Soap Batch Scaling Calculator

Scaling a working soap recipe is arithmetically simple but easy to get wrong by hand when a batch has many oils and additives: this calculator multiplies your original oil, lye (NaOH), and water weights by a single scale factor — 0.5 for a half batch, 2.0 to double it, and so on — preserving every ratio in the recipe exactly, since each ingredient is scaled by the identical factor. It reports the new oil, lye, and water weights individually, sums them into a scaled total batch weight, and estimates the number of standard 4 oz bars the scaled batch will yield by dividing that total by 4 and rounding down (a conservative estimate, since real bar molds and trim loss vary). It also reports the percentage size change relative to the original batch, purely as a convenience readout of (scale factor − 1) × 100.

The one thing this tool deliberately does not do is recalculate lye or water amounts from first principles — it assumes your original recipe was already correctly formulated (SAP value, superfat, and lye concentration all accounted for) and simply preserves those ratios at a new size. That means if your starting recipe had an error, scaling will faithfully reproduce that same error at the new batch size. A common mixup is scaling only the oils and forgetting the lye or water — this tool guards against that by scaling all three together — but it can't catch an internally inconsistent starting recipe, so it's worth double-checking your original superfat and water discount with a dedicated lye calculator before scaling up to a large batch.