Question 1

What is a lye calculator and why must I use one?

Accepted Answer

A lye calculator tells you exactly how much sodium hydroxide (NaOH) or potassium hydroxide (KOH) your specific blend of oils needs to turn into soap. Every oil has a different saponification (SAP) value, so guessing gives you either lye-heavy soap that can burn skin or a greasy separated mess. The soapmaking community rule is simple: never make soap without running the recipe through a lye calculator first, and cross-check important batches with a second calculator.

Question 2

How is the amount of lye calculated? (SAP values explained)

Accepted Answer

Each oil has a SAP value: the grams of lye needed to saponify one gram of that oil (for NaOH; divide KOH SAP by 1.4025 to convert). The calculator multiplies each oil's weight by its SAP, adds them up, then subtracts your superfat percentage. Example: 500 g olive oil (0.135) + 300 g coconut oil (0.190) + 200 g shea butter (0.128) needs 150.1 g NaOH at 0% superfat, or 142.6 g at 5% superfat. This tool uses a curated database of 200+ oils with verified SAP values.

Question 3

NaOH vs KOH — which lye do I need?

Accepted Answer

Use NaOH (sodium hydroxide, caustic soda) for solid bar soap and KOH (potassium hydroxide, caustic potash) for liquid soap and soft/cream soaps. They are not interchangeable: KOH needs about 1.4x more by weight for the same oils. Dual-lye recipes (typically 95% NaOH / 5% KOH) are used to boost lather in bar soap; this calculator has a dedicated dual-lye mode that computes each alkali on its own SAP basis.

Question 4

What is superfat (lye discount) and how much should I use?

Accepted Answer

Superfat — also called lye discount — is the percentage of oils deliberately left unsaponified by using less lye than the SAP math demands. It makes bars milder and more conditioning. Typical ranges: 3-5% for everyday body bars, 5-8% for facial bars or extra conditioning. Above about 8-10% the extra free oil can turn rancid (DOS — dreaded orange spots). Zero or negative superfat gives harsh, potentially lye-heavy soap. Hot process makers often add part of the superfat as luxury oils after the cook.

Question 5

How much water should I use? (3 methods + water discount)

Accepted Answer

This calculator offers three equivalent ways to set water: as a percent of oil weight (common default 33-38%), as lye concentration (common default 28-33%, meaning lye divided by lye-plus-water), or as a water-to-lye ratio (common default 2:1). A water discount — less water than default — speeds unmolding and shortens cure but accelerates trace, which can surprise beginners. More water slows trace and is friendlier for intricate swirls, at the cost of longer drying.

Question 6

What is lye purity and hidden superfat?

Accepted Answer

Store-bought lye is rarely 100% pure: NaOH is often around 97% and KOH around 90%, the rest being water and carbonates absorbed from air. If your recipe calls for 142.6 g of 100%-basis NaOH but your flakes are 97% pure, you must weigh 147.0 g. Weigh 142.6 g of the 97% material and your real superfat silently rises from 5% to about 7.8% — the hidden superfat. This tool's purity correction keeps your intended superfat honest.

Question 7

How do I add citric acid to soap? (extra lye needed)

Accepted Answer

Citric acid neutralizes part of your lye (it becomes sodium citrate, a chelator that reduces soap scum), so you must add extra lye to compensate: about 0.624 g NaOH (or 0.876 g KOH) per gram of citric acid. Ten grams of citric acid needs roughly 6.2 g extra NaOH. Forget this step and the citric acid consumes lye meant for your oils, raising the effective superfat and softening the bar. The calculator adds this automatically when you enter citric acid.

Question 8

How do I calculate how much soap fits my mold? (0.40 rule)

Accepted Answer

For a rectangular mold, multiply length x width x height in inches by 0.40 to get the ounces of oils your recipe needs; multiply by 0.70 instead for total batter capacity. A 10 x 3.5 x 2.5 inch loaf mold is 87.5 cubic inches, so it takes about 35 oz (992 g) of oils. For round molds use pi x radius-squared x height x 0.40. You can also fill the mold with water, weigh it, and let the calculator convert. The factor is adjustable because batter density varies.

Question 9

Can I double or halve a cold process soap recipe?

