The superfat and lye discount calculator takes the full lye a recipe would need at zero percent superfat, trims it by the percentage you choose, and shows the reduced lye plus the grams of oil left unsaponified in the finished soap. Superfat is the deliberate oil surplus that makes a bar milder; lye discount is the same idea named from the alkali side. This tool converts one into the other and hands you the exact weight to put on the scale.
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It is aimed at soapers who already have a base lye figure, from a chart or a lye calculator, and want to adjust how gentle the bar is without redoing the whole recipe. Nudge the superfat up for a dry-skin facial bar, or hold it low for a hard, long-lasting utility bar.
The math is short but easy to get wrong by hand, especially when you apply a discount to a number that was already discounted. This calculator keeps that from happening.
- ✂️ How to use the superfat and lye discount calculator
- Calculator fields explained
- 📊 Understanding the results
- 🧮 Calculation formulas
- 🎨 Practical examples
- 💡 Tips and best practices
- ⚠️ Common mistakes to avoid
- Applying the discount twice
- Setting superfat to zero by accident
- Reading free oil grams as the whole recipe
- Pushing superfat high on soft oils
- Assuming superfat and lye discount are different numbers
- Ignoring shelf life when raising superfat
- 🎯 When to use this calculator
- 🔗 Related calculators
- 📖 Glossary
- ❓ Frequently asked questions
- Is superfat the same as lye discount?
- What superfat should a beginner use?
- Why does more superfat make a softer bar?
- Can superfat go too high?
- Does superfat affect how long soap lasts?
- What base lye number do I enter?
- Do I superfat liquid soap the same way?
- Why is my lye removed figure larger than expected?
- ⚖️ Disclaimer
✂️ How to use the superfat and lye discount calculator
Start with the base lye amount. This is the lye your oils need at zero superfat, the full saponification value figure. If you have run a lye calculator at 0 percent, that number goes here. For a 1000 g mixed batch it often sits near 148 g of NaOH.
Enter the superfat percentage next. Five percent is the standard for an everyday bar. The field accepts whole numbers, and the sensible working range runs from 0 to about 20 percent depending on the recipe.
Add the total oil weight if you want the free oil reported in grams rather than as a percentage. On a 1000 g batch a 5 percent superfat leaves roughly 50 g of oil unsaponified, and seeing that in grams helps you judge whether the bar will feel greasy.
Feed this calculator a base lye taken at zero superfat, never a figure that already had a discount applied. Discounting an already-discounted number stacks the reduction and pushes the real superfat far higher than you intended.
Pick the input mode last. Superfat mode reads your entry as the oil surplus; lye discount mode reads it as the cut to the lye. The two produce the same weights, so a 5 percent superfat and a 5 percent lye discount give an identical result. The mode only changes the label.
Read the results from the top. The hero number is the discounted lye you weigh out. Below it sit the grams of lye removed, the free oil left in grams, and the effective discount, which confirms superfat and lye discount are the same value.
Calculator fields explained
Base lye at 0 percent superfat (grams) – the full lye demand of your oils before any discount. There is no default; it comes from your recipe or a lye calculator run at zero superfat. This is the number the discount is applied to.
Superfat percentage – the share of oils left unsaponified, entered as a whole number. Default is 5, with a working range of 0 to 20. Higher values give a milder, softer, shorter-lived bar.
Total oil weight (grams) – the combined weight of all oils, used only to express the free oil left as grams. Default is 1000. It does not change the lye figure.
Input mode (superfat or lye discount) – selects how your percentage is read. Default is superfat. Both modes return the same discounted lye; the choice is a matter of which vocabulary your recipe uses.
📊 Understanding the results
| Result | What it means | How to act on it |
|---|---|---|
| Discounted lye (g) | Lye to weigh after the superfat cut | This is the only number you measure; weigh it exactly |
| Lye removed (g) | Grams of lye taken out by the discount | Sanity-check it against your superfat percent |
| Free oil left (g) | Oil that stays unsaponified in the bar | Judge whether the bar will feel conditioning or greasy |
| Effective discount (%) | Confirms superfat equals lye discount | Should match the percentage you entered |
The discounted lye is the figure that matters. For a base of 148 g of NaOH at 5 percent superfat, the tool returns 140.6 g. You weigh that and nothing else; the reduction is already inside the number.
A 5 percent superfat on 1000 g of oils leaves about 50 g of free oil. That surplus is what softens the feel of the bar and buffers against small weighing errors. Push it higher and the bar gets creamier but softer; drop it toward zero and you lose the cushion that keeps a bar skin-safe.
Lye removed is a cross-check. At 5 percent on a 148 g base it should read 7.4 g. If it reads 14 g, you either entered 10 percent or fed the tool an already-discounted number, and the result is not what you think it is.
