Cold Process Soap Lye (NaOH) & Water Sizer: Exact Superfat Ratio
Getting lye weight wrong in cold process soap either leaves a harsh, unreacted-lye bar or an overly greasy one, and because sodium hydroxide is a corrosive chemical, precision and safety go hand in hand. This calculator sums saponification values (SAP) across your oil blend and applies your chosen superfat discount.
SAFETY: Sodium hydroxide is caustic — always wear gloves and eye protection, add lye to water (never water to lye), and work in a ventilated area. For this oil blend, weigh exactly 141.41g of pure NaOH into 282.8g of distilled water at 5.0% superfat.
| Component | Amount |
|---|---|
| Total oil weight | 1,000 g |
| Full saponification lye (0% superfat) | 148.85 g |
| Superfat discount | 5.0% |
| NaOH required | 141.41 g |
| Water:lye ratio | 2.0:1 |
| Water required | 282.8 g |
| Total soap batch weight | 1,424 g |
Formula & step-by-step maths
- OilSAPValue
- Grams of NaOH needed per gram of that oil
- Superfat%
- Percentage of oils left unreacted (excess fat)
SAFETY FIRST: Handling sodium hydroxide
Always add lye to water, never water to lye, to avoid a violent exothermic reaction that can splash caustic liquid. Wear chemical-resistant gloves and eye protection, work in a well-ventilated space, and keep the mixture away from children and pets at every stage.
What superfat percentage does to your bar
Superfatting leaves a small percentage of oils unreacted with lye, making the bar gentler and more moisturizing. Most cold process recipes use 5-8% superfat; higher percentages (10%+) can shorten shelf life and cause premature rancidity ("dreaded orange spots").
Why each oil has a different SAP value
Saponification values reflect how much lye is needed to fully convert one gram of that specific oil into soap — coconut oil needs almost 50% more lye per gram than olive oil because of its different fatty acid profile, which is why blend composition changes the total lye figure substantially.
Reading your hardness and cleansing scores
Coconut and palm oils contribute hardness and cleansing power but can be drying in high proportions; olive, shea, and castor contribute conditioning and lather stability. A balanced recipe typically keeps hardness and cleansing scores each in the 15-40 range out of 100 for a mild, long-lasting bar.
Common oil SAP values (NaOH, g/g)
| Oil | SAP value (NaOH) | Contributes |
|---|---|---|
| Olive oil | 0.135 | Conditioning, mild lather |
| Coconut oil | 0.190 | Hardness, cleansing, big lather |
| Palm oil | 0.141 | Hardness, stable lather |
| Shea butter | 0.128 | Conditioning, creamy lather |
| Castor oil | 0.128 | Lather boost, bubbly |
| Sunflower oil | 0.134 | Conditioning |
People also ask
What happens if I don't superfat at all?
A 0% superfat bar uses exactly the stoichiometric lye needed to saponify every oil molecule, but any measurement imprecision risks leftover free lye, making 5%+ superfat a common safety buffer.
Can I use this calculator for hot process soap?
Yes, the lye and water math is identical; hot process simply cooks the saponification reaction to completion faster using external heat rather than curing over weeks.
Why does my recipe call for less water than this calculator shows?
Some experienced soapers use a more concentrated lye solution (a smaller water:lye ratio) to speed trace and reduce cure time — just ensure you use enough water to fully dissolve the lye without recrystallizing.
Is store-bought drain cleaner an acceptable NaOH substitute?
No — always use 100% pure sodium hydroxide labeled for soap-making; drain cleaners often contain additives, other chemicals, or aluminum that are unsafe and unpredictable in soap.
How long should cold process soap cure before use?
Typically 4-6 weeks in a cool, dry, well-ventilated space, which allows excess water to evaporate and produces a harder, longer-lasting, milder bar.
Can I substitute distilled water with milk or tea?
Yes, many soapers replace some or all of the water with milk, tea, or other liquids, but these require slower lye addition and freezing techniques to prevent scorching or curdling.
Why is my finished soap soft and never hardens?
This is often caused by too high a superfat, too much soft oil (like olive oil) relative to hard oils, or insufficient cure time — check your oil ratio and let it cure longer.
What does 'zap testing' mean and is it necessary?
Zap testing (briefly touching cured soap to your tongue to check for a lye tingle) is a traditional but imprecise method; a properly calculated and cured recipe using this calculator should never need it as your primary safety check.
Can I scale this recipe up or down?
