Handmade Soap Lye Calculator | Sodium Hydroxide or Potassium Hydroxide and Water from Your Oils and SAP Values, Superfat (Lye Discount), Hardness, Cleansing and INS Value, Oil Amount from a Mold or Milk Carton, and a SAP Value Chart
The Handmade Soap Lye Calculator answers the two questions every cold process (CP) soap maker asks: how many grams of lye, and how many grams of water? Pick from 20 oils and fats, including olive, coconut, palm, rice bran, shea butter, castor, camellia and lard, and enter their weights. The tool adds up the lye each oil needs from its SAP value, then applies the superfat (the share of oil left unsaponified; a 90% saponification rate is the same as a 10% superfat) and the purity printed on your container to give the amount to weigh out. Water can be set as a multiple of the lye, a percentage of the oils, or a lye solution concentration. It works with sodium hydroxide for bar soap and potassium hydroxide for liquid (potash) soap. From the fatty acid profile it charts hardness, cleansing, conditioning, bubbly lather, creamy lather, iodine value and INS value, and it warns you when the superfat is too low or the lye solution is too strong. Enter the inside dimensions and pour height of a milk carton, a rectangular mold or a round mold to get the oil that fits exactly and rescale your recipe. There is also a SAP value chart that shows the lye for 100 g of each oil at 0, 5, 7 and 10% superfat, plus the basic steps and how to buy and handle lye safely.
Oil recipe
Enter your oils and their weights to calculate the sodium hydroxide (or potassium hydroxide) and water from their SAP values.
Start from a common recipe
| Oil | Weight (g) | Share | SAP value | |
|---|---|---|---|---|
g | 50.0% | 0.134 | ||
g | 25.0% | 0.183 | ||
g | 25.0% | 0.142 |
Lye and water settings
Lye
Saponification rate 93% (superfat 7%)
Enter the purity (assay) on the container label. Sodium hydroxide is usually 95–99% and potassium hydroxide 85–90%. If you don't know, leaving sodium hydroxide at 100% is fine.
How to set the water
Amounts to weigh
Sodium hydroxide
68.9 g
Distilled water
172.3 g
Total oils
500.0 g
Lye solution concentration
28.6%
Batter weight
741 g
| Oil | Weight (g) | Share | Sodium hydroxide to saponify (g) |
|---|---|---|---|
| Olive oil | 250.0 | 50.0% | 33.50 |
| Coconut oil | 125.0 | 25.0% | 22.88 |
| Palm oil | 125.0 | 25.0% | 17.75 |
Soap properties (estimate)
These are estimates from the fatty acid profile of the oils. A value inside the shaded range usually means an easy-to-use soap.
Sodium hydroxide is a hazardous chemical
Wear goggles, rubber gloves and long sleeves, and ventilate the room. Always add the lye to the water a little at a time, never the other way round. Do not use aluminium tools. If it touches your skin, rinse at once with plenty of running water; if it gets in your eyes, rinse with running water for at least 15 minutes and see an eye doctor.
ABOUT
About Handmade Soap Lye Calculator | Sodium Hydroxide or Potassium Hydroxide and Water from Your Oils and SAP Values, Superfat (Lye Discount), Hardness, Cleansing and INS Value, Oil Amount from a Mold or Milk Carton, and a SAP Value Chart
The Handmade Soap Lye Calculator works out the lye (sodium hydroxide for bar soap, potassium hydroxide for liquid soap) and water for cold process soap from the oils you use and their weights. It adds up each oil's SAP value and applies your superfat and the purity of your lye to give the amount to weigh out. It also shows hardness, cleansing, lather and INS value, the oil that fits a mold or milk carton, and a SAP value chart for 100 g of each oil.
How to use Handmade Soap Lye Calculator | Sodium Hydroxide or Potassium Hydroxide and Water from Your Oils and SAP Values, Superfat (Lye Discount), Hardness, Cleansing and INS Value, Oil Amount from a Mold or Milk Carton, and a SAP Value Chart
- 1Under “Start from a common recipe”, pick the basic three-oil soap, a Marseille-style soap and so on, or list your own oils with “+ Add an oil”
- 2Enter the weight (g) of each oil. To change only the total and keep the proportions, press 300 g, 500 g and so on
- 3Choose the lye (sodium hydroxide for bar soap, potassium hydroxide for liquid soap), then enter the superfat (5–10% is typical for skin) and the purity printed on the container
- 4Choose how to set the water: a multiple of the lye, a percentage of the oils, or a concentration
- 5Weigh out the lye and water shown under “Amounts to weigh”, and check that there are no warnings and that the soap properties fall within the ranges
- 6If your mold is fixed, enter its inside dimensions and pour height under “Oil from mold size”, then press “Rebuild the recipe for this amount of oil”
Great for
- Find out how many grams of lye and water a basic olive, coconut and palm soap needs
- Recalculate the lye after adapting a book recipe to the oils and amounts you have
- Change a recipe written as a “90% saponification rate” to a 5% or 7% superfat
- Decide how much oil fills a milk carton or the silicone mold you own
- Check whether a palm-free recipe has enough hardness and lather
- Work out the potassium hydroxide for liquid (potash) soap
- Look up the SAP values of olive oil, rice bran oil and other oils in one chart
Good to know
- A SAP value is the grams of lye needed to turn 1 g of oil into soap. The lye is calculated by adding up oil weight × SAP value for each oil, then subtracting the superfat. The SAP values in the chart are typical figures and can differ by a few percent between brands and origins.
