Free cold-process soap lye and cost calculator. Enter olive, coconut, and castor oil weights and costs, superfat percentage, and water ratio; outputs lye (NaOH) and water needed plus total batch and per-bar cost.
Built and maintained by Paul Clark, Redmoon Software
Use when planning a cold-process soap batch's ingredient cost and lye amount before mixing, especially when scaling a recipe up or down.
Dedicated lye calculators (like SoapCalc or Bramble Berry's) handle many-oil recipes precisely; this tool trades that flexibility for a fast three-oil cost-and-lye estimate.
⚠️ Lye (sodium hydroxide) is caustic. Wear gloves and eye protection, and always add lye to water — never water to lye. This 3-oil tool is for cost/ballpark planning only; run your actual recipe through a full multi-oil SAP-value lye calculator before making soap.
This batch (800 g oils) needs 115.6 g lye and 304.0 g water, costing $18.59 total — about $1.86 per bar across 10 bars.
Lye (NaOH) needed = (oil weight × its SAP value, summed across olive 0.135, coconut 0.183, and castor 0.128) × (1 − superfat %).
Water needed = total oil weight × water percentage (default 38% of oil weight, a common cold-process starting ratio).
Batch cost = oil cost (weight ÷ 453.6 g/lb × cost per lb, summed per oil) + lye cost + extras; cost per bar = total batch cost ÷ number of bars.
Superfat is the percentage of oils left unsaponified (uncombined with lye) on purpose, so the bar feels moisturizing instead of stripping. 5% is a common default for cold-process soap because it's gentle on skin without leaving so much excess oil that the bar turns soft or rancid faster.
Saponification (SAP) values are defined per gram or per ounce of oil, not per milliliter, because oils of different densities need different amounts of lye to fully react. Measuring oils by weight (a kitchen scale) rather than volume is standard practice for accurate, safe lye calculations.
Always add lye to water, never water to lye, since adding water to concentrated lye can cause a violent, splattering exothermic reaction. Wear gloves and eye protection, work in a ventilated area, and keep the mixture away from children and pets — lye (sodium hydroxide) is highly caustic and will burn skin and eyes on contact.
Input
400g olive oil ($8/lb), 300g coconut oil ($5/lb), 100g castor oil ($10/lb), 5% superfat, 38% water, $4/lb lye, 10 bars, $5 extras.
Output
Lye needed ≈ 108.9 g, water ≈ 304 g, batch cost ≈ $13.75, ≈ $1.38/bar.
Lye = (400×0.135 + 300×0.183 + 100×0.128) × 0.95 ≈ 108.9 g. Oil cost + lye cost + $5 extras totals about $13.75 for the 800g-oil batch, or $1.38 per bar across 10 bars.
Every oil has its own saponification value — the amount of lye required to convert a given weight of it into soap. Olive, coconut and castor all differ, so the lye quantity depends on the exact blend and not on the total oil weight. Changing the proportions without recalculating produces a wrong lye figure, and this is the single most important reason to run the numbers rather than reuse them.
Weights must be by mass, not volume. Oils differ in density and a recipe measured in cups will not saponify correctly.
Superfatting means deliberately using less lye than the oils could consume, leaving a small proportion unsaponified. The default here is 5%, which is the standard range for a body bar.
It does two jobs. It leaves free oils in the finished bar, which is what makes it feel conditioning rather than stripping. More importantly it is the margin for error: measurement is never perfect, and a small lye deficit guarantees no free lye remains in the finished soap. A bar made with excess lye is caustic and will burn skin.
Do not superfat below the normal range to make a harder bar, and do not treat this — or any calculator — as a substitute for accurate scales, protective equipment and a tested recipe. Lye is dangerous to handle and the arithmetic being right does not make the process safe.
Why cold-process recipes lean on olive, coconut, and castor oil, what each one contributes to a finished bar, and how the blend — not just the lye math — decides whether your soap is hard, bubbly, and gentle.
How SAP values, superfat percentage, and water ratio determine the exact lye amount a cold-process soap batch needs, with a worked example and an important lye-safety disclaimer.
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