How to Make Liquid Soap with Potassium Hydroxide (KOH): Recipe, Lye Math, Clarity Test and Dilution

Bar soap and liquid soap can start from the same bottle of oil. What decides whether you end up with a hard bar or a pourable soap is the alkali. Sodium hydroxide makes sodium soaps, which set hard. Potassium hydroxide makes potassium soaps, which stay soft and dissolve in water. Every liquid soap recipe that starts from oils, from a pump-bottle hand soap to a traditional castile, runs on KOH.

This guide gives a full recipe, then the parts most tutorials skip: how to do the lye math by hand, what purity to enter in a soap calculator and why, how to use a ready-made liquid KOH solution instead of flakes, the clarity test that tells you the paste is finished, how much water to dilute it with, and why homemade liquid soap turns cloudy. Where the numbers depend on your oils or your lot of KOH, we show the arithmetic so you can check it.

We sell potassium hydroxide as Technical Grade flakes and ACS Grade flakes, and as liquid solutions at 25%, 30% and 45%. The buying section near the end covers which one suits a soap batch.

Last reviewed 15 October 2026 against the sources cited below. This is a practical chemistry guide, not medical, legal or regulatory advice. How we researched it is set out at the end.

How do you make liquid soap with potassium hydroxide?

You make liquid soap by dissolving potassium hydroxide in water, blending that lye solution into warm oils, cooking the mixture into a translucent paste, and then diluting the paste with water. The cook takes a few hours; the dilution takes a day or two; and the result is a concentrated true soap you can thin to whatever consistency you like.

The short version of the method:

1. Weigh everything. Use a digital scale in grams. Soapmaking is done by weight, never by volume.
2. Make the lye solution. Add the KOH flakes slowly to the water, never water to the lye, in a stainless steel or heavy polypropylene container. It gets very hot.
3. Warm the oils in a slow cooker or a stainless pot.
4. Combine and blend with a stick blender until the mixture thickens to trace.
5. Cook on low, stirring now and then, until the paste turns translucent, usually two to four hours.
6. Run the clarity test. A spoonful of paste should dissolve clear in hot water.
7. Dilute the paste with hot deionized or distilled water, cover, and leave it to dissolve fully.

The sections below explain each step and what to do when one goes wrong.

Why does liquid soap use potassium hydroxide instead of sodium hydroxide?

Liquid soap uses potassium hydroxide because potassium soaps are far more soluble in water than sodium soaps, so they stay liquid or soft instead of setting into a hard bar. The fatty acids are the same either way; only the metal ion attached to them changes.

When a fat or oil meets a strong alkali it undergoes saponification. Each fat molecule (a triglyceride) splits into three soap molecules and one molecule of glycerin. With sodium hydroxide those soap molecules are sodium salts of fatty acids, which pack tightly together and crystallize into a firm bar. With potassium hydroxide they are potassium salts. The larger potassium ion keeps them from packing as tightly, so they dissolve in water and form a soft paste or a liquid.

That is why you cannot make true liquid soap from sodium hydroxide alone, and why grating a bar of soap into water gives a cloudy gel rather than a clear liquid soap. Some soapmakers use a little NaOH alongside KOH for soft “cream soap,” but a pourable liquid soap is a potassium soap. Our sodium hydroxide soap guide covers the bar side of the chemistry, and the complete potassium hydroxide guide covers how the two alkalis differ beyond soap.

The history runs the same way. Before industrial caustic soda, soapmakers leached wood ash to get potash, a potassium alkali, and the soap they made with it was a soft soap. Hard bars came with sodium. Our article on potash and America’s first patent tells that part.

How much potassium hydroxide do you need for liquid soap?

The amount of potassium hydroxide equals the weight of each oil multiplied by that oil’s KOH saponification (SAP) value, added up, reduced by your superfat, and then divided by the purity of your KOH. A lye calculator does this for you, but doing it once by hand shows you where every gram comes from.

A SAP value is the weight of alkali needed to turn one unit of a particular oil completely into soap. Each oil has its own because each has a different mix of fatty acids. Published tables list a NaOH value and a KOH value for each oil. Use the KOH column. If a table gives only NaOH values, multiply by 1.403, the ratio of the molar mass of KOH (56.11) to that of NaOH (40.00).

SAP values vary slightly between published tables and between batches of the same oil. These three reproduce the published Soapmaking Friend castile liquid wash recipe to the hundredth of a gram.