Yes — enter your actual oil weights for the batch size you want; the calculator scales lye, water, and totals proportionally as long as your oil ratios stay the same.
Do fragrance oils or essential oils affect the lye calculation?
No, fragrance and essential oils are typically added at trace after saponification math is set and don't require lye adjustment, though they do add to total batch weight.
Three worked examples
Same engine, three different starting points — useful if you want to see how sensitive the answer is before you type your own numbers in.
Example 1: olive oil 234 g, lye concentration (water:lye) "2:1 water to lye (33%)"
On the lower / more conservative end. SAFETY: Sodium hydroxide is caustic — always wear gloves and eye protection, add lye to water (never water to lye), and work in a ventilated area. For this oil blend, weigh exactly 132.94g of pure NaOH into 265.9g of distilled water at 5.0% superfat.
Example 2: olive oil 300 g, lye concentration (water:lye) "2:1 water to lye (33%)"
A typical middle-of-the-road setup. SAFETY: Sodium hydroxide is caustic — always wear gloves and eye protection, add lye to water (never water to lye), and work in a ventilated area. For this oil blend, weigh exactly 141.41g of pure NaOH into 282.8g of distilled water at 5.0% superfat.
Example 3: olive oil 390 g, lye concentration (water:lye) "1.5:1 water to lye (38%)"
On the higher / more demanding end. SAFETY: Sodium hydroxide is caustic — always wear gloves and eye protection, add lye to water (never water to lye), and work in a ventilated area. For this oil blend, weigh exactly 152.95g of pure NaOH into 229.4g of distilled water at 5.0% superfat.
Quick answers about the Soap Lye (NaOH) & Superfat
What exactly does the Soap Lye (NaOH) & Superfat work out?
Getting lye weight wrong in cold process soap either leaves a harsh, unreacted-lye bar or an overly greasy one, and because sodium hydroxide is a corrosive chemical, precision and safety go hand in hand. You enter olive oil, coconut oil, palm oil and shea butter (plus 3 more optional details) and the result panel updates straight away, so you can compare two or three versions of the same question in a few seconds.
What do I need before I start?
Only 7 fields: olive oil, coconut oil, palm oil, shea butter, castor oil, target superfat and lye concentration (water:lye). Nothing else is needed and nothing is stored.
How is it calculated — sAFETY FIRST: Handling sodium hydroxide?
Always add lye to water, never water to lye, to avoid a violent exothermic reaction that can splash caustic liquid. The same maths runs inside this page, so hand-checking the result on paper gives you the identical figure.
Why do two calculators give me different answers for soap Lye (NaOH) & Superfat?
Superfatting leaves a small percentage of oils unreacted with lye, making the bar gentler and more moisturizing. Different sites pick different assumptions, so always check which method a calculator states before you trust the gap between two numbers.
What does the "Common oil SAP values (NaOH, g/g)" table on this page tell me?
It is the reference range this tool works against — 6 rows from "Olive oil" (0.135) up to "Sunflower oil" (0.134). Use it to sanity-check whether the number you just calculated sits where you expected it to.
Which lye concentration (water:lye) should I pick?
The dropdown offers 3 choices — 2:1 water to lye (33%), 1.5:1 water to lye (38%) and 3:1 water to lye (25%). Pick the one that matches your real situation rather than the one you would like to be true; lye concentration (water:lye) usually moves the final figure more than any other single input, so it is worth running it twice with the option above and below your guess.
Do I have to press a button or reload the page to see the result?
No. Soap Lye (NaOH) & Superfat runs completely inside your browser, so the moment you change a value the cards recalculate — there is no submit step, no page reload and no waiting for a server round trip. That also means it keeps working on a weak or intermittent mobile connection.
Is it free, and do you keep what I type?
It is free with no sign-up, no app install and no usage limit. Nothing you enter into Soap Lye (NaOH) & Superfat leaves your device — the calculation is JavaScript running locally, so there is no upload of your figures to DrHint or anyone else.
Can I use it on a phone?
Yes — the layout stacks to a single column on small screens and the number fields open the numeric keypad on both Android and iOS. Many people bookmark this page or add it to their home screen and re-open it whenever the question comes up.
Anything to be careful about with the result?
A 0% superfat bar uses exactly the stoichiometric lye needed to saponify every oil molecule, but any measurement imprecision risks leftover free lye, making 5%+ superfat a common safety buffer. Treat the output as a well-grounded estimate for planning, not as a professional, legal or medical decision on its own.
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