- The superfat is the share of oil left unsaponified on purpose. A 90% saponification rate and a 10% superfat mean the same thing. Soap for skin usually uses 5–10%, so that variations in SAP values and weighing errors never leave excess lye.
- With 95% pure sodium hydroxide, the calculated amount is divided by 0.95 so you weigh out a little more. If you don't know the purity, leaving sodium hydroxide at 100% is fine, as the difference is absorbed by the superfat.
- Water is commonly 2–3 times the lye (a 25–33% concentration) or 30–38% of the oil weight. Less water makes soap set faster; more water makes trace and unmolding take longer.
- Potassium hydroxide SAP values are the sodium hydroxide values multiplied by the ratio of molecular weights (56.11 ÷ 40.00 ≈ 1.403). Potassium hydroxide sold in shops is often 85–90% pure, so the default purity is 90%.
- Hardness, cleansing, conditioning, bubbly lather, creamy lather, iodine value and INS value are estimates calculated from each oil's fatty acid profile (lauric, myristic, palmitic, stearic, ricinoleic, oleic, linoleic and linolenic acid), using the same approach and ranges as SoapCalc.
- Mold volume is converted to oil by treating the batter as having a density of about 1, so volume (ml) = batter weight (g). Oil, lye and water together weigh about 1.45 to 1.5 times the oil.
- Sodium hydroxide is classified as a deleterious substance under Japan's Poisonous and Deleterious Substances Control Act. Wear goggles and gloves, and always add the lye to the water. Selling handmade soap in Japan requires a cosmetics manufacturing licence, among other things.
- Your recipe is stored only in this browser and never sent to a server.
Questions this tool answers
Use Handmade Soap Lye Calculator | Sodium Hydroxide or Potassium Hydroxide and Water from Your Oils and SAP Values, Superfat (Lye Discount), Hardness, Cleansing and INS Value, Oil Amount from a Mold or Milk Carton, and a SAP Value Chart for searches like these:
- Calculate how much lye a handmade soap needs
- Work out lye and water from my oil recipe
- See how the lye changes when I change the superfat
- Look up the SAP values of olive oil or coconut oil
- Find out how much oil fits a milk carton or soap mold
- Check a recipe's hardness, cleansing and INS value
- Calculate potassium hydroxide for liquid soap
FAQ
- How do I calculate the lye for soap?
- For each oil, multiply its weight (g) by its SAP value, add the results, and subtract the superfat. For example, 250 g of olive oil, 125 g of coconut oil and 125 g of palm oil need 250 × 0.134 + 125 × 0.183 + 125 × 0.142 = about 74.1 g of sodium hydroxide for 100% saponification; at a 7% superfat that becomes 74.1 × 0.93 = about 68.9 g. With water at 2.5 times the lye, you need about 172 g of water.
- Is a 90% saponification rate the same as a 10% superfat?
- Yes. The saponification rate is the percentage of the lye needed for full saponification that you actually add, and the superfat (lye discount) is the percentage of oil left over; the two add up to 100%. When you enter a superfat, the tool also shows the matching saponification rate.
- How much lye do I need for 500 g of olive oil (100% olive soap)?
- With an olive oil SAP value of 0.134, full saponification takes 67 g of sodium hydroxide. That is about 62.3 g at a 7% superfat, about 60.3 g at 10% and about 63.7 g at 5%. Pure olive (castile) soap is soft and slow to harden, so allow at least six weeks to cure.
- How much water should I use relative to the lye?
- Two to three times the lye (a 25–33% lye concentration) is common, and the tool starts at 2.5 times. If you set water as a percentage of the oils, 30–38% is typical. Less water helps soap set and unmold sooner, but the batter can also thicken suddenly.
- How much oil fits in a 1-litre milk carton?
- A Japanese 1000 ml milk carton is about 7 cm square inside, so pouring to a height of 15 cm gives about 735 ml. The batter of oil, lye and water weighs about 1.48 times the oil, so about 500 g of oil is a good guide. Under “Oil from mold size” you can change the height and the number of molds.
- Where can I buy sodium hydroxide in Japan?
- It is sold at pharmacies and drugstores with a pharmacist on duty. Because it is a deleterious substance, bring photo ID and a personal seal (hanko), and fill in a transfer form with your name, address and purpose (such as soap making). It cannot be sold to anyone under 18, and the shop may need to order it in.
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