Soap lye calculator – safe NaOH and KOH for any oil blend

The soap lye calculator works out how much sodium hydroxide or potassium hydroxide a batch of oils needs, then tells you how much water to dissolve it in and how many bars the finished batch will yield. Every fat has its own saponification value, so a recipe that mixes olive, coconut and palm cannot be dosed by eye. You enter each oil and its weight, pick your lye type and superfat, choose how to set the water, and the tool returns the numbers you weigh out on the scale.

It is built for cold-process and hot-process soapers who blend several oils and want a lye amount that leaves a small, deliberate fat surplus. The NaOH mode covers hard bar soap; the KOH mode covers liquid and cream soap, where the numbers shift by roughly forty percent.

Nothing here is a fixed recipe. Change one oil weight and the required lye moves with it, which is why superfat is set as a percentage rather than a fixed weight, which is the whole reason a per-oil breakdown matters when you scale a formula up or swap a fat you have run out of.

✂️ How to use the soap lye calculator

Start by choosing the lye type. Bar soap uses sodium hydroxide, liquid soap uses potassium hydroxide, and the two are not interchangeable because potassium hydroxide is a heavier molecule and needs a larger mass to saponify the same oil. The default is NaOH, which is what most beginners reach for first.

Lisa Mandel

Add your oils one row at a time, selecting each fat and typing its weight in grams. The calculator keeps a running total at the top, so a recipe that should hit 1000 g is easy to check. Order does not matter; the tool sums the demand across every row.

Set the superfat next: the percentage of oil left unsaponified for a gentler bar. Five percent is the common starting point for a mixed recipe. A pure coconut oil bar is usually pushed to fifteen or twenty percent, because coconut oil strips skin at low superfat.

Weigh oils and lye to the gram, not the rounded number. A recipe that calls for 141 g of NaOH is not the same as 140 g once the batch is small; the missing gram changes the real superfat by close to a full percent on a 1000 g batch.

Then decide how the water is set. Concentration asks what percentage of the solution should be lye by weight, with 33 percent as a reliable default. Ratio asks for a water-to-lye figure such as 2:1. Both reach a safe solution; concentration gives finer control over how much water leaves the bars during cure.

Last, enter the bar weight you plan to cut, for example 100 g. The calculator divides the batch weight by this to estimate bar count. Then read the results from the top: the hero number is the lye mass, and below it sit the water mass, total batch weight, bar count and a per-oil breakdown of the lye demand.

Calculator fields explained

Lye type – selects NaOH for solid bar soap or KOH for liquid and cream soap. Default is NaOH. KOH results are scaled up by a factor near 1.40 and adjusted for purity, since flake KOH is typically only 90 percent pure.

Oil rows (oil and weight in grams) – one row per fat, each with an oil picked from the list and a weight in grams. Each oil carries its own saponification value, so this is the most important input for accuracy. There is no default oil.

Superfat (percent) – the share of oils left unsaponified, entered as a whole number. Default is 5, with a sensible working range of 0 to 20. Higher superfat means a milder bar and a shorter shelf life, since surplus oil can eventually go rancid.

Water method – a choice between lye concentration and water-to-lye ratio. Default is concentration. Concentration expresses lye as a percent of the solution; ratio expresses water as a multiple of lye weight.

Water value – the number that goes with the method above. For concentration the default is 33 (percent). For ratio a common entry is 2 (meaning twice the lye weight in water). Lower water, meaning higher concentration, speeds trace and shortens cure.

Bar weight (grams) – the target weight of a single cut bar, used only to estimate how many bars the batch yields. Default is 100 g. It does not affect the lye or water figures.

📊 Understanding the results

The lye amount is the figure everything else depends on. For a 1000 g batch of olive, coconut and palm at 5 percent superfat, the tool returns about 141 g of NaOH. That number already has the superfat discount applied, so you do not subtract anything further; you weigh 141 g of sodium hydroxide and no more.

The lye figure is the only result you cannot round without changing the soap. Water can drift by a few grams with no real consequence, and bar count is an estimate by design. Lye is chemistry, and an excess of even a few grams leaves free alkali that can burn skin.

