A soap and lye calculator tells you exactly how much sodium hydroxide (NaOH) or potassium hydroxide (KOH) and water you need for any cold process soap recipe. Lye is a caustic alkali that must be measured precisely — too little leaves unsaponified oil, too much makes harsh soap. Enter your oils and weights below to get an accurate lye calculation instantly.
Recipe Settings
Oils & Fats
Recipe Breakdown
| Oil / Fat | Weight | % of Recipe | SAP Value | Lye Needed |
|---|---|---|---|---|
| Total | 100% | — |
Safety Reminder
Always add lye to water — never water to lye. The mixture will heat up rapidly. Wear gloves and eye protection. Work in a well-ventilated area. Keep vinegar nearby to neutralize spills. Do not use aluminium containers.
How to Use the Soap & Lye Calculator
Cold process soap making requires precise chemistry. Every oil you use has a unique saponification (SAP) value — the amount of lye needed to convert exactly one gram of that fat into soap. This soap and lye calculator uses established SAP values to compute the exact NaOH or KOH quantity for your recipe, accounting for your chosen superfat level and water ratio. Accurate lye measurement is not optional: too much lye produces caustic soap that irritates skin, while too little leaves greasy, soft bars that never fully cure.
Step 1: Choose Your Lye Type
Select NaOH (sodium hydroxide) for solid bar soap — this is the most common choice for cold process soap makers. Choose KOH (potassium hydroxide) if you are making liquid or soft soap. Because commercial KOH is often sold at 90% purity rather than 100%, the calculator lets you enter the actual purity percentage so the lye amount is adjusted accordingly.
Step 2: Set Your Superfat Percentage
The superfat (or lye discount) is the percentage of oils that deliberately remain unsaponified in the finished soap, providing extra moisturizing benefits. The default of 5% superfat is a reliable starting point that produces a mild, well-lathering bar. Increase it to 8–10% for a gentler, more conditioning soap. Decrease it to 2–3% for a harder bar with stronger cleansing power. The calculator reduces the lye amount by exactly the superfat percentage you choose.
Step 3: Configure Your Water Amount
Choose between two water calculation methods. The Water:Lye Ratio method multiplies your lye amount by the ratio to get the water quantity — a 2:1 ratio (the default) gives a lye concentration of roughly 33%, which is safe and manageable for beginners. The Water as % of Oils method calculates water as a fixed percentage of total oil weight — typically 33–40% for cold process bars. Both methods produce professional results; pick whichever matches your recipe source.
Step 4: Add Your Oils
Click Add Oil to insert an oil row. Select the oil from the dropdown and enter the weight in grams or ounces. Each oil in the list has a pre-loaded SAP value. Build your recipe by adding as many oils as you need — a typical cold process recipe uses 2–5 different oils. Common choices include olive oil for conditioning, coconut oil for hard bars and fluffy lather, castor oil to boost bubble retention, and shea butter for a creamy, silky feel.
Step 5: Read Your Results
Click Calculate to see your complete soap recipe. The results show total oil weight, lye needed (already adjusted for superfat), water amount, the weight of unsaponified oils (superfat), and the total batch weight. The recipe breakdown table shows each oil's share of the formula as a percentage, its individual SAP value, and how much lye it contributes. Use these numbers to weigh your ingredients with a digital kitchen scale accurate to at least 1 gram (0.1 oz).
Lye Safety Essentials
Lye is highly caustic and must be handled carefully. Always wear nitrile gloves and safety glasses when working with lye. Always add lye to water — never pour water onto dry lye, as this can cause a violent spattering reaction. Mix in a stainless steel, high-density polyethylene, or heat-resistant glass container. Never use aluminium, as lye reacts violently with it. The lye-water solution will heat up to 80–90°C (180–200°F) immediately — set it aside to cool to 40–50°C before combining with your melted oils.
FAQ
Is this soap calculator free to use?
Yes, this soap and lye calculator is completely free with no signup, no account, and no usage limits. All calculations happen instantly in your browser — your recipe data is never sent to any server and stays private on your device.
Is my soap recipe data private?
Absolutely. Every calculation runs locally in your browser using JavaScript. Your oil amounts, recipe percentages, and lye figures are never transmitted to or stored on any server. Your formulations remain entirely on your device.
What is superfat and what percentage should I use?
Superfat (also called lye discount) is the percentage of oils left unsaponified in the finished soap, providing extra skin conditioning. A 5% superfat is the most common choice for a good balance of lather and mildness. Use 3-4% for a harder, longer-lasting bar, and 8-10% for a gentler, more moisturizing soap.
What is the difference between NaOH and KOH for soap making?
NaOH (sodium hydroxide) produces solid bar soaps through cold process saponification. KOH (potassium hydroxide) produces soft or liquid soaps because the resulting potassium salts are more water-soluble. KOH is typically sold at 90% purity, so this calculator automatically adjusts the amount needed based on the purity percentage you enter.
What water:lye ratio should I use?
A 2:1 water-to-lye ratio (33% lye concentration) is the most common starting point for cold process soap. Lower ratios (e.g. 1.5:1) produce a stiffer batter that unmolds faster but can cause lye-heavy issues. Higher ratios (2.5:1) give more working time but increase DOS (dreaded orange spots) risk. Beginners should start at 2:1.
How do I use the SAP values in this calculator?
SAP (saponification) values represent how much lye is needed to saponify one gram of a specific oil. Each oil has a unique SAP value — coconut oil requires more lye than olive oil, for example. This calculator uses established SAP values for each oil automatically. You only need to enter the weight of each oil you want to use.
Can I mix different oils in one recipe?
Yes, that is exactly what this calculator is designed for. Click 'Add Oil' to build a multi-oil recipe. Each oil contributes different properties: coconut oil for lather and hardness, olive oil for conditioning, castor oil to boost bubbles, shea butter for creamy lather. The calculator sums all the oil SAP values and gives you one total lye figure for the entire blend.
Why does the lye amount change when I adjust the superfat?
Increasing the superfat percentage reduces the lye amount by an equivalent fraction, leaving more unsaponified oil in the final bar. For example, a 5% superfat means you use 5% less lye than would be needed to fully saponify all the oils. This intentional lye discount is what gives cold process soaps their moisturizing quality.