Superfat and free oil are proportional to your oil weight, so the same percentage means very different grams at different batch sizes. Fifteen percent on a 400 g coconut batch is 60 g of oil; the same percentage on a 2000 g batch is 300 g, which can leave a soft, oily bar if the oils are slow to saponify.
The effective discount confirms the identity between the two terms. It exists so that a soaper who learned one vocabulary can trust the other. A 6 percent superfat is a 6 percent lye discount, and the field proves it every time.
Very high results in the free oil field, above roughly 12 to 15 percent of oil weight, are a signal to check your recipe. Some oils tolerate a big superfat; a bar heavy in soft oils at 18 percent may never fully firm up.
🧮 Calculation formulas
The discount is a single multiplication. The discounted lye is the base lye scaled down by the superfat fraction, and the lye removed is the base lye scaled by that same fraction.
Discounted lye = base lye x (1 – superfat / 100).
Lye removed = base lye x superfat / 100.
Because the lye is cut in proportion to the oils it would have saponified, the oil left unreacted tracks the superfat directly. Free oil in grams is the oil weight times the superfat fraction.
Free oil (g) = total oil weight x superfat / 100.
Superfat percent and lye discount percent are numerically equal by the convention every soap calculator uses, so no conversion sits between them. Work the example: a 148 g base at 5 percent gives 148 x 0.95 = 140.6 g discounted, 148 x 0.05 = 7.4 g removed, and on 1000 g of oils, 1000 x 0.05 = 50 g of free oil.
| Superfat / discount | Lye multiplier | Free oil per 1000 g oils | Typical use |
|---|---|---|---|
| 0 percent | 1.00 | 0 g | Test batches only, no margin |
| 2 percent | 0.98 | 20 g | Shaving soap, hard laundry bar |
| 3 percent | 0.97 | 30 g | Castile, salt bar base |
| 5 percent | 0.95 | 50 g | Everyday body bar, the default |
| 8 percent | 0.92 | 80 g | Facial and sensitive-skin bars |
| 10 percent | 0.90 | 100 g | Rich, conditioning bars |
| 15 percent | 0.85 | 150 g | High-coconut and salt bars |
| 20 percent | 0.80 | 200 g | Pure coconut bars only |
The free-oil figure is an estimate that assumes the lye reduction spreads evenly across all oils. In reality the last oils to saponify are the ones most likely to remain free, which is why a superfat built from stable, slow-oxidizing oils lasts longer on the shelf.
The values above are linear, so any superfat not listed is easy to place. Seven percent sits between the 5 and 8 rows: multiplier 0.93, and 70 g of free oil per 1000 g of oils.
| Soap type | Recommended superfat | Why |
|---|---|---|
| Shaving soap | 2 to 5 percent | Keeps lather stable and the puck firm |
| Castile (100 percent olive) | 3 to 5 percent | Olive is already gentle |
| Everyday body bar | 5 percent | Balanced mildness and shelf life |
| Facial and sensitive skin | 8 to 10 percent | Extra conditioning without going greasy |
| Salt bar | 15 percent | Salt makes the bar drying and brittle |
| Pure coconut bar | 15 to 20 percent | Coconut is strongly cleansing |
| Liquid soap (KOH) | 0 to 3 percent | Excess oil clouds and separates |
🎨 Practical examples
Each example lists the base lye, the superfat, the oil weight, and what the soaper does with the numbers.
1. Everyday body bar. Base lye 148 g, 5 percent, oils 1000 g. Discounted lye = 140.6 g, removed 7.4 g, free oil 50 g. A safe first cold-process bar with a comfortable margin.
2. Pure coconut bar. Base lye 71.2 g (400 g coconut), 20 percent, oils 400 g. Discounted lye = 57.0 g, removed 14.2 g, free oil 80 g. That 80 g of free oil in a 400 g batch is why coconut bars need such a high superfat. At 5 percent the same bar would scour skin.
3. Facial bar for dry skin. Base lye 148 g, 8 percent, oils 1000 g. Discounted lye = 136.2 g, removed 11.8 g, free oil 80 g. The extra surplus of free oil makes the bar gentler on the face without going soft.
4. Salt bar. Base lye 148 g, 15 percent, oils 1000 g. Discounted lye = 125.8 g, removed 22.2 g, free oil 150 g. Salt bars run high because added salt makes them drying and brittle otherwise.
5. Shaving soap. Base lye 148 g, 2 percent, oils 1000 g. Discounted lye = 145.0 g, removed 3.0 g, free oil 20 g. A low superfat keeps the lather stable and the puck firm under a brush.
I moved my main bar from 5 to 7 percent one winter because customers with dry skin kept asking for something milder. Two grams less lye on the batch, and the complaints stopped.
