1. Why KOH Makes Liquid & Soft Soap 💡

The single most important fact in soap making is this: the alkali you choose decides whether your soap is hard or soft. Potassium hydroxide (caustic potash) produces potassium soaps, which are soft, water-soluble and free-flowing - the basis of every genuine liquid soap, soft "green" soap, shaving cream and many shampoos.

Sodium hydroxide (caustic soda), by contrast, makes sodium soaps that set into firm bars. So if you want a liquid or soft product, KOH is not optional - it is the defining ingredient. For the wider comparison, see our KOH vs NaOH guide, and for the chemical itself, our pillar on what potassium hydroxide is.

2. The Saponification Reaction ⚗️

Saponification is the reaction in which a strong base splits the triglycerides in fats and oils into soap (a fatty-acid salt) and glycerol:

With potassium hydroxide, the fatty acids end up paired with potassium ions. Because potassium soaps are far more soluble in water than sodium soaps, the result stays soft or liquid instead of hardening. The glycerol produced alongside the soap is a natural humectant that gives potassium soaps their characteristically mild, moisturising feel.

3. SAP Values & the Lye Calculator 🔢

Every oil needs a specific amount of base to saponify completely. That amount is given by its saponification (SAP) value - and crucially, KOH SAP values are different from NaOH values.

• 🔹 Because KOH is heavier per mole (56.11 vs 40.00 g/mol), every recipe needs about 1.4× more KOH than the equivalent NaOH amount.
• 🔹 SAP tables and online lye calculators let you enter each oil by weight and automatically return the KOH needed. Always select "KOH" (not "NaOH") in the calculator.
• 🔹 SAP values are measured in the lab using ethanolic KOH - the same test described in our alcoholic KOH guide.

4. KOH Purity & the 90% Adjustment 🧱

Most soap makers use solid KOH flakes, which are typically about 90% pure (the rest is water and a little carbonate). Lye calculators assume 100% pure base, so you must correct for this:

• 🔹 Divide the calculator's KOH figure by your actual purity. For 90% KOH: actual KOH = calculated ÷ 0.90.
• 🔹 Many calculators have a "KOH purity" field - set it to 90% (or your COA value) and they do this automatically.

Skipping this step leaves your soap under-saponified and oily. Knowing your exact assay matters, which is why a reliable COA is important - see our KOH grades & purity guide and our concentration & purity explainer.

5. Superfatting & Lye Excess 🌿

Liquid soap making handles excess oil and base differently from bar soap:

• 🔹 Superfat means leaving a small percentage of oils unsaponified for mildness. Liquid soaps usually use a low superfat (around 0–3%) because too much excess oil makes the soap cloudy.
• 🔹 Lye excess is the opposite approach some liquid-soap methods use - a slight excess of KOH to ensure a clear soap, later neutralised with a mild acid such as citric or boric.
• 🔹 A clarity/zap test confirms the soap paste is fully cooked and not lye-heavy before dilution.

6. How to Make Liquid Soap, Step by Step 🧴

1. Calculate the KOH using a lye calculator set to KOH, then adjust for ~90% purity.
2. Mix the lye: add KOH to water (never the reverse) - it heats up strongly and may fizz, which is normal for KOH. See our solubility & exothermic guide.
3. Combine the lye with warmed oils and blend to a thick paste.
4. Cook the paste (e.g. in a slow cooker) until it turns translucent and passes a clarity test.
5. Dilute the paste with hot water to your desired thickness - this dilution stage is unique to liquid soap.
6. Neutralise & finish: adjust pH if needed, then add fragrance, colour and preservative once cool.

7. Is Potassium Hydroxide Safe in Soap? 🛡️

This is the question every new soap maker asks, and the answer is reassuring: there is no KOH left in finished, properly made soap. During saponification the potassium hydroxide is entirely consumed - converted into soap and glycerol. A correctly formulated and fully cooked soap contains no free caustic potash.

The risk is in the process, not the finished product. Treat the lye stage with respect and KOH soap making is entirely safe.

8. Troubleshooting Common Problems 🔧

9. Frequently Asked Questions ❓

🔹 Why use potassium hydroxide instead of sodium hydroxide for soap?

KOH makes soft, water-soluble potassium soaps - the basis of liquid soap, soft soap and shaving cream. NaOH makes hard bars.

🔹 Is there potassium hydroxide left in finished soap?

No. In a properly made, fully saponified soap the KOH is entirely converted to soap and glycerol, leaving no free caustic potash.

🔹 Can I make a bar of soap with KOH?

Pure KOH soap stays soft, so it doesn't form firm bars well. Some makers blend KOH with NaOH (a "dual lye" method) to fine-tune hardness.

🔹 How much more KOH do I need than NaOH?

About 1.4× more by weight, because KOH has a higher molar mass. Always use a lye calculator set to KOH.

🔹 What KOH purity should I use in calculations?

Most flakes are ~90% pure. Divide the calculated KOH amount by 0.90 (or set the calculator's purity field to 90%).

🔹 What is potassium soap base?

It's the soft soap paste made by saponifying oils with KOH, which is then diluted to make liquid soap or used as a base for other products.

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