The water figure follows from your method. At 33 percent concentration, 141 g of lye pairs with about 287 g of water, since the lye must make up a third of the solution. A 2:1 ratio asks for 282 g instead. The concentration route holds the ratio steady as recipes change.

A lye figure that looks far larger than the oil weight is a signal you left KOH selected for a bar recipe. Potassium hydroxide demands roughly 40 percent more mass than sodium hydroxide, so a KOH number used to weigh NaOH will drastically overdose the batch and produce a caustic, unusable soap.

The per-oil breakdown is the quietly useful part. For the example above it shows olive contributing about 67 g of raw lye demand, coconut about 53 g, and palm about 28 g. Swap palm for lard and you can see exactly how much demand you are replacing.

Total batch weight tells you volume. That 1000 g oil recipe becomes roughly 1428 g of batter once lye and water are added, which at 100 g per bar cuts into 14 bars. If your loaf mould holds 1200 g, you know to scale down or split the pour.

Very high superfat gives a smaller lye number and a softer bar; near-zero superfat pushes the lye up and risks a lye-heavy bar if your weights are even slightly off. The middle of the range gives a margin for real-world error.

🧮 Calculation formulas

Every oil has a saponification value, written here as the grams of NaOH needed to saponify one gram of that oil. The batch demand is the sum of each oil weight multiplied by its own value, and superfat trims that total.

Raw NaOH demand = sum over oils of (oil weight x SAP value).

Final NaOH = raw NaOH demand x (1 – superfat / 100).

To convert to potassium hydroxide, multiply the NaOH figure by 1.4025, the ratio of the two molar masses (56.11 divided by 40.00). Because flake KOH is usually 90 percent pure, the tool then divides by 0.90 to give the real weight you scoop.

Final KOH = final NaOH x 1.4025 / 0.90.

Water depends on method. Under concentration C (a percent), water = lye x (100 – C) / C. Under a water-to-lye ratio R, water = lye x R. Total batch weight is oils plus lye plus water, and bar count is that total divided by your bar weight, rounded down.

Work the example by hand to see it. Olive 500 g x 0.1345 = 67.25 g. Coconut 300 g x 0.178 = 53.40 g. Palm 200 g x 0.141 = 28.20 g. The raw demand is 148.85 g of NaOH. Apply 5 percent superfat by multiplying by 0.95, and you land on 141.4 g. That is the hero number the calculator prints.

Saponification values are averages. A given batch of olive oil can vary by a small margin depending on where and when the olives were pressed, which is one more reason to build in a superfat cushion rather than aim for zero surplus.

For the water at 33 percent concentration: 141.4 x (100 – 33) / 33 = 141.4 x 2.030 = 287.1 g. Total batch is 1000 + 141.4 + 287.1 = 1428.5 g. Divided by a 100 g bar weight, that is 14 bars with 28 g to spare.

🎨 Practical examples

Each scenario below lists the inputs, the calculation and what a soaper does with the result. All numbers use the saponification values in the table above.

1. Single-oil castile bar. Olive oil 500 g, NaOH, 5 percent superfat, 33 percent concentration. Lye = 500 x 0.1345 x 0.95 = 63.9 g. Water = 63.9 x 2.03 = 129.7 g. Batch = 693.6 g, about 6 bars at 100 g. The soaper unmoulds late, because pure olive soap stays soft for days.

2. Coconut-only bar for laundry or a rich lather. Coconut 400 g, NaOH, 20 percent superfat, 33 percent. Lye = 400 x 0.178 x 0.80 = 57.0 g. Water = 115.6 g. Batch = 572.6 g, about 5 bars. The high superfat is deliberate, since coconut at 5 percent would strip skin raw.

The first batch that taught me superfat was a 100 percent coconut bar at 5 percent. It cleaned like a degreaser and left my hands cracked by day three. The same oils at 20 percent are one of my best sellers.

3. Balanced three-oil beginner batch. Olive 500 g, coconut 300 g, palm 200 g, NaOH, 5 percent, 33 percent. Lye = 141.4 g, water = 287.1 g, batch = 1428.5 g, 14 bars. This is the reference recipe and a safe first cold-process pour.