6. Adjusting an existing recipe. A 5 percent recipe on a 148 g base gives 140.6 g. Bump it to 7 percent for winter and you get 137.6 g, 3 g less lye and 20 g more free oil per 1000 g. Small change, noticeably milder bar.
7. Production loaf scaled 3x. Base lye 444 g (3000 g oils), 5 percent. Discounted lye = 421.8 g, removed 22.2 g, free oil 150 g. The same percentage carries cleanly to a market-size batch; only the grams grow.
8. Zero superfat, edge case. Base lye 148 g, 0 percent, oils 1000 g. Discounted lye = 148 g, removed 0 g, free oil 0 g. No cushion at all, so a 2 g weighing error on the oils tips the bar lye-heavy. Reserved for controlled tests.
9. Lye discount framing. Enter 6 percent in lye discount mode on a 148 g base and you get 139.1 g, the exact result a 6 percent superfat produces. Proof the two labels describe one number.
💡 Tips and best practices
Always take your base lye at zero superfat before using this tool. The single most common error is feeding in a figure that already has a discount baked in, which quietly doubles the reduction. A base lye near 14 to 15 percent of oil weight for NaOH is the range to expect.
Match the superfat to the oils, not to a habit. Cleansing oils like coconut oil and babassu want a high superfat; gentle oils like olive oil can sit at 3 to 5 percent and still feel mild. The recommended-superfat table above is a faster starting point than guessing.
Write the superfat next to every saved recipe. Two bars from the same oils can behave differently at 5 and 8 percent, and only the note explains why one lathers richer and the other lasts longer.
Remember that superfat shortens shelf life. Every extra gram of free oil is a gram that can eventually go rancid, so a 15 percent bar made with soft, unsaturated oils may develop orange spots months sooner than a 5 percent bar. Add a little vitamin E or rosemary extract if you push the superfat high.
Keep facial bars in the 8 to 10 percent band. It is gentle enough for most skin without leaving the greasy film that a 15 percent body bar can. Test on yourself before scaling a new facial formula.
Do not chase a superfat below 3 percent for a normal bar. The margin exists to absorb scale error and oil variation, and shaving it away trades a mild convenience for a real burn risk if anything is off.
When you change one oil for another, re-take the base lye first, then apply the same superfat. The discount percentage is portable; the base lye is not, because different oils demand different amounts.
Use lye discount mode only if your source recipe speaks that language. Mixing the two vocabularies in your own notes is where confusion creeps in, so pick one and stay with it across a recipe book.
⚠️ Common mistakes to avoid
Applying the discount twice
Running an already-superfatted lye figure through the calculator applies the cut a second time, and the two reductions stack.
A 5 percent discount on a lye that already had 5 percent removed lands you near a 10 percent superfat, not 5. The bar can end up too soft to use and slow to cure. Always start from the zero-superfat base.
Confirm your base is a full saponification figure before you type it, and check that lye removed matches the percentage you asked for.
Setting superfat to zero by accident
A blank or zeroed superfat field returns the full lye with no cushion, which is a lye-heavy bar waiting for the smallest weighing slip.
Treat 5 percent as the resting default and only enter zero on purpose for a controlled test. The calculator will not warn you; the number simply comes back at full strength.
Reading free oil grams as the whole recipe
The free oil figure is the unsaponified surplus, not the total oil in the bar. Confusing the two leads soapers to think a 50 g result means a tiny batch.
Free oil of 50 g on a 1000 g batch means 5 percent of the oils stay free, not that the recipe uses only 50 g of oil. Read it as a slice of the whole, not the whole.
Keep the total oil weight field in view so the proportion stays obvious as you adjust the percentage.
Pushing superfat high on soft oils
A large superfat built from olive, sunflower oil or other soft oils can leave a bar that never firms up and goes rancid early.
If you want a high superfat, lean on stable oils and butters, and keep the soft-oil share modest. Fifteen percent on a coconut bar behaves very differently from fifteen percent on a sunflower bar.
Assuming superfat and lye discount are different numbers
Some soapers double-adjust because they think a 5 percent superfat needs a separate lye discount on top. It does not; they are the same figure.
Use one field and one mode. The effective discount output exists precisely to confirm the two are equal so you stop second-guessing it.
Ignoring shelf life when raising superfat
Bumping the superfat for a milder bar without changing the oils quietly cuts how long the bar lasts before it spoils.
Balance mildness against longevity. If a bar will sit unsold for months, a 5 percent superfat with an antioxidant beats a 12 percent bar that goes off first.
🎯 When to use this calculator
Reach for it whenever you want to change how gentle a bar is without rebuilding the recipe. You have the base lye already; this tool just applies the surplus and tells you the new weight and the free oil that comes with it.
It earns its place when you adjust a proven formula for a season or a customer. Moving from 5 to 8 percent for winter skin is a two-second change here, and the free oil figure tells you whether the bar will still firm up.
