oils in the calculator and let it recalculate the lye from scratch.