Then take three more steps:

1. Apply the superfat. A superfat leaves a small share of the oil unsaponified so there is no leftover lye. Liquid soap is usually made with a low superfat, 0 to 3%, because excess oil clouds the finished soap. At 3%: 107.4 × 0.97 = 104.2 g of pure KOH.
2. Correct for purity. Divide by the purity of your KOH. At 90%: 104.2 ÷ 0.90 = 115.8 g of flakes.
3. Set the water. Many liquid soapmakers use about three parts water to one part KOH by weight: 115.8 × 3 ≈ 347 g of water.

The published recipe we checked against, with the same 500 g of oils at a 5% superfat, calls for 113.37 g of KOH at 90% purity and 340.1 g of liquid. Our figures differ only because of the lower superfat.

The take: liquid soap lye math is oil weight × KOH SAP value, minus the superfat, divided by purity. For 500 g of 40/40/20 coconut, olive and castor oil at a 3% superfat that is 104.2 g of pure KOH: 115.8 g of 90% flakes, or 416.7 g of a 25% KOH solution with no extra water.

What KOH purity should you use in a soap calculator?

Use 90% unless the certificate of analysis for your lot shows a different assay, in which case enter that figure. Solid potassium hydroxide is never pure KOH. It pulls water and carbon dioxide out of the air, so the solid always carries some water and a little potassium carbonate.

Most soap calculators offer two settings for KOH, 100% and 90%, and the soapmaking community settled on 90% as the practical default for commercial flakes. ACS reagent-grade specifications set a minimum assay of 85.0% KOH, with potassium carbonate capped at 2.0%. A lot can be anywhere from that floor upward, and the actual figure is printed on its certificate of analysis.

If your lot’s certificate shows an assay of, say, 87%, divide the pure-KOH figure by 0.87 instead of 0.90. For our recipe that is 104.2 ÷ 0.87 = 119.8 g of flakes rather than 115.8 g. Four grams sounds trivial, but it is the difference between a soap that tests clear and one that tests cloudy.

Keep the container sealed between batches. An open bag or jar of KOH keeps absorbing moisture and carbon dioxide, so its real purity drifts downward and the flakes clump.

Can you make liquid soap with liquid potassium hydroxide solution?

Yes. A potassium hydroxide solution sold at a stated concentration can replace both the flakes and the water you would dissolve them in, and it removes the hottest and most hazardous step of the process: dissolving solid KOH. You calculate how much solution delivers the pure KOH your recipe needs, then add any extra water.

The math uses the solution’s concentration in place of the purity figure. Confirm on the product’s technical data sheet or certificate of analysis that the percentage is by weight; for the examples below we assume it is.

A 25% solution is, in effect, a ready-mixed liquid soap lye at roughly the three-to-one water ratio many soapmakers use. The 45% solution is more concentrated, so you dilute it with water before it goes into the oils. Add the solution to the water, not the other way round; diluting concentrated caustic still releases heat.

The one trade-off: a solution is mostly water, so shipping it costs more per gram of KOH than shipping flakes. For a hobby batch that hardly matters. For a small soap business making many batches, flakes are usually the cheaper source of KOH, and a solution is the easier and safer one.

How do you cook liquid soap paste?

Liquid soap is almost always made by hot process: the oils and lye are blended to trace and then cooked on low heat until saponification is complete, which turns the mixture into a translucent paste. Cooking speeds up a reaction that would otherwise take weeks at room temperature.

1. Set up. Goggles and chemical-resistant gloves on; long sleeves; the room ventilated. Use stainless steel, heat-resistant glass or heavy polypropylene. Never aluminum: KOH attacks aluminum and releases hydrogen.
2. Dissolve the KOH. Put the water in the container first, then add the flakes slowly while stirring. The solution can reach near-boiling temperatures and gives off caustic mist; do not lean over it. If you are using a 45% or 30% solution, add it to the measured water instead.
3. Warm the oils. Melt the coconut oil, then add the olive and castor oil in a slow cooker on low or a stainless pot over low heat.
4. Combine. Pour the lye solution into the oils and stick-blend in short bursts. Liquid soap batters reach trace fast, then pass through stages that look like applesauce and then thick taffy. A stiff mass that is hard to stir is normal.
5. Cook. Cover and cook on low, folding the paste over every 20 to 30 minutes. As saponification finishes it turns from opaque to translucent, like amber petroleum jelly. Most batches take two to four hours; a high-olive or all-olive batch takes longer.
6. Test. Run the clarity test below before you add any dilution water.

