If you have been making soap for any length of time, you already know that the lye calculator is the most important tool in your workflow. Get it right and you have a safe, balanced bar. Get it wrong and you have a caustic disaster that strips skin, crumbles in weeks, or goes rancid before you can sell a single unit.
There are dozens of lye calculators on the internet. Some have been running since the early 2000s. Others launched recently with polished interfaces and ambitious feature lists. Most of them will get you close enough on a basic recipe. But "close enough" covers a wide range, and the differences between these tools become glaring the moment you move beyond a simple three-oil bar soap recipe.
In this post we are going to do something that no lye calculator website usually does: give you a genuinely honest comparison of the tools that exist, including the ones that compete directly with ours. We will walk through what each calculator does well, where it falls short, who it is designed for, and where it is likely to frustrate you. Then we will explain exactly what we built with the SoapCalc.pro Advanced Lye Calculator, why we built it that way, and where it still has room to improve.
Before comparing tools, it is worth making sure we are all working with the same vocabulary. If you have been making soap for years, feel free to skip ahead.
The saponification value — how much lye is required to fully convert one gram of a specific oil into soap. Swap oils without recalculating and your soap will be either caustic or greasy.
Sodium hydroxide (NaOH) makes solid bar soap. Potassium hydroxide (KOH) is used for liquid, cream, and shaving soaps. They have different SAP values — they cannot be interchanged.
Deliberately using less lye than required to fully saponify all oils. A 5% superfat leaves free oil in the finished bar, improving skin feel and providing a safety buffer against SAP variation.
The amount of water you dissolve your lye in, expressible as a water-to-lye ratio, percentage of oil weight, or lye concentration percentage. Different traditions use different conventions.
Hardness, Cleansing, Conditioning, Bubbly lather, Creamy lather, Iodine, and INS — calculated from your oil blend's fatty acid profile to predict finished bar performance.
Oils are composed of fatty acids. Lauric and myristic give fluffy lather; oleic acid conditions; linoleic softens and conditions but shortens shelf life. Each oil's profile determines its contribution to a recipe.
SoapCalc launched in 2001 and effectively defined what a lye calculator is. The soap property display — hardness, cleansing, conditioning, bubbly, creamy, iodine, INS — that every other tool now replicates was pioneered here. For that alone, SoapCalc deserves enormous credit. It recently received a rebuild and is now mobile-friendly, addressing a long-standing frustration.
The honest limitation is reliability. There were outages in July 2025 and February 2026, during which the site was down with no communication to users. Many soapmakers switched to other calculators during that period. The team has since added a status page, but if you are running a production business, an unreliable calculator is a serious problem.
Feature-wise, SoapCalc does the core calculation well. What it does not do: save recipes, show a visual property graph, offer a fragrance calculator, support mold sizing, allow lye purity input, or support custom oils. It is a clean, fast, single-purpose tool.
SoapmakingFriend is one of the most feature-complete calculators available and has built a genuine community around it. It offers a smart recipe builder with real-time calculations, mold calculator, fragrance and additive tracking, and recipe saving. The community dimension is real — you can browse public recipes shared by other soapers, copy and customise them, and get support from a forum.
The friction point is the paywall. The free plan allows saving only two recipes — which SMF itself effectively acknowledges is just for testing. Custom oils, inventory tracking, and full batch cost analysis all require the paid tier. Some users have reported app bugs (premium subscribers seeing upgrade prompts incorrectly, mold calculator errors), which the team has been responsive about.
The Bramble Berry calculator assumes 90% purity for KOH and 97% for NaOH — the purities commonly available to consumers. That tells you exactly who this calculator is designed for: someone buying supplies from Bramble Berry and making their first few batches. It calculates lye and water correctly. It does not show soap property scores, fatty acid profiles, a visual chart, or any advanced features. There is no saving, no custom oils, no mold calculator.
This is not a criticism — it is a deliberate design choice for their audience. Once you start adjusting recipes yourself, you will quickly outgrow it.
FormulaAlchemy is part of a broader professional formulation platform — alongside an SPF calculator, HLB calculator, INCI checker, and formulation manager — which gives it a different character from the soap-only tools. The lye calculator itself is genuinely feature-rich for a free tool: it supports NaOH, KOH, and dual-lye blending (with an adjustable NaOH/KOH split slider), adjustable lye purity for both lye types, all three water input methods (ratio, percentage of oils, lye concentration), fatty acid breakdown with visual range bands, soap quality property scores, a mold volume calculator, fragrance and colorant dosage, fluid substitution, a batch scaling tool, and a surfactant/additive section. The oil database covers 100+ oils and no account is needed for any of these core features.
