Soap lye calculator
Formulation guide
From oils to batch sheet
Each step mirrors a real cold-process session—and matches the labels in the studio above.
-
Choose your fats
Start with Starter Formulations for a balanced base, or add oils from the Botanical Library. Adjust percentages in the Active Recipe Ledger until the blend totals 100%.
In the calculator Starter Formulations · Botanical Library · Active Recipe Ledger
-
Size your batch
Enter Total Oils under Formulation Base, or open the Mold Volume Calculator in the ledger to estimate oil weight from a rectangular mold’s length, width, and height (SoapCalc 0.40 oz/in³ oil factor).
In the calculator Formulation Base · Mold Volume Calculator
-
Set superfat and water
Under Additives, set your superfat (lye discount). Five percent is our default for a balanced bar; a higher superfat leaves more conditioning oils in the finished soap. Adjust water under Lye Solution if you work by ratio or concentration instead of percent of oils.
In the calculator Lye · Lye Solution · Additives
-
Print, email, or save
When the oil list totals 100%, open Recipe Summary for oils, lye, water (g and ml), and batch total in g / oz / lb. Print a batch sheet, email a copy, or Save to My Recipes when you are signed in.
In the calculator Recipe Summary · Print Recipe Sheet · Email Recipe · Save to My Recipes
Your recipe book
My Recipes
A free account keeps every formula in one place—reopen, refine, print, and share without rebuilding from scratch.
-
Save and reopen
Store formulas from the calculator. Open any saved recipe to tweak oils, superfat, or batch size, then update with one click.
-
Print from your book
Print a batch sheet directly from My Recipes when you are ready to weigh at the bench, without rebuilding the formula.
-
Photos and shared links
Attach a photo of cured bars to remember how a batch turned out. Share a read-only link when you want students or friends to view a formula.
Questions
Frequently asked
Clear answers on lye math, saving recipes, and mold sizing.
How is lye calculated for cold process soap?
Multiply each oil weight in grams by its NaOH saponification value (SAP), sum the results, then multiply by (1 minus your superfat percentage). Terra rounds lye up to the nearest 0.1 g for safety.
Is this calculator only for solid bar soap?
This tool is designed for cold-process bar soap using sodium hydroxide (NaOH). An optional potassium hydroxide (KOH) mode is available for liquid soap formulations, with KOH-specific SAP values and a 90% purity adjustment. Always match your alkali type to the soap you are making.
Can I save my custom recipes?
Yes. Sign in to save a recipe to My Recipes, then reopen it in the calculator, update the formula, print a batch sheet, or duplicate it for a new batch. You can also add a photo of your cured bars and share a read-only link when you choose.
What is a typical superfat percentage for bar soap?
A superfat (or lye discount) of 5% to 8% is standard for most body bars. This leaves a safe portion of unsaponified oils to provide moisture and conditioning, ensuring the soap is gentle on the skin.
How does the mold calculator work?
Enter your mold length, width, and height in centimetres in the Mold Volume Calculator. Terra estimates total oil weight from cavity volume using the SoapCalc rule of about 0.40 ounces of oils per cubic inch (≈0.69 g/cm³), which accounts for water and lye in the batter—not pure oil density. Bar thickness is optional cut size only. For round or irregular molds, set total oils manually or measure a rectangular equivalent.