4. Butter-rich bar, set by ratio. Olive 320 g, coconut 200 g, shea 160 g, castor 120 g, NaOH, 5 percent, water-to-lye 2:1. Raw lye = 43.04 + 35.60 + 20.48 + 15.43 = 114.55 g, times 0.95 = 108.8 g. Water = 217.6 g, batch = 1126.4 g, about 11 bars. The 2:1 ratio keeps the water level predictable as you tweak the blend.

5. Fast-trace batch for a tight design window. Same 800 g blend as above but at 40 percent concentration. Lye stays 108.8 g; water drops to 108.8 x 1.50 = 163.2 g. Batch = 1072 g. Less water means the batter thickens sooner, which suits a simple pour but punishes an intricate swirl.

6. Production loaf scaled 3x. Triple the reference recipe: olive 1500 g, coconut 900 g, palm 600 g, NaOH, 5 percent, 33 percent. Lye = 424.2 g, water = 861.3 g, batch = 4285.5 g, about 42 bars. A loaf this size needs a mould rated over 4 kg, so most sellers split it into two pours.

7. Liquid soap paste with KOH. Coconut 200 g, olive 200 g, castor 100 g, KOH, 3 percent superfat, water-to-lye 3:1. NaOH-equivalent raw demand = 35.6 + 26.9 + 12.86 = 75.36 g, times 0.97 = 73.1 g. Convert: 73.1 x 1.4025 / 0.90 = 113.9 g of KOH. Water = 3 x 113.9 = 341.7 g. The 90 percent purity step alone adds about 11 percent more KOH. Liquid soap runs low superfat so the paste stays clear when diluted.

8. Substitution when palm runs out. Take the reference recipe and replace palm 200 g with lard 200 g. Palm demanded 200 x 0.141 = 28.2 g; lard demands 200 x 0.138 = 27.6 g. The raw total drops by 0.6 g, the final lye moves from 141.4 g to about 140.8 g, and the bar is nearly identical. Close saponification values make lard and palm easy swaps.

9. Edge case, zero superfat. The reference recipe at 0 percent gives the full 148.85 g of NaOH with no fat cushion. If your scale reads 2 g light on the oils, the batch tips lye-heavy and can be harsh to the skin. This is why zero superfat is left to experienced soapers checking a specific hardness target.

💡 Tips and best practices

Always add lye to water, never water to lye, and do it with gloves and eye protection in a ventilated space. The calculator gives you the masses; your safety habits handle the heat and fumes. This is the one rule with no flexibility.

Weigh with a scale that reads to at least one gram, and tare between each oil. Small kitchen scales that jump in 5 g steps are the hidden cause of most failed first batches, because the superfat you think you set is not the superfat you got. A 0.1 g jeweller’s scale is worth buying once you sell.

Keep coconut in check. A blend above 30 percent coconut wants a higher superfat to stay skin-friendly, and the calculator lets you test that by nudging the superfat field and watching the lye fall.

Save your three or four proven recipes with their exact lye and water outputs written on the card. Re-running the calculator each time still catches a mistyped weight, but a written baseline tells you instantly when a number looks wrong.

Use concentration rather than ratio once you are comfortable. It holds the water behaviour steady across recipes, so a swirl that worked at 33 percent works again next month even after you change oils. Ratio is fine for learning; concentration scales better with a growing recipe book.

Match your water to your design. A detailed multi-colour swirl wants more water and a slower trace, near 30 percent concentration. A quick single pour or a batch you need to unmould tonight wants less water, near 38 to 40 percent.

Round the lye down, never up, if your scale forces a choice. Landing a gram under the target nudges the effective superfat up slightly, which is safe; a gram over eats into your cushion and moves toward lye-heavy.

For KOH liquid soap, expect a longer process and a cook stage. The lye and water numbers here get you a correct paste, but dilution rates and sequestering are separate steps the calculator does not cover.