The Pro tier — unlocked with a 7-day free trial on sign-up — adds batch history, PDF export, AI blend recommendations, and custom SAP values. The free tier is unusually complete by the standards of the category. The tool is mobile-first and runs without an app download. As a newer platform, it has less of the community history and long-standing trust of SoapmakingFriend, and the broader multi-tool focus means soap-specific depth in areas like masterbatching and community recipe sharing is not the priority.
| Feature | SoapCalc | SMF | Bramble Berry | FormulaAlchemy | SoapCalc.pro |
|---|---|---|---|---|---|
| NaOH / KOH support | ✓ | ✓ | ✓ | ✓ | ✓ |
| Dual lye (NaOH + KOH blend) | ✓ | ✓ | ✗ | ✓ | ✓ |
| Adjustable lye purity | ✗ | ✓ | ✗ | ✓ | ✓ |
| Water as ratio / % / concentration | ✓ | ✓ | ✗ | ✓ | ✓ |
| Soap property scores | ✓ | ✓ | ✗ | ✓ | ✓ |
| Fatty acid breakdown | ✓ | ✓ | ✗ | ✓ | ✓ |
| Visual property graph with ranges | ✗ | ✓ | ✗ | ✓ | ✓ |
| Custom SAP override per oil | ✗ | 💲 | ✗ | Pro | Pro |
| AI-Powered Recipe Optimizer | ✗ | ✗ | ✗ | Pro | Pro |
| Mold / batch size calculator | ✗ | ✓ | ✗ | ✓ | ✓ |
| Fragrance dosage calculator | ✗ | ✓ | ✗ | ✓ | ✓ |
| Fluid substitution calculator | ✗ | ✓ | ✗ | ✓ | ✓ |
| Save recipes / batch history | ✗ | 💲 | ✗ | Pro | Pro |
| Batch comparison side-by-side | ✗ | ✗ | ✗ | Pro | Pro |
| Download SOP / PDF export | ✗ | ✗ | ✗ | Pro | Pro |
| Cost tracking per oil | ✗ | 💲 | ✗ | Pro | Pro |
| Custom oils (user-defined) | ✗ | ✓ | ✗ | ✓ | ✓ |
| Mobile-friendly | ✓ | ✓ | ✓ | ✓ | ✓ |
| Server-side calculation | ✗ | ✗ | ✗ | ✗ | ✓ |
| Free tier available | ✓ | ✓ | ✓ | ✓ | ✓ |
💲 = paid or membership-only feature
We did not set out to build a calculator. We set out to stop being frustrated by the existing ones. SoapCalc.pro started as a formulation consultancy, and the tools that existed either lacked the features professionals need, locked those features behind expensive subscriptions, or required switching between multiple platforms to complete a single formulation workflow.
Here is what we built and why each decision was made.
Every other calculator runs in your browser, making the entire oil database visible to anyone with developer tools open. Ours sends inputs to our server via REST API; only results return to the browser. The database never leaves the server.
Most "dual-lye" tools require two separate oil batches. Ours lets you set the NaOH/KOH split percentage on a single oil blend — which is how cream soaps are actually formulated.
Water ratio, percentage of oil weight, and lye concentration. Different traditions use different conventions. We support all three so you are never doing mental conversions before you start.
A score of 42 on hardness means nothing without context. Our Chart.js graph overlays your recipe's scores on recommended ranges for each property — adjust oils and the chart updates in real time.
Set your target property values, select your available oils, and our optimisation algorithm finds blend percentages that bring your recipe as close as possible to those targets simultaneously — factoring in cost per kilogram if you need it.
Add oils with your own SAP values, soap property scores, and fatty acid profiles. These are stored privately in your account and appear in the search dropdown alongside built-in oils. The shared database is never touched.
Generate a PDF structured as a Standard Operating Procedure. Compare two or more saved batches side-by-side with a full property score table — essential when you are trying to understand why last month's recipe performed differently.
Enter a price per kilogram for each oil. See total ingredient cost broken down per oil. The AI-Powered Recipe Optimizer optimisation can factor in your prices to find the best-performing blend at the lowest cost.
We said we would be honest, and we meant it.
Every lye calculator on this list — including ours — works from average SAP values compiled from published references, laboratory measurements, and historical soap making literature. They are accurate enough for safe soap making when combined with an appropriate superfat, but they are not exact constants. The actual SAP value of any real batch of oil will vary depending on crop, harvest season, processing method, and supplier. This is why superfat exists. A 5% superfat does not just make your soap more conditioning — it provides a margin of error that absorbs the natural variability in your ingredients. Any calculator that implies its SAP values are exact is overstating its precision.
There is no single right answer, and anyone who tells you there is probably has a financial interest in which one you choose.
Use Bramble Berry for your first recipe following one of their tutorials, then move to FormulaAlchemy once you start adjusting things yourself. It is free, requires no account, and covers everything an intermediate maker needs.
The best free option available right now. If you want a community and do not mind paying, SoapmakingFriend is worth the subscription.
You need cost tracking, batch records, and documented processes. SMF's paid tier handles this reasonably well. Our Pro tier was built specifically for this use case.
Optimising blends, working with non-standard oils, tracking costs, running comparison batches, producing team documentation — the only tool with AI-Powered Recipe Optimizer, server-side calculation, and cost-aware optimisation.
Available free under Formulation Tools. No account required to calculate. Log in for custom oils. Upgrade to Pro for AI-Powered Recipe Optimizer, batch history, cost tracking, batch comparison, and SOP export.