soapformulator
EN

The stripe under the menu is your recipe. It moves as you type.

Calculators

Lye calculator

NaOH, KOH or dual lye with purity and superfat. Water as a lye concentration, a percentage of oils or a ratio, all three shown at once. Citric acid and milks handled with real chemistry. Every number has an explanation, and every oil value names its source.

Batch
A recipe is its proportions; this is the batch size. Or size it from your mould, below.
Resize this recipe

—

Resizing multiplies every weight by one factor and leaves the percentages exactly as they are. Nothing is re-derived from a rounded figure, which is how a resize elsewhere in this category once turned 25 oz of palm oil into 857.

Size the batch to a mould
cm
cm
cm
Weigh a filled mould once and calibrate. Whipped soap is lighter.

Oils to fill it with this recipe: — (batch —). The old "in³ × 0.4" rule would say —.

Oils, butters and waxes
Oil%After cook
Total 0
Weigh from an oil master batch

A blend of oils you mix in bulk and keep on the shelf. Give its oils and their shares, and the calculator says how much of it this recipe takes and what to weigh on its own.

Lye
The lye discount. 5 % is the usual bar default; liquid soap 0–3 %.
Food/technical grade is 98–100 %.
Water
33 % is a forgiving default for cold process.
Additives and liquids
Picking one fills in a suggested dose and when it goes in. Lye it takes is added back, and water it brings counts toward the water.
Fragrance or essential oil
Category 9 for bar and liquid soap.
Allergens, from the supplier’s declaration

Paste the allergen declaration for your lot: one allergen per line, its name and then its percent of the fragrance. The ones over 0.01 % of the finished soap go on the ingredient list under the names EU law labels them by.

Words used here:

Colour and design

Split the batter into portions for layers or a swirl, each with its own colours. The calculator weighs each portion and its colours, and writes the steps into the method.

Notes and photos
Saved with the recipe and shown on its page to anyone with the link.
Photos

Measured values from a certificate of analysis

Published saponification values are ranges, because fats vary with origin, season and refining. If your supplier gave you a certificate of analysis for the lot in front of you, enter its measured value and the uncertainty for that oil collapses to it.

ready

Sodium hydroxide (NaOH)
—g

—

Water
—g

—

Lye concentration—% solution
Water : lye— : 1
Water, % of oils—%
Total batch—g
Effective superfat—%

Recipe

Soap qualities

Words used here:

Hardness — 29–54
Cleansing — 12–22
Conditioning — 44–69
Bubbly lather — 14–46
Creamy lather — 16–48
Iodine — 41–70
INS — 136–165
How to read this

The shaded band is the conventional range. These are a community convention summing fatty-acid groups, not a measurement; the formula page says what they ignore.

What if one oil moved? And where the fatty acids come from
Now With the change

Where the fatty acids come from

Each oil’s share of each fatty acid, in percent of all the fatty acids in the recipe.

Make this batch

Log it in your batch log and take what it uses from your stock, in one go.

Fatty acids

Explain every number

    Calculated at soapformulator.com, every value sourced. Check the figures against your own scale. Sodium and potassium hydroxide are caustic: wear eye protection and gloves, and add lye to water, never water to lye.

    The formula, and where it comes from 
    Lye
  KOH₁₀₀  = Σ(oilᵢ × SAPᵢ ÷ 1000)                  # SAP in mg KOH per g
  KOH_sf  = KOH₁₀₀ × (1 − superfat ÷ 100)
  NaOH    = KOH_sf × 39.997 ÷ 56.106 ÷ (purity ÷ 100)
  KOH     = KOH_sf ÷ (purity ÷ 100)
  Dual: KOH share f of the saponification → KOH = KOH_sf × f, NaOH = KOH_sf × (1 − f) × 0.7129

Acids in the pot
  NaOH_add = mass × purity × (n × 39.997 ÷ M_acid)   # citric 0.6246 g/g, acetic 0.6660, lactic 0.4440