⚠️ Common mistakes to avoid

Using the wrong lye type

Selecting KOH and then weighing that figure as NaOH, or the reverse, is the most damaging error in soap making. The two differ by about 40 percent in mass for the same oils.

A NaOH bar recipe weighed against a KOH number overdoses the batch by roughly 40 percent. That is a lye-heavy soap that can chemically burn skin, not a bar to salvage. Throw it out and start over.

Confirm the lye type at the top of the form before you weigh anything, and cross-check that the lye figure is a believable fraction of your oil weight, usually between 12 and 18 percent for NaOH.

Skipping the superfat entirely

Leaving superfat at zero, or forgetting it exists, produces a technically correct but unforgiving bar with no margin for scale error.

Set 5 percent as your habit for mixed recipes and only deviate on purpose. The calculator applies it automatically once entered, so the mistake is almost always a blank or zeroed field.

Reading a SAP chart for KOH as if it were NaOH

Some published tables list saponification values for potassium hydroxide, which are larger. Plug those into a sodium hydroxide batch and every number inflates.

If your lye figure comes out near 20 percent of oil weight for a normal bar recipe, you are probably reading KOH values as NaOH. Stop and check the source of your numbers before weighing.

Let the calculator handle the conversion internally rather than mixing chart sources by hand, which is where the two systems get crossed.

Forgetting KOH purity

Potassium hydroxide flakes are typically 90 percent pure, and a calculation that assumes 100 percent leaves the soap under-saponified with excess free oil.

The tool divides by 0.90 for you, so trust its KOH output rather than a hand figure that skipped the purity step. If you buy 95 percent KOH, that changes the divisor.

Guessing oil weights instead of weighing

Measuring oils by volume, in cups or spoons, breaks the whole calculation because saponification runs on mass, not volume.

Every oil row is a weight in grams for a reason. A cup of coconut oil and a cup of olive oil weigh different amounts and demand different lye, so the scale is not optional.

Ignoring the batch weight against the mould

Calculating a beautiful recipe and only then discovering the batter overflows the mould wastes the batch and the ingredients.

Check the total batch weight result against your mould capacity before mixing lye. A 1428 g batch will not fit a mould rated for 1000 g.

Changing water to fix a trace problem after mixing

Adding water to a batch that is tracing too fast dilutes the recipe unpredictably and can seize the soap.

Set the water correctly at the start. If a design needs more working time, lower the concentration next batch rather than improvising mid-pour.

🎯 When to use this calculator

Reach for it any time you create or change a recipe. A new oil blend, a different superfat, a switch from bar to liquid soap, or a scale-up all change the lye demand, and none can be carried over from a previous batch by memory.

It matters most when you substitute. The per-oil breakdown shows whether a swap is a rounding difference, as with palm oil and lard, or a real shift that moves your total lye by several grams. It matters again when you scale, since doubling a recipe by hand is easy to get wrong once four or five oils are involved, and one mis-multiplied row throws the superfat off across the batch.

I run the calculator even for recipes I have made fifty times. Not because the recipe changed, but because my fingers mistype a weight often enough that the two-second check has saved me more than one ruined loaf.

You can skip it only for an identical repeat of a recipe whose lye and water you have already written down. Even then, a quick re-entry catches the day you type 5000 g of olive instead of 500.

🔗 Related calculators

• Superfat and lye discount calculator
• Lye concentration and water calculator
• Oil blend properties calculator
• Soap mould volume calculator
• Soap fragrance usage calculator
• Melt and pour batch calculator
• Liquid soap dilution calculator
• Soap cost per bar calculator

📖 Glossary

Saponification – the reaction between fats and a strong alkali that produces soap and glycerin.

SAP value – the mass of lye needed to saponify one gram of a specific oil, unique to each fat.

NaOH – sodium hydroxide, the alkali used for solid bar soap.

KOH – potassium hydroxide, the alkali used for liquid and cream soap, heavier per molecule than NaOH.

Superfat – the percentage of oils left unsaponified to make a milder, more conditioning bar.

Is lye discount the same as superfat? In practice yes, they describe the same reduction from two directions. Superfat names the leftover oil; lye discount names the cut in alkali that creates it. The calculator uses one field for both.

