Cold Process Soap Lye & Batch Size Calculator

Step 1 — Oils & Butters

SAP values sourced from established soap-making references (Maurer 2010, Cavitch 2001). All calculations run in your browser — no data is sent to any server.

Step 2 — Lye Type & Superfat

5% superfat is a common starting point for skin-safe bar soap. Water:lye ratio of 2.0 means 2 g water per 1 g lye — range 1.5–2.5 covers most recipes. KOH 90% accounts for typical commercial purity.

Step 3 — Mold Size (optional)

Enter mold dimensions to calculate how much oil you need to fill it, or leave blank to use the weights above.

Recipe Output

Saved Recipes

How it works

Cold process soap is made by combining oils or butters with a lye solution. The lye (NaOH for bars, KOH for liquid) reacts with fats in a process called saponification. Each oil has a SAP value — the grams of NaOH needed to fully saponify 1 gram of that oil. A superfat leaves a small percentage of oils un-saponified for extra skin feel.

Example: 500 g olive oil (SAP 0.134) at 5% superfat needs 63.65 g NaOH and 127.3 g water (2:1 ratio) → total batch 690.95 g. Mold volume from dimensions is used to scale the recipe if you choose.

Frequently asked questions

What is a SAP value and why does it matter?

SAP (saponification value) is the number of grams of NaOH (or KOH) needed to fully convert 1 gram of a specific fat or oil into soap and glycerin. Every oil has a different fatty-acid profile, so the SAP value varies — coconut oil (0.190) needs much more lye than olive oil (0.134). Using the wrong SAP value produces soap that is either lye-heavy (harsh, caustic) or heavily superfatted (soft, rancid). This calculator uses values from peer-reviewed soap-making references.

What superfat percentage should I use?

A 5% superfat is the standard starting point for most bar soaps — it leaves enough free oils for a mild, moisturizing feel while keeping the soap firm and stable. Go higher (8–10%) for facial or baby soaps that need extra gentleness; go lower (1–3%) for shaving soaps or shampoo bars that need a cleaner lather. Never go to 0% unless you are making cleaning soap (laundry, dish) — a small margin of safety also compensates for any weighing imprecision. Liquid soap made with KOH is often made at 3–5% superfat to avoid a cloudy finish.

Should I use NaOH or KOH, and what is the 90% purity adjustment?

NaOH (sodium hydroxide) produces hard bar soap. KOH (potassium hydroxide) produces soft paste or liquid soap. Commercial KOH is typically sold at 90% purity, not 100%, so you need to use more of it per gram of oil. This calculator automatically applies the factor: KOH amount = NaOH equivalent × 1.559 (which is the MW ratio 56.11/40.00 divided by 0.90 purity). If you have 100% pure KOH, multiply the KOH result by 0.90 to adjust back down.

How does the mold size calculator work?

Enter your mold's inner dimensions in centimeters. The calculator estimates the required total batch weight using a soap density of 0.90 g/cm³ — this accounts for the fact that cured soap is slightly lighter than water. The oil weight ratio is then back-calculated from your chosen oil blend percentages to scale the whole recipe to fill the mold exactly. If you leave the mold blank, the calculator uses the weights you entered directly.

Can I save and reuse my recipes?

Yes. Click "Save recipe" to store the current recipe in your browser's localStorage — it stays there until you clear your browser data. You can load any saved recipe back into the calculator with one click, export all saved recipes as a JSON file for backup, or print the current recipe card. No account or internet connection is needed; all data stays on your device.