Melt-and-Pour Soap Calculator: Base, Spill and Fragrance

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Opens on the book's Market Batch. Change any number to your own and press Calculate.

Cavities

Cavity size from

Water in one cavity g, to the brim you will fill to

Or measure a rectangular cavity

Length cm

Width cm

Depth cm

The base and the scent

Base density about 1 g per mL

Spill allowance %

Fragrance % of the base; the supplier's maximum rules

Or a rate in oz per lb optional: shown as a percent

Base block size lb (2 lb = 907.2 g)

Enter the cavities and press Calculate.

The formula

Cavity volume = the water it holds (1 g of water = 1 mL), or length × width × depth in cm = mL
Base in the soaps = cavities × volume × about 1 g/mL
Base to melt = that × (1 + spill %)
Fragrance = base to melt × fragrance %. A rate in oz per lb ÷ 16 × 100 = the same as a percent

The book's worked example, Chapter 1. Step 1 of the Market Batch is 12 guest soaps in 60 mL cavities: 12 × 60 = 720 g of base, plus 5 % for spills = 756 g to melt (26.67 oz), and 2 % fragrance = 15.12 g. One 2 lb block covers it with 151.2 g left over.

Melt the base gently and keep it under about 60 to 70 °C (140 to 160 °F, the supplier's figure rules); never let it boil. Check a base is vegan and palm-free by its ingredient list, and ask the supplier in writing. Prices are illustrative US supplier prices from October 2026, shown without a currency sign; put in your own. Everything stays in your browser: no sign-up, nothing stored.