How to resize a soap recipe for a different mold
Every ingredient in a soap recipe scales by the same factor — oils, lye, water, fragrance, additives. Work out what the new batch needs to be, divide by what the recipe makes, and multiply every line by that. The only judgment call is which weight you are matching: the oil weight (what molds are sized by, and what this site's mold calculator gives you) or the total batch weight (oils + lye + water, what actually fills the mold). The tool above works from whichever you have; for a mold, use oil weight.
The classic bar is written for 1,000 g of oils (1,466 g of batter). A 10 × 3.5 × 2.5 in loaf takes 956 g of oils, so the factor is 956 ÷ 1,000 = 0.956. Lye becomes 143.8 × 0.956 = 137.5 g, water 279 g, and the batch 1,402 g. Run the same four oils through the recipe builder at 956 g and it prints 137.5 g of lye — the same number to a tenth of a gram, because saponification is linear.
Does lye scale linearly? Yes — and here is the proof
Lye is a fixed fraction of each oil's weight (its SAP value), so half the oils need exactly half the lye. Scaling a recipe by a factor and recomputing it from scratch give the same lye to rounding error (137.47 g vs 137.48 g above). What scaling cannot do is change the oils: swap shea for palm, or push coconut from 25% to 30%, and the lye has to be recalculated, because the new blend has a different SAP. Resize for a mold here; change the recipe in the builder.
The classic bar, scaled to common molds
| Target | Oils | Factor | Lye (NaOH) | Water | Batch |
|---|---|---|---|---|---|
| 10 × 3.5 × 2.5 in loaf | 956 g | × 0.956 | 137.5 g | 279 g | 1,402 g |
| 12 × 3.5 × 2.5 in loaf | 1,148 g | × 1.148 | 165.0 g | 335 g | 1,682 g |
| 9 × 12 × 1.25 in slab | 1,476 g | × 1.476 | 212.2 g | 431 g | 2,163 g |
| 12 × 100 mL cavities | 800 g | × 0.800 | 115.1 g | 234 g | 1,173 g |
| 100 g test batch | 100 g | × 0.100 | 14.4 g | 29 g | 147 g |
Olive 35 / coconut 30 / palm 30 / castor 5, 5% superfat, 33% lye concentration, 3% fragrance; molds at 33% concentration via the mold calculator. Computed by this site's engine.
Scaling down: test batches and rounding
A 100 g test batch of the recipe above needs 14.38 g of lye. Round that to 14 g and you have shaved 2.6% off the lye — the superfat just went from 5% to about 7.5%, which is harmless; round it up to 15 g and the superfat drops to about 0.9%, which is a meaningfully different bar. The tool rounds lye to 0.1 g and everything else to the gram for that reason. Below roughly 300 g of oils a 0.1 g scale stops being a luxury.
Mold volume to oil weight assumes the batter is poured to the stated height with no headroom and nothing left in the pot. Real pours leave batter on the stick blender, the spatula and the bowl — 2–4% on a small batch. If your molds keep coming up a hair short, scale to a target about 3% higher than the mold calculator says; if they overflow, you are filling past the height you measured.
When the recipe is in percentages
A recipe given only as percentages (olive 35%, coconut 30% …) has no size yet, so there is nothing to resize — take it straight to the recipe builder, which turns percentages into weights for any oil total and computes the lye. This tool is for recipes already written in weights, including the ones with lye and water already on the card; it passes percent lines through unchanged.