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.
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 (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.
输入
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.
输出
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.