Soap Making Lye (Saponification) Calculator

Enter the weight in ounces of every oil in the recipe and leave unused oils at 0. Set a superfat (5% is the usual starting point) and a lye concentration (30% to 33% is typical for cold process), then read the lye, water and total batch weight.

For the three-oil bar above, the calculator works oil by oil: olive 12 × 0.1353 = 1.6236 oz, coconut 10 × 0.1910 = 1.91 oz, palm 10 × 0.1420 = 1.42 oz. That sums to 4.9536 oz, the lye needed to saponify every gram of oil. A 5% superfat keeps 95% of it: 4.9536 × 0.95 = 4.71 oz NaOH.

At 30% concentration the lye is 30% of the lye-water solution, so the solution weighs 4.706 ÷ 0.30 = 15.69 oz and the water is the rest, 10.98 oz. Add 32 oz oils, 4.71 oz lye and 10.98 oz water for 47.69 oz, about 3 lb of raw soap before curing. The per-oil lye breakdown in the results shows coconut oil carrying the largest share of the lye even though olive oil is the heaviest ingredient.

Lye before superfat = Σ(weight of oil *i* × SAP of oil *i*). SAP values here are ounces of NaOH per ounce of oil, so the same numbers work in grams per gram.

Lye after superfat = lye before superfat × (1 − superfat ÷ 100). The superfat field accepts 0-20%.

Water = lye ÷ (concentration ÷ 100) − lye. The concentration field accepts 20-40%.

Total batch weight = total oils + lye + water. Fragrance, colour and additives are not included.

The SAP table. The eight NaOH values built into the tool match the saponification chart published by Certified Lye, which states it was compiled in 2006 from values supplied by oil distributors and processors: olive oil 0.1353, coconut oil (refined, 76°) 0.1910, palm oil 0.1420, castor oil 0.1286, shea butter 0.1296, sunflower oil 0.1358, avocado oil 0.1337 and sweet almond oil 0.1367. The lookup rule is simple: each oil you fill in is multiplied by its row in that table, and an oil with no entry contributes nothing.

Keep the three-oil recipe and raise superfat to 8%. Lye falls to 4.56 oz and water, still at 30%, to 10.63 oz, a 47.19 oz batch. The difference from the 5% bar is only 0.15 oz of lye, which is exactly 3% of the 4.95 oz full-saponification figure. That small weight is why superfat must be measured on a scale that reads to at least a tenth of an ounce or a gram: a kitchen scale that rounds to the nearest quarter ounce can erase the whole difference between the two bars.

Scaling up changes the weights, not the logic. A 48 oz batch of 20 oz olive, 16 oz coconut, 4 oz castor and 8 oz shea butter needs 7.313 oz of lye before superfat. At 8% superfat and 33% concentration that becomes 6.73 oz NaOH and 13.66 oz water, a 68.39 oz batch. The higher concentration uses proportionally less water, so this batch traces faster than the first one would.

Many older recipes size water as a fixed share of the oils, often 38%. That habit hides how strong the lye solution really is. For the 32 oz three-oil recipe, 38% of oils is 12.16 oz of water, which puts the 4.71 oz of lye at a 27.9% concentration. The same 38% applied to the 48 oz four-oil batch gives 18.24 oz of water and a 26.9% solution. One rule, two different strengths, because the lye-to-oil ratio changed with the blend.

Sizing by concentration fixes the strength directly. For the three-oil bar at 5% superfat, 33% concentration means 9.55 oz of water, 30% means 10.98 oz, and 25% means 14.12 oz. More water gives longer working time for swirls but a softer bar at unmolding and a longer cure; less water speeds trace and can reduce soda ash, but leaves less time to pour.

Coconut oil's SAP value is about 41% higher than olive oil's (0.1910 against 0.1353), because its short-chain fatty acids have a lower molecular weight and need more lye per ounce. Run 32 oz of pure olive oil at 5% superfat and the tool asks for 4.11 oz of lye; the same weight of coconut oil needs 5.81 oz.

Swap them without recalculating and both mistakes are real. Using the olive-oil lye on coconut oil leaves an effective superfat of about 33%, a greasy bar that goes rancid early. Using the coconut-oil lye on olive oil supplies 34% more lye than the oil can consume even at zero superfat, a caustic bar that is not safe on skin. Any change to the oil list needs a new calculation.

The CDC's NIOSH Pocket Guide to Chemical Hazards lists sodium hydroxide as causing eye and skin burns, and gives first aid as flushing skin with water immediately and irrigating eyes immediately. In practice: goggles rather than glasses, gloves, long sleeves, and a sink within reach. Mix outdoors or under an extractor, because the first fumes irritate the airway.

Weigh lye into a dry container, then pour it slowly into the measured water in a heat-safe stainless steel or polypropylene jug. Never use aluminium, which reacts with lye. Stir until clear, label the jug, and keep children and pets away while it cools.

SAP values vary between batches, suppliers and refining methods, and the built-in table covers only eight oils. For a sale product, check each oil's SAP value with its supplier and confirm the lye with a second calculator.

The tool calculates sodium hydroxide for bar soap only. Liquid soap uses potassium hydroxide, which needs different SAP values. It does not model sodium lactate, milk, sugar or fragrance, which can accelerate trace or overheat a batch without changing the lye weight. It is not a substitute for safe handling of a corrosive chemical.

Soap makers who also pour candles can use the Candle Making Wax & Fragrance Calculator, which weighs fragrance oil in as a percentage of wax the same way this tool weighs superfat against lye. The Epoxy Resin Calculator handles another two-part mix where the ratio decides whether the result cures properly, and the Homebrew ABV & Priming Sugar Calculator brings the same weigh-it-exactly discipline to fermentation and carbonation.