Soap Calculator

Work out exactly how much lye and water your soap recipe needs, and see how the bar will turn out, for cold process, hot process and liquid soap. It's free, there's no sign-up, and every number updates as you type. Rather leave the lye to us? Shop our handmade soaps.

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How to use this soap calculator

1. Pick your oils and enter each one as a percentage of the total, or switch to weights if you already know them.
2. Choose your lye: sodium hydroxide (NaOH) for bar soap, potassium hydroxide (KOH) for liquid soap, or both for cream soap.
3. Set the superfat and the water. A 5% superfat and a 33% lye concentration are common starting points for bars.
4. Add fragrance and additives, read the notes beside the weigh-out, then weigh exactly what it shows.

What it can do

• Lye and water for cold process soap, hot process soap (including superfat added after the cook), liquid soap with paste dilution, and cream soap made with both lyes.
• 149 oils, butters and fats, plus your own oils using the SAP value from your supplier's spec sheet.
• Bar qualities (hardness, cleansing, conditioning, bubbly and creamy lather, iodine and INS) shown against the usual target ranges, plus the full fatty acid profile.
• Fragrance and essential oil blends, each checked against the IFRA maximum for soap from your supplier's certificate.
• Additives, including the extra lye that citric acid uses up.
• Premixed (masterbatch) lye solutions, sizing a batch to your mould or number of bars, cost per bar, saved recipes, share links and printable recipe cards.

How the numbers are worked out

Every oil needs a different amount of lye to turn into soap, given by its saponification (SAP) value. The calculator uses each oil's SAP value and the standard 1.403 ratio between potassium hydroxide and sodium hydroxide, then applies your superfat and lye purity. Bar qualities come from each oil's fatty acid profile. The calculations are checked against published worked examples and hand calculations.

SAP values are averages and vary a little between harvests and suppliers. Your superfat is the safety margin, so keep it above 1%, and run any new recipe through a second lye calculator before you make it.

Soapmaking terms

Superfat: the share of oils left unsaponified because you use slightly less lye than full saponification needs. It adds a safety margin and makes a milder bar.

Lye concentration: lye as a percentage of the lye solution. 33% is the same as a 2 : 1 water to lye ratio. Less water means faster trace and a shorter cure.

SAP value: how much lye one gram of an oil needs to turn into soap.

Iodine and INS: iodine measures how unsaturated the oils are; high-iodine blends make softer bars that go rancid sooner. INS is the SAP value minus the iodine value.

Masterbatch: lye solution mixed ahead of time, usually 50% lye and 50% water by weight.

Citric acid: helps against soap scum in hard water, but it uses up lye. Each gram of anhydrous citric acid neutralizes about 0.62 g of sodium hydroxide, so the calculator adds that back for you.