Use it to settle the superfat-versus-lye-discount confusion once. Enter a number in each mode, watch the identical result, and you can stop worrying about which term your recipe used.
The day I realised superfat and lye discount were the same number, half my recipe notes suddenly made sense. I had been treating them as two separate dials for a year.
You can skip it for a first-time recipe you are building from oils, where a full lye calculator already applies the superfat in one step. This tool shines on adjustment, not on the initial build.
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📖 Glossary
Superfat – the percentage of oils left unsaponified to make a milder, more conditioning bar.
Lye discount – the cut in lye below full demand, numerically equal to the superfat percentage.
Base lye – the full lye a recipe needs at zero superfat, before any discount.
Saponification – the reaction between fats and a strong alkali that produces soap and glycerin.
Free oil – the unsaponified oil remaining in the finished bar, felt as conditioning.
Is a higher superfat always better? No. Above about 10 percent the extra mildness comes with a softer bar and a shorter shelf life, so the right superfat depends on the oils and how the bar will be used.
NaOH – sodium hydroxide, the lye used for solid bar soap.
KOH – potassium hydroxide, the lye used for liquid and cream soap.
Rancidity – the oxidation of free oils over time, often seen as orange spots.
Cure – the weeks a finished bar rests to lose water and finish hardening.
Cleansing oil – an oil like coconut that is strongly cleansing and usually paired with a higher superfat.
Lye-heavy – a bar with excess unreacted alkali, harsh or caustic to skin.
❓ Frequently asked questions
Is superfat the same as lye discount?
Yes. Superfat names the leftover oil; lye discount names the matching cut to the alkali. A 5 percent superfat and a 5 percent lye discount produce the same lye weight.
This calculator offers both modes only so you can use whichever term your recipe does. The effective discount output confirms they land on the same number every time.
What superfat should a beginner use?
Five percent for a general body bar. It gives a comfortable margin against weighing error while keeping the bar firm and long-lasting.
Go to 8 percent for facial bars and 15 to 20 percent for high-coconut bars. Stay at 2 to 3 percent for shaving soap and hard laundry bars.
Why does more superfat make a softer bar?
Free oil is liquid or soft fat that never became solid soap, so the more of it you leave, the less firm the bar. At 5 percent the effect is small; at 15 percent it is easy to feel.
On 1000 g of oils, 15 percent leaves 150 g of free oil, which is why salt bars need added hardeners and a long cure to stay usable.
Can superfat go too high?
Yes. Above roughly 12 to 15 percent, most recipes turn soft, greasy and quick to spoil, because the surplus oil oxidizes over time.
The practical ceiling depends on the oils. A coconut bar tolerates 20 percent; a soft-oil bar may misbehave at 10.
If you want a very mild bar, raise the butter content rather than pushing superfat past what the oils can carry.
Does superfat affect how long soap lasts?
It affects two lifespans. A higher superfat gives a softer bar that wears down faster in the shower, and it shortens shelf life because more free oil can go rancid.
A 5 percent bar with an antioxidant like rosemary extract will usually outlast a 12 percent bar made from the same oils by several months.
What base lye number do I enter?
The lye your oils need at zero superfat, which you get from a lye calculator run at 0 percent or from a saponification chart. It should be around 14 to 15 percent of oil weight for sodium hydroxide.
Never enter a figure that already had a superfat applied. Doing so double-counts the discount and can push a 5 percent target to a real 10 percent.
Do I superfat liquid soap the same way?
Liquid soap made with KOH usually runs a low superfat, often 0 to 3 percent, so the paste stays clear when diluted. Excess oil clouds liquid soap and can separate.
The discount math is identical; only the target percentage is lower. Enter your KOH base lye and a small superfat.
Why is my lye removed figure larger than expected?
Either the superfat is set higher than you meant, or the base lye you entered was already discounted. The lye removed should equal the base times the superfat fraction.
On a 148 g base at 5 percent, lye removed is 7.4 g. If it reads closer to 15 g, recheck both the percentage and the base.
⚖️ Disclaimer
This calculator provides educational soap-making estimates. It is a planning aid for hobby and small-batch soapers, not a substitute for careful weighing, testing and safe handling of caustic materials.
Free oil is an approximation that assumes the lye reduction spreads evenly across all oils, and real bars vary with the specific fats used, their freshness and your cure. Always work from a base lye taken at zero superfat and verify your figures against your oils.
Any judgement about shelf life or bar hardness is general guidance, not a guarantee. Results depend on your oils, additives and storage.
Before committing an expensive batch, make a small test bar and confirm it behaves as expected. Sodium hydroxide and potassium hydroxide are caustic; wear eye protection and gloves, work in ventilation, and add lye to water rather than water to lye.
