Using a lye calculator: every number explained

A lye calculator does one job that no one should do by eye: it works out exactly how much lye a particular set of oils needs, and how much water to dissolve it in. Ours does a good deal more than that, and every figure on the page can look like something you must act on. Most of them are not. This guide goes through the lye calculator field by field and result by result, using a real recipe, so you know which numbers to weigh, which to read and which to leave until later.

Why every recipe goes through a calculator

The lye in a recipe is correct only for the oils in that recipe, at those weights, with that lye. Change any of them and the right figure changes too, so a recipe is only as trustworthy as the arithmetic behind it.

Printed recipes contain mistakes. Among the soapmaking books we reviewed for this site, one printed about 30 % more sodium hydroxide than its oils could use at all, and others gave no lye figure whatever. Treat any recipe from a book, a friend or a forum as a starting point, and check it.

Substitutions catch people out even when the recipe was right. Swap the coconut oil in our everyday bar for olive oil, keep the old lye figure, and the bar that was meant to have a 5 % superfat carries free lye; the oils guide has the full table. The two lyes are not interchangeable either. Weigh the everyday bar’s 144.1 g of lye as 90 % potassium hydroxide instead of sodium hydroxide and you get a superfat of about 39 %, a soft mess; do the reverse with a potassium recipe and the soap is badly lye-heavy.

If a recipe is already written down somewhere, the importer reads it pasted as text, from a spreadsheet or from another calculator, shows you every line it understood and every line it did not, and opens the result in the calculator.

The recipe we will use

Our everyday bar is the most repeated beginner formula there is: olive oil 40 %, coconut oil 30 %, palm oil 30 %. We will use 1000 g of oils and leave every other setting at its default. (Our classic olive, coconut and palm bar is the same idea with 5 % castor oil, written up with a method and a photograph.)

The form has six numbered sections, in the order a recipe is written.

1. Batch

Units switches between grams and ounces, converting every weight on the page. Grams are the better choice: a gram is a finer step than a tenth of an ounce.

Oils entered as sets how you type the oils. In percent, you give each oil’s share and a Total oil weight, which is the batch size (1000 g by default). In weight, you type each oil’s own weight and the total field disappears. Switching converts what is already there, so the recipe stays the same.

Process is Cold process or Hot process. For a first batch, leave it on cold process; the hot process guide explains what the other setting does.

2. Oils, butters and waxes

+ Add oil opens a search: start typing and pick the oil you actually have. Type its share or weight in the row. In percent mode the total under the list has to reach 100 % (it says “adds up”); until it does, the page shows “needs 100 %” and calculates nothing, rather than guessing.

Every oil comes with its published saponification (SAP) range, the lye it needs per gram. The calculator works with a representative value, usually the middle of the range, and carries the whole range through to the uncertainty panel described below. Each oil’s page in the oils reference names the document its range comes from.

3. Lye

Lye is NaOH · bar (sodium hydroxide, for bar soap), KOH · liquid (potassium hydroxide, for soft and liquid soap) or Dual, a mix of the two for shaving and cream soaps, with a KOH share field. A first bar uses NaOH.

Weigh the lye as is Dry for beads or flakes, or Master batch if you keep lye already dissolved at a known strength. Beginners weigh dry lye.

Superfat, % is the lye discount: at 5 %, the lye is enough for 95 % of the oils and the rest stays in the bar as oil. Five per cent is the default and the usual figure for a bar. Each point of superfat is 1 % of the lye the oils could take, here about 1.5 g:

Everyday bar, 1000 g of oils, water at a 33 % lye concentration, from our calculator.

Choose KOH · liquid and the superfat drops from 5 to 3 % by itself, because liquid soap is made leaner. The superfat guide covers where to set it for other soaps.

NaOH purity, % defaults to 100, and the note under it puts food and technical grade sodium hydroxide at 98 to 100 %. If your supplier states a purity, enter it and the calculator weighs out more to make up the difference. At 98 % the everyday bar needs 147.1 g. Getting it wrong in either direction moves the superfat by about two points: leave the field at 100 % with 98 % lye and you get 6.9 % instead of 5 %, which is harmless; enter 98 % when your lye is really 100 % and you get 3.1 %.