Some recipes add alcohol to speed up the reaction. We leave it out: it adds a flammable vapor to a hot pot and is not needed for a good batch.

How do you know when liquid soap paste is done?

The paste is done when a small sample dissolves in hot water to a clear or nearly clear solution with no oily film or milky cloud. That is the clarity test, and it is the most useful check in liquid soapmaking.

1. Put about a teaspoon (5 g) of paste in a clear glass.
2. Add about two tablespoons (30 g) of near-boiling distilled or deionized water and stir until it dissolves.
3. Hold the glass against a light. Clear or slightly hazy and amber means the paste is fully saponified. Milky, opaque, or with droplets of oil on the surface means it is not.

If the test is cloudy, cook for another 30 to 60 minutes and test again. If it stays cloudy after a long cook, the likely causes are too little KOH for those oils (often a purity overestimate) or a high superfat; see the cloudy-soap section below.

Clarity does not prove the paste is free of excess lye: a lye-heavy paste can test clear. Check the pH of the diluted soap with pH strips or a meter. True soap is alkaline, typically in the 9 to 10 range. A reading well above that points to excess KOH. Some old guides suggest touching the paste to your tongue (“zap test”). We don’t recommend it: pH paper is cheap and gives you a number.

How do you dilute liquid soap paste?

Dilute liquid soap paste with hot deionized or distilled water, starting at about one part paste to one part water by weight and adding more water in small amounts until the soap is as thin as you want. Then cover it and leave it alone: thick paste can take overnight or longer to dissolve completely.

• Start with equal weights. Break the paste into pieces, pour hot water over it, cover, and wait. Stirring hard only makes foam.
• Adjust in steps. Add water a little at a time once the first portion has dissolved. A pump-bottle hand soap usually ends up between 1 : 1 and 1 : 2 paste to water by weight. Foaming dispensers need a much thinner soap.
• High-olive soaps take more water. A coconut-heavy paste and an olive-heavy paste of the same weight do not end up the same thickness.
• Let it settle. Many soapmakers let diluted soap sit for one to two weeks so any haze settles out; this is called sequestering. Pour off the clear soap.
• Undiluted paste keeps. Paste stored in a sealed container can be diluted in small portions as needed.

Why deionized or distilled water? Tap water carries calcium and magnesium. They react with soap to form insoluble calcium and magnesium soaps, the same scum that rings a bathtub, and in a bottle that shows up as cloudiness and sediment. Our distilled vs deionized water guide explains the difference between the two.

Why is my homemade liquid soap cloudy?

Homemade liquid soap is usually cloudy because some oil is still unsaponified, because the water had minerals in it, or because the recipe uses oils that form soaps which cloud at room temperature. Each cause has a different fix.

A soap that is cloudy because of unreacted oil can usually be rescued by cooking it again with a measured extra amount of KOH. A soap that is cloudy because of tap water cannot be fixed by cooking; only time (sequestering) and pouring off the clear layer help.

What is the glycerin method for liquid soap?

The glycerin method replaces some or all of the water in the lye solution with glycerin, which makes the batter come together faster and helps keep the finished soap clear. Glycerin is already a product of saponification, so adding more does not change what the soap is.

The traditional version, sometimes called the pharmacist’s method, heats KOH directly in glycerin until it dissolves. It is the riskiest way to make liquid soap. A widely cited soapmaking forum safety warning puts it plainly: “If the hot caustic mixture spills onto the skin, severe chemical and thermal burns will result in seconds.” The same warning notes that the mixture can foam up suddenly and that overheated glycerin breaks down into irritating fumes.

The safer variant most experienced soapmakers now use: dissolve the KOH in an equal weight of water at room temperature first, then add glycerin to make up the rest of the liquid. You keep most of the speed without heating caustic glycerin. If you try it, use USP Grade glycerin and treat the mixture exactly as you would any hot lye solution. Our glycerin guide covers the grades.

How do you neutralize excess lye in liquid soap?

If a pH test shows excess KOH, you can neutralize it with a measured amount of citric acid solution stirred into the diluted soap. The key word is measured: neutralizing too much converts some of the soap back into free fatty acids, which clouds it.

