Everything you need to know about using SoapCalc.pro — from first batch to professional production.
The Basics
New to lye calculators or soap making? Start here.
A soap lye calculator computes the exact amount of lye (sodium hydroxide NaOH for bar soap, or potassium hydroxide KOH for liquid soap) needed to fully saponify a given blend of oils and fats. You enter your oil weights, choose a superfat percentage, and the calculator outputs precise lye and water amounts using the SAP (saponification) value of each oil. Without accurate lye amounts, soap can be either caustic (too much lye) or greasy and soft (too little).
NaOH (sodium hydroxide) produces solid bar soap. KOH (potassium hydroxide) produces soft, paste, or liquid soap. The two lyes have different saponification values and cannot be swapped without recalculating. Dual-lye recipes blend both — typically to achieve a semi-solid texture like a traditional shaving soap. SoapCalc.pro supports all three modes and automatically applies the correct saponification math for each lye type.
Superfat — also called lye discount — is the percentage of oils deliberately left unsaponified in the finished soap. A 5% superfat means 5% of your oils remain as free fatty acids, making the soap more moisturising and providing a safety buffer against natural variation in SAP values. Most solid bar soap recipes use 3–8% superfat. Higher values (8–10%+) create a richer, more conditioning bar but can reduce lather performance and shorten shelf life.
The saponification (SAP) value is the amount of lye required to fully convert one gram of a specific oil into soap. Every oil has a different SAP value because each has a different fatty acid composition and average molecular weight. SAP values in calculators like ours are average figures compiled from published literature and laboratory data — actual values vary slightly by crop, season, and supplier, which is why a superfat is always recommended.
These are scores derived from the fatty acid profile of your oil blend that predict how the finished soap will behave:
Hardness — how firm the bar will be. Target: 29–54. Cleansing — how well it removes dirt and oil. Target: 12–22. Conditioning — how moisturising it feels on skin. Target: 44–69. Bubbly lather — large, fluffy bubbles. Target: 14–46. Creamy lather — stable, dense lather. Target: 16–48. Iodine — indicates softness and shelf life risk. Target: 41–70. INS — overall balance score. Target: 136–170.
These are guidelines, not rules. Many beloved soaps fall outside the typical ranges.
The amount of water you dissolve your lye in, expressed in three ways — all of which SoapCalc.pro supports:
Water:lye ratio — parts of water per part of lye by weight (e.g. 2:1 means twice as much water as lye). Water as % of oil weight — water expressed as a percentage of your total oil weight (e.g. 38%). Lye concentration — percentage of lye in the lye solution by weight (e.g. 33% means one third is lye, two thirds is water).
Different soap making traditions use different conventions. The calculator lets you work in whichever you prefer.
Using the Calculator
How to get the most out of the calculator features.
No. All core features — lye calculation, fatty acid breakdown, soap quality property chart, mold volume calculator, fragrance dosage, and fluid substitution — are completely free and require no account or sign-up. You only need an account to save recipes or access Pro features.
Type the name of any oil into the oil search field and select it from the dropdown. Enter the weight or percentage for that oil. Add as many oils as your recipe requires — the calculator updates results in real time after each change. You can switch between grams and ounces at any point using the unit toggle.
The database includes 100+ oils and fats: olive oil (all grades), coconut oil (76° and 92°), palm oil, shea butter, castor oil, avocado oil, sweet almond oil, babassu oil, lard, tallow, cocoa butter, mango butter, sunflower oil, canola, rice bran, hemp seed, argan, neem, jojoba, meadowfoam, sea buckthorn, and many more. Pro users can also add custom oils with their own SAP values and fatty acid profiles.
Enter your mold dimensions (length × width × height for rectangular molds, or diameter × height for cylindrical molds) and your desired fill percentage. The calculator returns the total oil weight needed to fill that mold, which you can then use as your batch size. You can also specify multiple molds for a single batch.
Fluid substitution lets you replace part or all of the water in your lye solution with another liquid — milk, tea, aloe vera, beer, coffee, or any other water-equivalent fluid. The calculator adjusts water and fluid amounts so your total liquid weight stays correct. This is useful for making milk soaps or botanical soaps where the liquid contributes to the recipe's character.
Our calculations use well-established SAP values from published soap making references and are accurate to the precision of those averages. Results are computed server-side — the full oil database and calculation logic never leave our server — which means there is no risk of client-side rounding errors. That said, SAP values are averages; actual values vary by batch and supplier. Always use an appropriate superfat (3–8%) as a safety margin, and weigh your ingredients accurately with a digital scale.
Yes. SoapCalc.pro is fully responsive and works on any modern mobile browser. You can also install it as a Progressive Web App (PWA) on Android or iOS — tap "Add to Home Screen" in your browser menu for an app-like experience with offline support for the interface.
Pro Features
What Pro unlocks and how each feature works.
