Calculate exact lye (NaOH or KOH) and water amounts for cold process and hot process soap — with built-in superfat and water ratio controls.
Saponification is the chemical reaction between a fat or oil and a strong alkali (lye) that produces soap and glycerin. Every oil has a unique saponification value — often called a SAP value — which tells you exactly how many ounces of lye are needed to fully saponify one ounce of that oil. Coconut oil, for example, has a high SAP value of 0.190 because its fatty acid chains are shorter and require more lye per ounce than long-chain oils like olive (0.134). This calculator multiplies your oil weight by its SAP value, sums across all oils in your blend, then applies your superfat discount and generates the final lye and water numbers.
Superfat (also called lye discount) is the percentage of oils you intentionally leave unsaponified in your finished soap. A 5% superfat means you use 5% less lye than the full saponification amount, leaving 5% of your oils as free fatty acids in the bar. This creates a more moisturizing, skin-conditioning soap and also provides a safety buffer against weighing errors. Most homestead soapmakers use 5% as their standard. Go lower (0–2%) for laundry soap or shampoo bars where you want maximum cleansing. Go higher (7–10%) for facial soap, baby soap, or very dry skin. Superfat above 10% can shorten shelf life because unsaponified oils eventually go rancid.
Bar soap uses sodium hydroxide (NaOH, also called caustic soda or lye) which creates a hard, solid bar. Liquid soap uses potassium hydroxide (KOH, or potash lye) which produces a paste that dissolves into liquid soap when diluted with water. KOH SAP values are simply NaOH values multiplied by 1.403. KOH is typically sold at 90% purity, so if your KOH is 90%, divide your calculated KOH amount by 0.90 to get the real-world weight you need to measure. This calculator assumes 100% purity — check your supplier's purity spec and adjust accordingly.
The water in a soap recipe is not part of the saponification reaction itself — it's the solvent that dissolves the lye so it can mix with the oils. More water means a thinner lye solution that gives you more working time before the batter reaches trace. Less water (water discount) produces a faster trace, harder unmolded bars, and faster cure time, but can be tricky for swirled or intricate designs. A 33% water-to-oil ratio is a good working default. Experienced soapmakers doing milk soaps, salt bars, or castile (100% olive) often adjust water significantly.
Cold process soap must cure for 4–6 weeks after unmolding. During this time, residual saponification completes, excess water evaporates, and the bar hardens and mildifies. Cutting too early produces soft bars with a high pH that can burn skin. Hot process soap (cooked in a slow cooker or oven) finishes saponification during cooking and can be used within days, though a 1–2 week rest still improves bar quality and hardness. Both methods use the same lye calculations.
It depends entirely on which oils you use. For a typical 32 oz (2 lb) batch using a blend of coconut oil and olive oil at 50/50, you'd need approximately 4.3 oz of NaOH with a 5% superfat. Use this calculator with your exact oil blend to get the precise amount — small errors in lye measurement directly affect finished soap quality and safety.
Excess lye — called "lye heavy" soap — produces a bar with an elevated pH that can cause skin burns and irritation. Signs include white ash on the surface, separation of liquid from the batter, or a "zap" when you touch the finished bar to your tongue (a mild tingle means residual lye; a strong sting means the soap is not safe). This is why accurate measurement with a digital scale and a reliable lye calculator matters so much.
Yes, but you must recalculate your lye amount whenever you change the oil lineup, because every oil has a different SAP value. Swapping olive oil for sunflower oil in equal amounts requires only a tiny lye adjustment (0.134 vs 0.134 — they're the same), but swapping olive for coconut changes your lye requirement significantly. Always recalculate rather than assuming a 1:1 swap is safe.
Trace is the point at which your soap batter has emulsified enough that a drizzle of batter poured over the surface leaves a visible trail (like a "trace") rather than sinking in immediately. Light trace is the consistency of thin pudding — ideal for intricate swirls. Medium trace is like yogurt. Heavy trace is like mashed potatoes — good for textured tops but moves fast. Stick blender in short bursts gives you the most control.
A water discount means using less water than the traditional full-water amount (typically 38% of oil weight). Using 33% water instead of 38% is a 13% reduction in water, producing harder bars that unmold sooner and cure faster. For most homestead soapmakers this is a simple quality-of-life improvement. Avoid water discounts when making soap with a high percentage of oils that trace quickly (coconut, palm) unless you're experienced, as the batter can seize before you finish pouring.