Water
  % of oils:          water = oils × p ÷ 100
  lye concentration:  water = lye × (100 − c) ÷ c
  water : lye ratio:  water = lye × r
  liquids:            plain water = max(0, water − Σ liquidᵢ × water%ᵢ)

Qualities (community convention)
  hardness = lauric + myristic + palmitic + stearic
  cleansing = lauric + myristic
  conditioning = oleic + linoleic + linolenic + ricinoleic
  bubbly = lauric + myristic + ricinoleic
  creamy = palmitic + stearic + ricinoleic
  INS = Σ wᵢ × (SAPᵢ − IVᵢ)

Fragrance
  % of oils = fragrance ÷ oils × 100;  PPO (oz per lb of oils) = fragrance ÷ oils × 16

Mold
  batch = V × ρ × fill;  oils = batch ÷ (1 + lye/oil + water/oil + additives/oil + fragrance/oil)
    • AOCS Official Method Cd 3-25, "Saponification Value of Fats and Oils". American Oil Chemists' Society. Defines SAP as mg KOH required to saponify 1 g of sample.
    • IUPAC Commission on Isotopic Abundances and Atomic Weights, "Atomic weights of the elements 2013" (Pure Appl. Chem. 88(3), 2016). Na 22.990, K 39.098, O 15.999, H 1.008 → NaOH 39.997 g/mol, KOH 56.106 g/mol.
    • Dunn, Kevin M. Scientific Soapmaking: The Chemistry of the Cold Process. Clavicula Press, 2010. Chapters on saponification stoichiometry, lye discount and lye purity.
    • Community convention: "full water" = 38 % of oil weight, the default that most hobby calculators have used since the 2000s; roughly a 27–28 % lye solution for a typical bar recipe. — A convention, not a chemical constant. Lye concentration is the better control variable because it does not drift with the oil blend.
    • Neutralisation stoichiometry: citric acid is triprotic (3 OH⁻ per molecule); acetic and lactic acids are monoprotic; disodium EDTA takes 2, sodium bicarbonate 1 (to carbonate) and magnesium sulfate 2 (to magnesium hydroxide). g NaOH per g = n × 39.997 ÷ M.
    • Community convention for soap qualities, in continuous use in hobby lye calculators since c. 2005: hardness = lauric + myristic + palmitic + stearic; cleansing = lauric + myristic; conditioning = oleic + linoleic + linolenic + ricinoleic; bubbly lather = lauric + myristic + ricinoleic; creamy lather = palmitic + stearic + ricinoleic. Recommended bands: hardness 29–54, cleansing 12–22, conditioning 44–69, bubbly 14–46, creamy 16–48, and iodine value 41–70. — A convention, not a measurement. It ignores superfat, water, cure and additives, and treats every fatty acid in a group as equal. The iodine value itself is measured (AOCS Cd 1-25); the 41–70 band beside it belongs to this convention, not to the method.
    • McDaniel, Robert S. Essentially Soap. Krause Publications, 2000. INS = saponification value (mg KOH/g) − iodine value; recommended range 136–165, ideal 160.
    • Empirical default. Fresh cold-process soap batter has a density close to water, ≈ 0.95–1.0 g/mL; the community rule "in³ × 0.4 = oz of oils" implies about 0.69 g/mL of oils, which is ≈ 1.05 g/mL of batter at 38 % water and 5 % superfat. Calibrate by weighing a filled mold once. — Density varies with whipping, fragrance, temperature and how full the mold is poured. Treat the default as a starting point and calibrate.

    How to read the results

    Lye is what you weigh out, after purity. Superfat is applied to the lye, not the oils: all the oils go in, a few percent of them never meet a hydroxide. Water is computed from the lye as weighed. If you replace water with milk, beer or aloe, the plain water shown is what is left after the liquid's own water is counted, and the batch total still includes the whole liquid.