The arithmetic follows from the chemistry. Citric acid has three acidic hydrogens, so one molecule neutralizes three molecules of KOH:

• Citric acid monohydrate: about 1.25 g per gram of excess KOH (210.14 ÷ (3 × 56.11)).
• Citric acid anhydrous: about 1.14 g per gram of excess KOH (192.12 ÷ 168.33).

Dissolve the citric acid in a little hot water and add it a few drops at a time, checking pH as you go. The better fix is upstream: get the purity figure right and use a small superfat, and you rarely need to neutralize at all. Most soapmakers who run a 0 to 3% superfat with an accurate purity setting skip this step. We stock Citric Acid Anhydrous ACS Reagent Grade and USP citric acid monohydrate.

How do you handle potassium hydroxide safely?

Handle potassium hydroxide with splash goggles, chemical-resistant gloves and covered skin, add it to water rather than water to it, and keep it away from aluminum, acids and children. KOH is corrosive: it causes severe skin burns and serious eye damage, and the NIOSH Pocket Guide lists eyes, skin and the respiratory system as its target organs.

• Eyes first. Goggles, not glasses. Alkali eye splashes are among the most serious chemical injuries.
• No aluminum, zinc or tin. KOH attacks these metals and gives off hydrogen. Use stainless steel, glass or polypropylene and HDPE.
• Ventilate. Dissolving KOH throws off caustic mist. NIOSH sets a ceiling exposure limit of 2 mg/m³ for KOH in air.
• Water for skin and eyes. Flush skin or eye contact with plenty of running water and get medical help for any eye exposure. Do not pour acid on a skin burn.
• Label and store it sealed. Keep KOH in its original labeled, closed container, out of reach of children, away from acids, and never in a food or drink container.

Read the safety data sheet on the product page before your first batch.

Can you sell homemade liquid soap?

Yes, and in the United States a true soap sold only as soap is regulated by the Consumer Product Safety Commission rather than the FDA. The line is set by the FDA’s definition of soap in 21 CFR 701.20, and it is narrower than many new soapmakers expect.

The FDA says: “To be regulated as ‘soap,’ the product must be composed mainly of the ‘alkali salts of fatty acids,’” its cleaning action must come from those salts, and it must be labeled and marketed only as soap. “If your product meets FDA’s regulatory definition of soap, it’s regulated by the Consumer Product Safety Commission (CPSC), not by FDA.” A KOH liquid soap made from oils fits the first two conditions. Add synthetic detergents, or make cosmetic or drug claims for it, and it falls under FDA rules as a cosmetic or a drug instead.

The CPSC, for its part, notes: “If the product meets the definition of a hazardous substance under the FHSA, then it must bear precautionary labeling.” A properly made, diluted liquid soap usually does not, but a lye-heavy one might. If you plan to sell, read both agencies’ guidance in full; this section is a summary, not legal advice.

Buying potassium hydroxide for soap making

For a soap batch, the choice is between flakes and a ready-made solution, and between Technical Grade and ACS Grade. Technical Grade flakes are the economical default. ACS Grade is made to a published reagent specification, including the minimum assay discussed above. A solution saves you the step of dissolving solid KOH.

Our recipe uses about 116 g of flakes, so a 2 lb (907 g) pack covers seven or so batches of that size. Flakes come in 2 lb and 4 lb packs and 55 lb bags; the solutions run from a quart to a 330-gallon tote. A safety data sheet is on every product page, and a certificate of analysis showing the assay of your lot is available on request. For buying at production scale, see our potassium hydroxide sourcing guide.

Sources

1. Frequently Asked Questions on Soap, U.S. Food and Drug Administration
2. 21 CFR 701.20, Detergent substances, other than soap, intended for use in cleansing the body, Electronic Code of Federal Regulations
3. Soap Business Guidance, U.S. Consumer Product Safety Commission
4. NIOSH Pocket Guide to Chemical Hazards: Potassium hydroxide, CDC / NIOSH
5. Potassium Hydroxide (CID 14797), PubChem, National Library of Medicine
6. Potassium hydroxide pellets, ACS reagent (PX1480) specification, MilliporeSigma
7. Classic Castile Liquid Wash recipe, Soapmaking Friend
8. A safety caution about heating KOH and glycerin, Soapmaking Forum (2016)
9. KOH purity, Soapmaking Forum