How to Use SoapMath

Lye & Oil Calculator for Cold Process, Liquid & Dual Lye Soap

SoapMath is a lye calculator that takes the guesswork out of cold process soapmaking. It calculates the exact amount of lye and water needed for any oil blend, handles NaOH (bar soap), KOH (liquid soap), and dual lye (cream soap) recipes, and generates a complete fatty acid profile and soap quality report for every formula. It contains SAP values for 143 oils and fats.

Step 1: Lye Selection

The first choice in SoapMath is your lye type. This determines what kind of soap you're making.

NaOH — Cold Process Bar Soap

Sodium Hydroxide makes hard bar soap. The most common choice for handmade soap. Bars need 4–6 weeks to cure fully.

KOH — Cold Process Liquid Soap

Potassium Hydroxide makes a soft paste that is diluted with water to become liquid soap. SoapMath adjusts for KOH purity (typically 90%). Enter your KOH purity percentage from your supplier.

Dual Lye — Cream Soap

A blend of NaOH and KOH for cream soap or shaving soap. Set the NaOH/KOH ratio and SoapMath calculates the correct amount of each. Common starting split: 70% KOH / 30% NaOH.

Step 2: Water Calculation

SoapMath lets you set your water in three ways. All three control the same thing — how concentrated your lye solution is.

Water as % of Oils

The traditional way. 38% is standard. Lower % = less water = faster trace, firmer bar sooner. Higher % = more working time, longer cure.

Lye Concentration

What % of the lye solution is lye (the rest is water). 33% is a common benchmark. Higher % means less water — same effect as lower water-as-%-of-oils.

Water:Lye Ratio

Parts of water per part of lye. A 2:1 ratio (2 parts water to 1 part lye) is roughly equivalent to 33% lye concentration.

Water replacements

If you're using milk, aloe vera, tea, or beer instead of plain water, enter the same amount — the liquid is still what dissolves the lye. Freeze milk first and add lye slowly to prevent scorching. You can split your liquid between plain water and a replacement in Step 5.

Step 3: Superfat & Fragrance

Superfat (lye discount) is the percentage of oils intentionally left unsaponified. These free oils remain in the finished bar and contribute skin feel and conditioning.

0–2% Superfat

Cleansing bars, shaving soap. Maximum lather, minimal free oils.

5% Superfat

Standard starting point. Well-balanced bar, good shelf stability.

8–10% Superfat

Conditioning and skin-loving. Shorter shelf life; use antioxidants.

Fragrance / Essential Oil

Typically 1–3% of oil weight. Enter your % and SoapMath calculates the exact weight to measure out.

Higher superfat does not automatically mean a "better" bar — it depends on the oils chosen. A high-oleic oil at 8% superfat will last much longer than a high-linoleic oil blend at 5%.

Step 4: Oil Weight

Set your batch size by entering your total oil weight. You can enter this in grams, ounces, or pounds. All lye, water, and additive amounts in the results scale from this number.

Calculate from Mold Weight

If you know your mold's total capacity, enter the finished soap weight your mold holds and SoapMath back-calculates the oil weight (finished soap is roughly 60% oils).

Step 5: Additives, Colorants & Water Replacements

Optional ingredients. These are added at or near trace (not with lye water), except water replacements which go in the lye solution.

Colorants

Oxides, micas, and ultramarines — dispersed in a small amount of oil before adding at trace.

Clays

Kaolin, French Green — 1 tsp per pound of oils for skin benefit and to help fragrance anchor.

Exfoliants

Oatmeal, coffee grounds, poppy seeds — added at light trace.

Water Replacements

Substitute part or all of your water with milk, aloe vera, tea, beer, or other liquids. SoapMath calculates how much of each to use based on your water setting.

Step 6: Oil Selection

Add oils to your recipe from the dropdown. SoapMath contains 143 oils and fats with both NaOH and KOH SAP values. Enter each oil as a percentage (total must equal 100%) or as a weight.

Help Me Pick Oils

A 4-step wizard that recommends which oils to use based on your preferences (bar hardness, lather type, conditioning level, gentleness) and any restrictions (palm-free, nut-free, animal-free). Start here if you're building a recipe from scratch.

Help Me Pick %

An optimizer that calculates the ideal percentage of each oil in your blend to hit your target bar properties. Best used once you already know which oils you want to use.

Common beginner blend

70% Olive Oil, 20% Coconut Oil, 10% Castor Oil. Good balance of conditioning, cleansing, and lather. Adjust coconut oil up for more cleansing, down for more moisturizing.

Reading the Results

Click Generate Results to get a complete printable recipe. The results include:

Tips for Success

Weight is Everything

Always weigh your lye on a dedicated digital scale. Never use volume measurements for lye — even a small error can result in a lye-heavy bar.

Safety First

Add lye to water, never water to lye — adding water to lye causes a violent volcanic reaction. Always pour lye into your liquid, not the other way around.

Check Stability

Iodine value under 70 is a guideline for a stable bar with good shelf life. Use the Soap Quality section in results to check yours.

Prevent DOS

If your bar develops "dreaded orange spots" (DOS), it's rancidity. Reduce high-PUFA oils (hemp, rosehip), increase antioxidants, or lower superfat.

Frequently Asked Questions

Tip: Open SoapMath and try it now

Open SoapMath and calculate your first lye-safe recipe while following this guide.

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