Total Oil Weight
1,000g
About 35.3 oz of oils in this batch.
Quick Start
Load a sample recipe
One click loads a proven beginner recipe with standard superfat. Then fine-tune oils, lye, and costs for your own batch.
Step 1
Lye & liquid settings
5% is standard for body bars. 0% is for laundry/cleaning soap. Superfat leaves extra oils unsaponified for mildness and a safety buffer.
33% is standard. Higher % = harder bar faster, but accelerates trace. Lower % = more water and a slower unmold.
NaOH makes hard bars. KOH makes liquid or soft soap. They are never interchangeable.
%
% of total oil weight. 3% is standard for body bars; check your fragrance maker’s IFRA maximum.
Step 2
Oils & butters
Pick oils from the built-in database — SAP values are already filled in. Values convert automatically when you switch modes.
Enter amounts as
g
Your percentages are multiplied against this total. A common beginner batch is 900–1000 g of oils.
Recipe total: 100%
Register a custom oil or butter
Find the NaOH SAP value on your supplier’s documentation. The KOH value is auto-derived (NaOH × 1.403), the industry-standard conversion.
Step 3
Cost settings (for business sellers)
Optional, but essential for Etsy or market sellers. Defaults shown are examples — enter your real costs for accurate COGS.
$
Step 4
Get your recipe & lye
Results update automatically as you type. Saving keeps your recipe on this device only.
Results
Your batch recipe & costs
Lye (NaOH)
141.6g
Weigh to 0.1 g. About 5.0 oz after superfat discount.
Water / Liquid
287g
≈ 287 ml · 10.1 oz at 33% concentration.
Fragrance Oil
30g
About 1.1 oz at 3% of oil weight.
Total Batch Weight
1,459g
Oils + lye + water + fragrance.
Estimated Bars
12 bars
Based on a standard 4 oz / 113 g mold.
Total Batch Cost
$30.85
Materials + packaging.
Cost Per Bar
$2.57
Materials + packaging only.
Notes & Warnings
These are mathematical estimates based on standard SAP values. Always weigh ingredients on a 0.01 g scale, verify with pH strips, and test every new recipe before use or sale.
Plain English
Why soap math works this way
Workshop
Your shopping checklist
The core kit that makes consistent, skin-safe soap possible. No aluminum anywhere in your setup.
Supplies
Recommended Soap Making Gear & PPE
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Learn
The soap maker’s guide
How Saponification Works: The Chemistry of Soap
Every oil or butter is made of triglycerides — glycerol molecules with three fatty acid chains attached. When sodium hydroxide meets those fats, it breaks the bonds and converts them into glycerol and fatty acid salts: soap. This reaction only works at precise ratios, which is why every soap recipe is a chemistry problem first and a craft second. SAP values express how many grams of lye fully saponify one gram of a specific fat — coconut oil (0.178 NaOH) needs far more lye than olive oil (0.135 NaOH) at the same weight. It’s the same weigh-don’t-guess ratio discipline we use in our Coffee Brew Ratio & Recipe Calculator.
Why You Must Never Use Aluminum Pots for Soap Making
Sodium hydroxide dissolves aluminum in a vigorous reaction that releases hydrogen gas. An aluminum pot filled with lye solution will pit, darken, and can eat holes in itself while filling your workspace with flammable gas and contaminating your soap with metal salts. Safe materials: stainless steel, heat-safe borosilicate glass, polypropylene or HDPE plastic, and silicone. If a pot is lightweight and non-magnetic, assume it’s aluminum and keep it away from lye.
Understanding SAP Values and Superfatting
A SAP value tells you how much lye fully reacts with a gram of oil. Running a recipe at exactly that amount is “zero superfat” and leaves no margin for error — a slightly off scale reading could mean leftover, harsh lye in your bar. Superfatting discounts the lye by a set percentage (this calculator’s slider). At 5% superfat you use 5% less lye than the math demands, guaranteeing some oils remain untouched: a milder, more conditioning bar with a built-in safety buffer. Body bars typically run 4–8%. Because everything is expressed as percentages of oil weight, scaling a recipe up or down is trivial — the same logic as our Baker’s Percentage Calculator.