A Pro pass unlocks the following features for the duration of your pass:
Batch history — save unlimited recipes to your account and retrieve them any time. PDF SOP export — download a print-ready Standard Operating Procedure for any saved recipe. Custom oil database — add your own oils with full SAP values, fatty acid profiles, and soap property scores. Custom SAP override — set a specific SAP value per oil to match your supplier's certificate of analysis. AI-Powered Recipe Optimizer — set target property scores and let the optimizer find the oil blend that best hits them. Batch comparison — compare two or more saved recipes side by side with a full property score table. Cost tracking — enter a price per kg for each oil and see total batch cost broken down by ingredient.
You set your target soap quality property values (hardness, cleansing, conditioning, lather, etc.), select the oils you want to work with, and optionally enter a cost per kg for each oil. The optimizer finds the blend percentages that bring all your property scores as close as possible to your targets simultaneously. If you have entered oil costs, it factors those in to find the best-performing blend at the lowest cost.
The SOP (Standard Operating Procedure) export generates a print-ready PDF for any saved recipe. It includes all ingredient weights, lye and water amounts, fatty acid analysis, soap property scores, and a step-by-step production notes section. It is designed to be handed to a production assistant or filed as documentation for a small soap business.
Pro users can add oils not in the built-in database — unusual botanical oils, locally sourced fats, blended butters, or any ingredient with a known SAP value. You enter the NaOH SAP value, KOH SAP value, and fatty acid profile (lauric, myristic, palmitic, stearic, oleic, linoleic, linolenic percentages). The custom oil then appears in the oil search alongside built-in oils and is stored privately in your account.
No. Your saved recipes and custom oils remain in your account after your Pro pass expires. You will not be able to save new recipes or access Pro features until you renew, but everything you saved while Pro is still there and visible. Renewing a pass immediately restores full access.
Yes. Every new registered account automatically receives 7 days of Pro access at no charge — no payment details required. The trial starts when your account is created and gives you full access to all Pro features so you can evaluate them before purchasing a pass.
Pricing & Orders
How passes are priced, purchased, and activated.
Yes. Core lye calculations, fatty acid breakdowns, mold volume, fragrance dosage, and soap quality property charts are completely free with no account required. Pro features — batch history, PDF SOP export, custom SAP values, AI-Powered Recipe Optimizer, and batch comparison — require a Pro pass starting from $4.99 USD for 30 days. Every new account also gets 7 free Pro days automatically.
All passes include the same full set of Pro features. Longer passes simply offer a lower effective daily rate.
Click any pricing plan on the pricing section to open a WhatsApp message with your order details pre-filled. After you send the message and confirm payment, Pro access is activated manually — usually within a few hours. You can also email Email or call WhatsApp if you prefer.
Pro access is activated manually after payment is confirmed, typically within a few hours. If your access has not been activated within 24 hours of confirmed payment, contact us at Email or WhatsApp and we will resolve it promptly.
Refunds are available within 48 hours of activation if you have not used any Pro features. After 48 hours or after Pro features have been used, passes are non-refundable. See our Refund Policy for full details. To request a refund, contact Email.
No. Passes are one-time purchases for a fixed period. There is no automatic renewal and no recurring charge. When your pass expires you simply purchase another if you want to continue with Pro access. Your saved data is never deleted when a pass expires.
Safety
Lye is a hazardous chemical. Please read this section carefully.
Yes. Lye (both NaOH and KOH) is a caustic alkali that causes severe chemical burns on contact with skin or eyes. It also produces significant heat when dissolved in water. Always treat lye with respect:
— Wear chemical-resistant gloves, safety goggles, and long sleeves.
— Always add lye to water — never water to lye — to control the exothermic reaction.
— Work in a well-ventilated area; the dissolving process releases fumes.
— Keep children and pets out of the workspace.
— Have running water accessible in case of skin or eye contact.
— Store lye in a sealed container away from moisture and children.
When made correctly with an appropriate superfat, cold process and hot process soap is safe to use. The saponification reaction fully converts lye into soap salts — there is no lye left in the finished bar. However, our calculator uses average SAP values, and actual values vary by oil source. Always use a 3–8% superfat to provide a safety margin. If you are unsure about a new recipe, test with a small batch first and cure for the full recommended period (4–6 weeks for cold process).
Too much lye (lye-heavy): the soap will be caustic and will zap the tongue (the "zap test"). Do not use it. A small excess can sometimes be addressed by rebatching with additional oils, but significant lye excess means the batch should be safely disposed of.
Too little lye (oil-heavy): the soap will be soft, greasy, and may not fully harden. It is safe to use but may have a shortened shelf life. This is the more common outcome of a superfat that is too high.
Always double-check your measurements before mixing. Weigh your lye and oils on a digital scale accurate to at least 1 gram.
Lye absorbs moisture from the air over time, which dilutes its effective concentration. A bag of NaOH labelled 100% may in practice be 97–99% pure if it has been stored for some time or improperly sealed. SoapCalc.pro allows you to enter a custom lye purity percentage so the calculator compensates by increasing the lye weight accordingly. Consumer-grade NaOH is typically 97–99% pure; KOH is commonly 90–92%. Check your supplier's specification sheet or SDS for the exact figure.
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