Soap Recipe Calculator
Multiply each oil's weight by its saponification value, discount the total by your superfat percentage, and that is the lye needed; water is then a chosen ratio of the lye. This soap recipe calculator runs the SAP maths for NaOH bar soap and KOH liquid soap across 24 oils and butters, with the superfat discount and water amount made explicit — and the safety rules up front, because lye is not optional equipment.
Oils & butters
5% is standard for bar soap
2:1 to 3:1 is typical
Recipe
NaOH assumed 100% pure. Weigh everything on a digital scale; never use volume measures for lye.
▸How this was worked out
Olive oil: 500 g × 0.134 SAP × 0.95 superfat = 63.65 g NaOH
Coconut oil (76°): 250 g × 0.19 SAP × 0.95 superfat = 45.13 g NaOH
Palm oil: 150 g × 0.141 SAP × 0.95 superfat = 20.09 g NaOH
Shea butter: 100 g × 0.128 SAP × 0.95 superfat = 12.16 g NaOH
lye total = 141.03 g
water = 141.03 × 2 = 282.1 g
Use a scale with precision suitable for the batch and follow a validated recipe. Cure cold-process bars according to that recipe. A pH strip alone does not establish that soap is free of excess lye; never taste soap to test it.
Shopping list
- Olive oil500 g
- Coconut oil (76°)250 g
- Palm oil150 g
- Shea butter100 g
- Sodium hydroxide (lye)141 g
- Distilled water282.1 g
CraftCalc · Soap recipe
NaOH at 5% superfat · 2:1 water
Olive oil: 500 g
Coconut oil (76°): 250 g
Palm oil: 150 g
Shea butter: 100 g
NaOH: 141 g · Water: 282.1 g · Batch: 1,423.1 g
Add lye to water, never water to lye. Goggles + gloves. Cure 4–6 weeks.
craftcalc.io — free maker calculators with the arithmetic shown
How is lye calculated from a soap recipe?
Every oil has a saponification value — the grams of lye needed to fully saponify one gram of that oil. Olive oil’s NaOH SAP is 0.134: 500 g of olive oil needs 500 × 0.134 = 67 g of sodium hydroxide to convert completely. A recipe with several oils is the sum of each: lye = Σ (oil weight × its SAP value). KOH values run about 1.4× the NaOH ones because potassium hydroxide is heavier per molecule of lye.
The superfat (lye discount) reduces the calculated lye amount to leave some oil unsaponified. It is a recipe margin, not proof that a finished batch contains no excess lye: SAP values, lye purity, weighing and mixing must also be correct. A 5% superfat multiplies the lye by 0.95. Water is then a separate choice: a water:lye ratio of 2:1 is the modern default (faster unmoulding, less gel-phase drama), where older recipes quote water as 30–38% of oil weight. Both modes are here; they converge around 2.5–3:1 for typical recipes.
What should I do after a lye exposure?
Sodium hydroxide can severely burn skin, eyes and the mouth. Carefully brush away dry lye while protecting the eyes, remove contaminated clothing, and flush exposed skin with water for at least 15 minutes; flush exposed eyes with water for at least 30 minutes and get urgent medical help. Do not use vinegar or another acid to neutralise lye on the body, and do not taste a suspect batch. Follow the product’s safety data sheet and contact local poison control or emergency services. See the CDC/ATSDR sodium hydroxide exposure guidance.
Worked example: a balanced 1 kg bar-soap recipe
A classic trio-and-butter batch: 500 g olive, 250 g coconut (76°), 150 g palm, 100 g shea — 1,000 g of oils at 5% superfat with 2:1 water. Lye per oil: olive 500 × 0.134 = 67.0; coconut 250 × 0.190 = 47.5; palm 150 × 0.141 = 21.2; shea 100 × 0.128 = 12.8. Sum: 148.5 g at zero discount; at 5% superfat: 148.5 × 0.95 = 141 g NaOH. Water at 2:1: 282 g distilled water. Total batch: about 1,423 g — roughly twelve 100 g bars after cure.
Procedure in brief: weigh the water, then slowly add the lye to it (never the reverse) in a ventilated space, stir until dissolved, and let both phases cool to roughly 38–43°C / 100–110°F. Pour the lye solution into the melted oils, stick-blend to a light trace, pour into the mould, insulate for 24 hours, unmould and cut, then cure 4–6 weeks. The SAP values used here match the standard published chart (Bramble Berry / Soap Queen) — see the methodology.
Frequently asked questions
What does superfat actually do?
It discounts the calculated lye amount to leave some oil unsaponified. A 5% discount multiplies the zero-superfat lye amount by 0.95. It does not guarantee safety if the oil identity, SAP values, lye purity, weighing or mixing are wrong. Use a tested recipe and verify the actual ingredients and measurements.
Why can't I swap NaOH and KOH?
Sodium hydroxide makes hard bars; potassium hydroxide makes soft or liquid soap. They have different SAP values (KOH's are ~1.4× higher) and different physical results — a bar-soap recipe run with KOH will not harden. Use the lye type the recipe was designed for.
Why add lye to water and not water to lye?
Dissolving lye is violently exothermic. Pouring water onto a pile of lye can flash-boil the surface layer and erupt caustic solution at you. Lye into water, added slowly with stirring, dissipates the heat safely. The mnemonic: 'snow falls on the lake' — lye (snow) onto water (lake).
Does KOH purity really vary?
Yes — flake potassium hydroxide is typically ~90% pure (the rest is water and carbonates), so the calculator divides the KOH amount by 0.90. Sodium hydroxide is sold essentially pure. Check your supplier's certificate of analysis; some KOH suppliers state their purity and you can enter it.
How do I know the soap is safe to use?
Use a tested recipe, confirm every oil and lye measurement, and follow an appropriate soap-testing procedure before use or sale. Cure time or a pH-strip reading alone is not a safety certificate, and extra curing does not correct every recipe error. Do not taste or touch your tongue to soap that may contain excess lye; keep a suspect batch out of use and seek qualified soap-testing advice.
Can I substitute one oil for another?
Only by re-running the lye math — every oil has its own SAP value. Replacing 100 g of olive oil (NaOH SAP 0.134) with 100 g of coconut oil (0.190) raises the zero-superfat NaOH requirement by 5.6 g, or 5.32 g at 5% superfat. Change the oil in this calculator and let it recompute; never reuse an old lye number after changing the recipe.
Formulas and standard tables are sourced in the methodology. Results are estimates — measure twice, cut once.