Cold Process vs. Hot Process Soap Making
Cold process (CP) mixes oils and lye water at modest temperatures (often 35–45 °C), then lets the reaction complete inside the mold over 24–48 hours. It produces a smooth, swirl-friendly batter, but bars need 4–6 weeks of curing to harden and mellow. Hot process (HP) cooks the batter until saponification finishes before molding. The texture is rustic and opaque, but the soap can be used much sooner since heat drives the chemistry to completion. This calculator’s lye math applies to both methods.
How to Safely Handle and Store Sodium Hydroxide
Buy lye in clearly labeled HDPE containers and keep it there — sealed, dry, and out of reach of children and pets, ideally in a locked cabinet. Lye absorbs moisture from the air and cakes, which makes accurate weighing harder and increases splash risk. When mixing: work in a ventilated area or outdoors, wear goggles and nitrile gloves, add lye to water slowly while stirring, and never lean over the container — the solution can briefly exceed 90 °C. Keep running water nearby; for skin contact the only correct first aid is copious water, never vinegar. If you’re building curing racks, a dedicated soap station, or a market booth for your batches, our Woodworking Cut List & Board Foot Calculator and Deck & Fence Material Calculator can estimate the materials for those builds.
FAQ
Frequently asked questions
What happens if I put water into lye instead of lye into water?
The reaction is violently exothermic. Water poured onto a pile of lye can flash-boil at the contact point and erupt, spraying concentrated caustic liquid and steam out of the container. Always add lye flakes or pearls slowly into the water while stirring gently, in a ventilated area, wearing full PPE.
Why is my soap seizing or ricing when I add fragrance?
Certain fragrance oils — especially florals, spice blends, and anything alcohol-heavy — accelerate trace dramatically or cause the batter to curdle (“rice”). Soap at a lower temperature, stir in the fragrance by hand or with very brief pulses at light trace, and check reviews for that specific scent. If it rices, a few firm stick-blender passes usually bring it back together.
What is the difference between NaOH and KOH?
Sodium hydroxide (NaOH) produces hard, solid bar soap. Potassium hydroxide (KOH) produces soft or liquid soap that is later diluted with water. They have different SAP values and are never substitutes for each other. Select the correct one in Step 1 and this calculator applies the right values automatically.
How long does cold process soap need to cure?
Most recipes cure 4–6 weeks on a rack with airflow. During cure, excess water evaporates, the bar hardens, lather improves, and the pH mellows slightly. High-olive soaps like Castile benefit enormously from 6–12 months — the long wait is why true Castile is prized.
Can I use vinegar to neutralize a lye burn?
No. Neutralizing a caustic burn with an acid triggers its own heat-producing reaction on already damaged tissue and can make the injury worse. The correct first aid is to flush with copious amounts of running water for at least 15 minutes, remove contaminated clothing, and seek medical care for anything beyond a minor splash. Vinegar is only useful for cleaning equipment residue after a batch is fully saponified.
How do I know if my soap is lye-heavy and safe to use?
Test cured soap with pH strips (aim for roughly pH 8–10) using a small sample dissolved in distilled water, or perform the traditional “zap test” — a lye-heavy bar gives a sharp electric-like tingle on the tongue. Any zap, stinging on skin, or pH above 10 means the batch should not be used on the body. Accurate scales, a reliable calculator, and a sensible superfat are your three layers of protection.
What is “ash” on top of soap and how do I prevent it?
Soda ash is a harmless white, powdery film of sodium carbonate that forms when unsaponified lye on the surface reacts with carbon dioxide in the air. It’s cosmetic, not dangerous. Prevent it by spraying the poured surface with 99% isopropyl alcohol, covering or insulating the mold, and avoiding very cool soaping temperatures. Existing ash can be steamed off or planed away.
Can I substitute one oil for another without recalculating lye?
Never. Every oil and butter has its own SAP value. Swapping, say, coconut for palm at the same weight changes the lye requirement significantly, and the result can be a lye-heavy bar that burns skin. Any change — new oil, different weight, different superfat — means recalculating the whole recipe.
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Multi-batch inventory for oils, lye & fragrance
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