Calculate NaOH or KOH lye, water, oils, fragrance, superfat, and batch size for handmade soap recipes. Supports cold process bar soap, liquid soap, melt and pour, surfactant blends, simple formulas, and bath bombs. Uses the same formula logic as BulkWork Production Suite — saving recipes and production management require the full system.
The calculator gives you the formula. A careful production process gets you a safe, consistent bar.
Work on a clear, stable surface in a well-ventilated area. Keep children, pets, food, and drinks away from your soap-making space. Have paper towels and cleanup materials ready before you start.
Put on safety goggles, chemical-resistant gloves, long sleeves, and closed-toe shoes before handling sodium hydroxide, potassium hydroxide, or raw soap batter. Lye is caustic and can cause serious burns.
Measure oils, water, lye, fragrance, colorants, and additives by weight on a digital scale. Volume measurements are not accurate enough for soap formulas — always use grams or ounces by weight.
Pour your measured distilled water into a heat-safe, lye-safe container. Slowly add lye into the water while stirring. Never pour water onto dry lye. The solution will heat to 65–90°C and may fume — do not lean over it.
Melt solid oils and let all oils cool to your working temperature. When oils and lye solution are both at the right temperature, pour the lye solution into the oils and blend to trace using a stick blender.
Pour batter into the mold, record the batch date and all formula notes, then unmold when firm. Cure bar soap on a ventilated rack for 4–6 weeks before use or sale. Curing completes saponification and evaporates excess water.
This is an educational guide. Always follow your supplier's safety data sheets and local regulations before making or selling soap.
For lye-based bar soap, recipes are scaled from total oil weight. The total batch weight is higher because it includes oils plus water, lye, fragrance, and additives. This calculator shows both.
Fresh soap weighs more than cured soap because water evaporates during the 4–6 week cure period. Plan your packaging weight around cured bars, not the fresh batch total.
Switch to percentage mode in the calculator to lock your formula ratios, then enter a new total oil weight target. The calculator will recalculate all ingredient weights while keeping lye, water, and fragrance percentages consistent.
Everything a home or small-batch soap maker typically needs. Links go to Amazon so you can check current prices and availability.
Affiliate disclosure: This page contains affiliate links to Amazon. If you purchase through one of these links, we may earn a small commission at no extra cost to you. We only link to supplies we would actually recommend.
Food-grade lye for cold process and hot process bar soap. Must be 100% pure — no drain cleaner blends.
View on Amazon →Required for liquid soap. Use 90% KOH flakes and enter the purity in the calculator for accurate lye amounts.
View on Amazon →Minerals in tap water can interfere with the lye reaction. Always use distilled water for consistent, predictable batches.
View on Amazon →Accurate to 0.1 g. Always weigh oils, lye, and water — never use volume measurements for soap making.
View on Amazon →Brings your batter to trace in minutes. A stainless steel shaft is easiest to clean and lye-safe.
View on Amazon →Useful for hand-stirring before switching to the stick blender, and for mixing colorants or fragrance into a small amount of oil first.
View on Amazon →Check lye water and oil temperatures without touching the liquid. Aim for 90–120 °F before combining.
View on Amazon →Loaf or slab molds release without lining. Size your mold to match the total batch weight from the calculator.
View on Amazon →Used to line wooden molds so soap releases cleanly. Freezer paper (plastic side in) works great and is cheap by the roll.
View on Amazon →Mix your lye solution in a dedicated stainless or heavy HDPE pitcher. Never use aluminum — lye reacts with it dangerously.
View on Amazon →For melting and combining your oils. Dedicate these bowls to soap making only — do not reuse for food after lye contact.
View on Amazon →Scrape every bit of batter into the mold and fold in colorants cleanly. Lye-safe and easy to wash. Keep a dedicated set for soap only.
View on Amazon →Non-negotiable when handling lye. Use chemical-splash goggles and nitrile or rubber gloves every single batch, no exceptions.
View on Amazon →Lye will eat through regular clothing. Wear a dedicated long-sleeve apron every time you work with raw lye or fresh soap batter.
View on Amazon →The two most common base oils. Coconut adds lather and hardness; olive adds a mild, conditioning bar. Buy in bulk to save money.
View on Amazon →Even at 5–10% of your oil weight, castor oil significantly boosts lather and helps other oils foam. A staple in most recipes.