Lye concentration – the percentage of the lye solution made up of lye by weight, with water as the remainder.

Water-to-lye ratio – water weight expressed as a multiple of lye weight, such as 2:1.

Trace – the point at which soap batter thickens enough to leave a trail on its own surface.

Cure – the weeks a finished bar rests to lose water and finish hardening.

Cold process – making soap without external heat after the lye and oils meet.

Hot process – cooking the soap batter to force saponification to complete before moulding.

Free alkali – unreacted lye left in a bar, the hazard that superfat is designed to prevent.

❓ Frequently asked questions

Can I mix NaOH and KOH in one recipe?

Yes, and it is common for cream soaps and some shaving soaps that want a soft, spreadable texture. You calculate the NaOH portion and the KOH portion separately, then combine.

A frequent split is 30 percent KOH and 70 percent NaOH by lye demand, though the exact ratio drives how soft the final soap is. This calculator handles one lye type per run, so you would compute each part and add the results.

Why does the lye number change when I change superfat?

Superfat is applied as a direct reduction to the raw lye demand, so a higher superfat means less lye and more leftover oil. At 5 percent the calculator multiplies the raw figure by 0.95; at 10 percent by 0.90.

On the 1000 g reference batch, moving from 5 to 10 percent superfat drops the NaOH from about 141 g to 134 g. That 7 g swing is the whole difference between a standard bar and a very mild one.

What water amount should a beginner use?

Start with 33 percent concentration or a 2:1 water-to-lye ratio, which land close to each other and give a forgiving, slow-tracing batter. Both leave enough working time for a first pour.

Once you are comfortable, drop toward 38 percent for firmer, faster batches. More water is safer for learning; it simply means a longer cure.

Do saponification values ever change?

The published averages are stable, but real oils vary slightly by source, season and refining. This is a small effect, usually well under one percent of the lye figure.

The practical takeaway is to keep a superfat cushion rather than chase a perfect zero. The cushion absorbs both oil variation and your own weighing error in one move.

If you ever switch to a pomace olive oil or a differently refined coconut, expect a minor shift and let superfat cover it.

Why is KOH heavier than NaOH for the same oils?

Potassium is a heavier atom than sodium, so a potassium hydroxide molecule has more mass. Saponifying the same fat therefore needs more grams of KOH than NaOH, by a factor of about 1.40.

The calculator applies that factor automatically when you select KOH, then divides by 0.90 for typical flake purity, so you do not do the conversion by hand.

Can I trust the bar count exactly?

Treat it as a close estimate, not a promise. It divides total batch weight by your bar weight and rounds down, but real bars lose weight as they cure and cutting is rarely perfectly even.

A batch that calculates to 14 bars at 100 g will often yield 14 slightly lighter bars after a four-week cure, since water evaporates during that time.

What happens if I set superfat too high?

The bar becomes very mild but softer, slower to harden, and quicker to develop rancidity because there is more free oil to oxidize. Above about 15 percent for most recipes you trade shelf life for gentleness.

Facial bars sometimes run 8 to 10 percent on purpose, while a general body bar rarely needs more than 5 to 7 percent.

Does fragrance or additive weight affect the lye?

No. Lye is calculated only against the oils that saponify. Fragrance oils, essential oils, colorants and additives like oats or clay do not react with the alkali and are not entered as oils here.

Add those on top of the recipe using their own usage rates, typically around 3 to 5 percent of oil weight for fragrance, calculated in a separate tool.

⚖️ Disclaimer

This calculator provides educational soap-making estimates based on average saponification values. It is a planning aid for hobby and small-batch soapers, not a substitute for careful weighing, testing and safe handling of caustic materials.

Saponification, oil purity and lye purity vary between suppliers and batches, so the figures are close approximations rather than guarantees. Work with a superfat margin and verify your oil and lye against supplier data. Any bar-count output is a rough estimate, not business or financial advice, and yields depend on your moulds, cutting and cure time.

Before committing an expensive oil blend or a large batch, make a small test batch 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.