KOH purity, % defaults to 90, because potassium hydroxide flakes are usually 85 to 95 % pure. Set it to 100 and the calculator warns you that the recipe will probably come out short of lye.

4. Water

Specify water as gives three ways to say how much water goes in, and they describe the same quantity:

• Lye concentration is the lye as a share of the lye solution: 33 % means 33 g of lye in every 100 g of solution.
• % of oils is the water as a share of the oil weight, the older convention.
• Water : lye is the ratio of the two weights.

Switching method puts that method’s usual starting value in the box: 33 %, 38 % or 2 : 1. Whichever you use, the results show all three:

Everyday bar, 1000 g of oils, 5 % superfat, from our calculator.

100 g of sodium hydroxide25 %300 g · 3.00 : 1slow; ash and shrinkage likelier28 %257 g · 2.57 : 130 %233 g · 2.33 : 133 %203 g · 2.03 : 1our default, the everyday middle35 %186 g · 1.86 : 140 %150 g · 1.50 : 1a water discount: faster trace45 %122 g · 1.22 : 150 %100 g · 1.00 : 1master batch; crystallises < 12 °CWater as 38 % of the oils, the old default, is about a 27 % solution for a typical recipe whose lye is 14 % of its oils.

We default to lye concentration because it stays put when the oils change. “38 % of oils” is a 27.5 % solution for the everyday bar but a 25.3 % solution for pure olive oil and a 31.4 % one for pure coconut, since those oils need different amounts of lye. A 33 % solution traces at a pace a beginner can manage, gels predictably, and shrinks less during cure than the old 38 % of oils. The calculator notes anything under 25 % as dilute and warns above 40 %, where trace comes fast and bars can crumble. The water guide goes further.

5. Additives and liquids

+ Add an additive opens a list of additives and liquids. Picking one fills in a suggested dose and the stage it goes in at, both of which you can change. The chemistry is handled for you: citric acid, for instance, uses up 0.62 g of sodium hydroxide per gram, which the calculator adds back, and a milk or juice counts its own water toward the recipe’s water. A first batch needs nothing here. The additives guide and the liquids guide cover the options.

6. Fragrance or essential oil

Pick from the list offers the essential oils we have data for, or Other fragrance oil for one of your own. Picking one fills in a Load, % of 3 % of the oils, where suppliers’ rates for soap begin, or less if the oil’s IFRA limit for soap is lower; the note under the field says which. Basis switches the load between of oils and of batch. IFRA max, % takes the category 9 limit (bar and liquid soap) from your supplier’s IFRA certificate, and the allergens from the supplier’s declaration can be pasted in below so that they reach the ingredient list.

At 3 % of the oils, the everyday bar takes 30.0 g of fragrance, which is 2.05 % of the finished 1466.7 g batch. The scenting guide explains limits and allergens.

Below the form

Three folded panels sit under the six sections. Measured values from a certificate of analysis takes the SAP value your supplier measured for the lot you bought, which removes that oil’s uncertainty. Resize this recipe scales it to a new oil weight, a new batch weight or a number of bars of a given weight, multiplying every amount by the same factor so the percentages cannot drift. Size the batch to a mould works out the oils from a mould’s size: its default loaf of 25 × 9 × 6 cm holds 939.6 g of oils of the everyday bar, where the old rule of thumb of 0.4 oz of oil per cubic inch would say 934.2 g.

Reading the results

The two figures you weigh

Sodium hydroxide (NaOH) is 144.1 g. It is the lye as you weigh it, purity already allowed for, and the line under it repeats the superfat. Water is 292.6 g, with its share of the oils (29.3 %) underneath. Those two figures and the oil weights are the recipe.

The tiles

Lye concentration (33.0 % solution), Water : lye (2.03 : 1) and Water, % of oils (29.3 %) are the same water three ways. Total batch (1436.7 g) is everything in the pot, which is what the mould has to hold. Effective superfat (5.0 %) matches the superfat unless milk fat or an oil added after a hot process cook raises it.