Accepted Answer

Yes — soap recipes scale linearly, so doubling or halving works exactly as long as every ingredient, including fragrance and additives, is multiplied by the same factor. That is what the Batch Scaler tab does, converting between percentages and grams in either direction. One caution: very large batches behave differently in the pot (trace and temperature), so scale up in steps if you are new.

Question 10

How much fragrance or essential oil can I add to soap?

Accepted Answer

Fragrance is measured as a percent of total oil weight. Common usage rates are 3-6% for fragrance oils and 1-3% for essential oils in cold process soap, but the hard limit is each scent's IFRA safe-use rate for wash-off products — always check the supplier's IFRA certificate and stay under it. The Fragrance tab converts between percent, grams, and ounces-per-pound. Never exceed IFRA limits to make a stronger scent; it risks skin sensitization.

Question 11

How long does cold process soap need to cure?

Accepted Answer

Cold process soap typically needs 4-6 weeks of curing in a cool, ventilated spot before use. During cure, water evaporates so the bar gets harder, milder, and longer-lasting. Hot process soap is saponified by the cook and technically safe sooner, but it still benefits from 1-2 weeks of drying. The Cure Date tab turns your made-on date into a ready date. Soap used before its cure is soft, dissolves fast, and can be harsher on skin.

Question 12

What do the soap quality numbers mean? (hardness, INS, etc.)

Accepted Answer

The predictions estimate bar character from the recipe's fatty-acid profile: hardness (saturated acids; aim 29-54), cleansing (lauric+myristic; 12-22 — higher strips more oil), conditioning (unsaturated acids; 44-69), bubbly and creamy lather (14-46 and 16-48), iodine (41-70; lower means a harder, longer-lived bar), and INS (136-165; higher predicts a harder bar). They are formulation guides from published soapmaking references, not lab measurements — always test the actual bar.

Question 13

Is making soap with lye safe?

Accepted Answer

Lye is caustic and demands respect, but millions of hobbyists use it safely with basic precautions: goggles, gloves, long sleeves, and good ventilation; always add lye TO water (never the reverse); mix in a well-ventilated area; never use aluminum containers or utensils (lye reacts with aluminum); keep vinegar nearby for surfaces but flush skin with running water, not vinegar; keep children and pets away. Run every recipe through a lye calculator and cross-check with a second one. This tool is a planning aid, not professional safety advice.

Question 14

Can I use this soap calculator in Indonesia or Malaysia?

Accepted Answer

If you searched for a soap calculator indonesia option — yes, this works for you. It is metric-first: enter oils in grams, mold dimensions in centimeters, and get lye and water in grams. Untuk pembuat sabun Indonesia: superfat sabun adalah persentase minyak yang sengaja tidak tersabunkan (kelebihan minyak) agar sabun lebih lembut — kalkulator ini menghitungnya otomatis, termasuk koreksi kemurnian NaOH yang sering hanya 97%. The same applies to Malaysian makers working in grams. No signup, works offline in your browser.

Question 15

What is TFM in soap?

Accepted Answer

TFM (Total Fatty Matter) is the percentage of a soap bar that is actual fatty matter — the classic quality grade used in India and other markets. Grade 1 toilet soap requires 76% or higher TFM. Handmade cold process soap usually lands around 70-80% because the natural glycerin stays in the bar. This calculator shows a rough TFM estimate for your recipe (cured bar, water evaporated, glycerin retained) so you can compare against commercial grades.

Question 16

Can this calculator make liquid soap?

Accepted Answer

Yes. Switch the Recipe tab to KOH mode, or use the dedicated Liquid Soap tab: it computes KOH for your oils (with KOH purity correction — KOH is commonly only 90% pure), then the dilution step, where paste is typically diluted to around 25-40% soap concentration with distilled water. Liquid soap is usually made at 0-3% superfat and sequestered about two weeks so cloudiness settles. Do not use NaOH for liquid soap — it makes solid bars, not liquid.

Soap Lye Calculator — Lye, Recipe & Mold Suite

Recipe & lye calculator

Oil database — 200+ SAP values

Superfat lab — purity-corrected

Hot-process post-cook superfat split

Soap mold → recipe sizer

Liquid soap (KOH) + dilution

Batch scaler — % ↔ grams

Fragrance / essential-oil usage rate

oz-per-pound ↔ percent converter

Cure-date calculator

How the math works

Frequently asked questions

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