    Citric acid forms sodium citrate in the pot and eats lye doing it: 0.62 g of NaOH per gram. Leave that out and the superfat quietly rises by about four points for every 1 % citric — a batch asking for 5 % lands nearer 9 %. The exact figure follows from the blend, because it is the extra lye as a fraction of what the fats need. The calculator adds it back and shows the step.

    Mould sizing starts from the mould's volume and the batter's density, then splits the batch mass into oils, lye, water, additives and fragrance in this recipe's own proportions. The "× 0.4" rule of thumb assumes full water and 5 % superfat; it drifts as soon as your recipe does not.

    Safety: sodium and potassium hydroxide are caustic. Weigh lye on a scale that reads to the gram, add lye to water and never water to lye, wear eye protection and gloves, and verify any recipe you did not calculate yourself.

    Common questions

    Why do you show water three ways?

    Because they are three ways of stating one quantity, and the most common first-batch confusion is not realising that. Water as a percentage of oils, a lye concentration and a water-to-lye ratio each pin down the same gram figure once the lye is known; change any one and the other two update. The familiar defaults are not the same amount, though: for an everyday olive, coconut and palm bar, water at 38 % of oils is about a 27.5 % lye solution, while a 33 % solution is about 29 % of the oils, or 2 : 1. Lye concentration is the better control: it does not drift when you change the oils.

    Do your numbers match SoapCalc?

    The saponification arithmetic is identical (NaOH = oil × SAP × 40.0/56.1 × (1 − superfat)). Where results differ, it is the oil values: we keep the published range for each oil and calculate with a representative value, and every oil page names the document the value came from. Open the "Explain every number" drawer to see exactly what was used, and the oils reference to compare.

    I replaced the water with milk and another calculator said the liquid required was negative. What happened?

    Milk is about 88 % water, so 300 g of milk carries 264 g of water. If the recipe only calls for 250 g of water, a calculator that subtracts gets a negative number. This one tells you the overshoot instead, and counts the milk's fat toward the effective superfat.

    What is a master batch, and how do I use one here?

    A master batch is lye dissolved ahead of time at a known strength, usually 50 %: equal weights of lye and water. In the Lye section, weigh the lye as Master batch and enter its strength. The recipe then lists the master batch to weigh and the extra water to add, and the soap stays the one you built. With dual lye the master batch holds the NaOH and the KOH is weighed dry.

    Which lye concentration should a beginner use?

    33 %. It traces at a manageable pace for cold process, gels predictably, and shrinks less during cure than "full water" at 38 % of oils, which is about a 27.5 % solution for a typical bar recipe. Go weaker (28–30 %, more water) for designs that need slow trace, unless the recipe is mostly olive oil, which can separate; hot process usually takes more water too. Go stronger (35–40 %) for firmer unmoulding, less soda ash and a shorter cure, and expect faster trace.

    Why does the total batch not match my own arithmetic?

    It should, to the gram: batch = oils + lye as weighed + plain water + every additive at its full mass (a milk's water included; a colour infused into the oils and strained out adds nothing) + fragrance. Some calculators leave custom liquids out of the total. Open the explain drawer and the "Total batch" step shows the sum.

    My supplier gives the fragrance rate in oz PPO. What is that?
    PPO means ounces per pound of oils, a ratio by weight: 1 oz per pound is a sixteenth of the oils, 6.25 %. So 0.7 oz PPO is 4.4 % of the oils, and 5 % of the oils is 0.8 oz PPO. Each scent’s PPO shows under its load, and an additive measured in spoons shows its spoons per pound of oils, the way US books give them.

    Rate this tool

    No account needed. We keep the stars and nothing about who gave them.

    Goes well with

    All tools
    Oil & butter reference Saponification values, iodine, fatty acids — as published ranges with the source on every row
    Formulas & sources The exact equations behind every result, with citations

    Hear when something new ships

    A short note whenever something new ships. No newsletter, nothing sold, one click to stop.