View on Amazon →Popular skin-conditioning butters that add creaminess and a harder bar. Use at 5–15% of total oils for best results.
View on Amazon →Use soap-safe fragrance oils rated for cold process. Enter the fragrance weight or percentage in the calculator above.
View on Amazon →A natural alternative to fragrance oils. Lavender, peppermint, and eucalyptus are popular choices and hold well in cold process soap.
View on Amazon →Mica powders and oxide pigments are lye-stable. Mix into a little oil before adding to your batter for even color throughout the bar.
View on Amazon →Spritz on top of freshly poured soap to prevent soda ash. Also useful for cleaning tools and wiping down work surfaces.
View on Amazon →Add to your cooled lye water to help bars unmold faster — especially useful in summer or with high-olive-oil recipes that stay soft longer.
View on Amazon →Used to test whether liquid soap paste is fully saponified before diluting. Look for strips that read in the 7–14 range.
View on Amazon →A wire or blade loaf cutter gives consistent bar thickness. Cut after 24–48 hours once the soap has firmed up enough to hold its shape.
View on Amazon →Kraft belly bands, shrink wrap, or printed labels finish your bars for gifting or selling. Required if selling — check your local cosmetic labeling rules.
View on Amazon →The full BulkWork Production Suite adds saved formulas, production work orders, ingredient inventory, batch scaling, label design, and printable production sheets — all self-hosted on your own server.
A soap calculator is a tool that uses saponification values (SAP values) to calculate the exact amount of lye, water, and other ingredients needed to make a batch of soap. It takes your oil or butter weights, lye type (NaOH for bar soap, KOH for liquid soap), superfat percentage, and water method, then outputs a safe, balanced formula. Without a calculator, lye amounts must be worked out by hand from tables, which increases the risk of calculation errors.
Superfat (also called lye discount) is the percentage of oils left unsaponified — unreacted with lye — in a finished soap formula. A 5% superfat means the lye is calculated to react with only 95% of the oils, leaving 5% free for conditioning. Superfat between 3% and 8% is standard for bar soap. Higher superfat (above 10%) makes soap softer, reduces lather, and shortens shelf life. Lower superfat (below 2%) can make soap feel drying and is more common in commercial formulas.
To calculate lye for soap: (1) list every oil and its weight in grams, (2) multiply each oil weight by its NaOH or KOH saponification value, (3) add all results together to get raw lye needed, (4) multiply by (1 minus superfat ÷ 100) to apply the lye discount, (5) divide by lye purity if your lye is not 100% pure. For example, 500 g of olive oil (SAP value 0.134) at 5% superfat and 100% pure NaOH: 500 × 0.134 × 0.95 = 63.65 g NaOH. This calculator performs all steps automatically from your ingredient list.
A good starting batch for cold process bar soap is 500–1000 g total oil weight, which typically yields 8–14 bars at 100–120 g each after cure. Smaller batches of 300–500 g are easier to manage while learning trace, scent acceleration, and unmolding. Scale up once your formula is tested and stable. For melt and pour, a 500 g base block is a common beginner size. This calculator shows estimated bar yield and total batch weight automatically.
To mix water and lye safely: (1) work in a well-ventilated area and wear goggles, gloves, and long sleeves, (2) measure distilled water into a heat-safe, lye-safe container — avoid aluminum, which reacts with lye, (3) slowly pour lye into the water while stirring — never pour water onto dry lye, which can cause a dangerous surge, (4) the solution will heat to 65–90°C quickly and may release fumes, so do not lean over it, (5) let it cool to your working temperature before combining with oils, and label the container clearly while cooling.
NaOH (sodium hydroxide) produces hard bar soap. KOH (potassium hydroxide) produces liquid or paste soap. NaOH has a higher SAP value efficiency per gram, while KOH is typically sold at 90% purity, so the calculator must account for purity when computing lye weight. You cannot substitute one for the other without recalculating, since they have different molecular weights and saponification values. This calculator supports both and adjusts automatically when you switch lye type.
The free public calculator lets you calculate formulas and print or download results as a work-order summary. Saving recipes, generating saved production work orders, managing ingredient inventory, scaling batch records, and designing labels are features of the full BulkWork Production Suite system.
Learn more in the soap calculator articles, or view the BulkWork Production Suite product page for saved formulas, work orders, inventory, and labels.