Check these

If anything in the recipe needs a second look, a list appears here, each line marked stop, check or note. A negative superfat is a stop; a very weak or very strong lye solution, a potassium hydroxide purity of 100 %, a liquid carrying more water than the recipe wants, or a fragrance over its IFRA limit each get a line. The everyday bar at its defaults has none, so the list stays hidden. Read it before you weigh anything.

Recipe

The list you weigh from: each oil with its grams and share, then the lye, the water, any additives and fragrance, and the total batch.

Soap qualities

Seven bars, each against a shaded band. For the everyday bar they read Hardness 45 (band 29–54), Cleansing 21 (12–22), Conditioning 51 (44–69), Bubbly lather 21 (14–46), Creamy lather 25 (16–48), Iodine 52 (41–70) and INS 161 (136–165). The first five are sums of the blend’s fatty acids; iodine and INS come from the oils’ iodine and saponification values. The arithmetic is exact, but the idea that a good bar sits inside those bands is a long-standing community convention. Treat the bands as a sensible target for an everyday bar: pure olive oil castile scores “too soft” and is one of the oldest soaps there is. If a value falls outside its band, a Suggest a swap list offers oils to trade at the same weight, and the lye is worked out again for the new blend. The fatty acids guide and the iodine and INS guide explain what each number counts.

How well is this pinned down?

This panel shows how far the published data can be trusted. A SAP value is a range, because the same oil varies with where and how it was grown and refined, and the calculator has been using the middle of each range. Your bottle could sit anywhere in it. For the everyday bar the panel reads:

• ± 3.7 % on the lye, in the heading.
• Superfat you asked for: 5.0 %.
• Worst case, if the oils saponify low: 1.4 %.
• Best case, if they saponify high: 8.4 %.
• Lye the published ranges allow: 138.8 g – 149.4 g.

The worst case takes every oil at the bottom of its range at the same time, which real oils do not do together, so it is a limit rather than a likely result. It is the reason the superfat exists. Set the everyday bar’s superfat to 3 % and the worst case drops to −0.7 %: the panel then says could be lye-heavy and names 3.7 % as the lowest superfat at which the published data can rule that out. At 5 %, the worst case is still soap. A measured value from a certificate of analysis replaces an oil’s range with a single figure and narrows the band.

Further down

How to make this is a method built from what is in your recipe, so an additive that needs care brings its own step. Cost prices the batch once you have priced your ingredients on the cost page. Make this batch logs it in your batch log if you are signed in. Ingredient list drafts the INCI list for the EU, Great Britain, the US, Canada or Australia, as a draft for your safety assessor rather than a finished label. Fatty acids lists the blend’s profile, and Explain every number shows every step of the arithmetic.

Saving, printing and sharing

The calculator keeps your working recipe in your browser, so a reload or a closed tab loses nothing (a private window is the exception). Start fresh clears it and goes back to the starter recipe, our first-batch palm-free bar.

• Copy a link to this recipe copies a web address with the whole recipe inside it. Anyone who opens it sees the same recipe, and nothing is stored on our side.
• Copy as text copies the recipe as plain text, with every weight, the settings, the qualities and the link, ready to paste into notes or a message.
• Print gives a clean sheet for the bench, with spaces to write the date made, a batch code and your initials.
• Save and get a link stores the recipe and gives you two links: a link to share, which shows the recipe without letting anyone change it, and your edit link. Keep the edit link: it is shown once, and it is the only way to change that recipe later. No account is needed. If you are signed in, a saved recipe joins the community recipes under your public name, unless you choose Private.

Before you weigh

Run down the page once more before anything goes on the scale:

• The oils are the ones in your cupboard, including the grade, and in percent mode they add up to 100 %.
• The lye type matches the tub, and its purity is what the label says.
• The superfat is 5 % unless the recipe gives a reason for something else.
• The water method and value are what you meant.
• Nothing is waiting under Check these.

Then print the recipe or copy it to your notes, and take it to the bench. The first batch guide picks up from there.