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Why soap recipes need resizing
Soap recipes are written for one batch size, but your mold is almost never that exact size. A recipe that makes 1,000 grams of oils will overfill a small mold and underfill a large one, and neither result is acceptable. Scaling a recipe up or down solves this, and the arithmetic is simple: divide the new total by the old total to get the scale factor, then multiply every ingredient by that factor. A recipe scaled from 1,000 grams to 500 grams uses a factor of 0.5, so 600 grams of olive oil becomes 300 grams, 300 grams of coconut oil becomes 150 grams, and 100 grams of shea butter becomes 50 grams. This resizer does that multiplication for you, for up to three ingredients, so you can scale any recipe in seconds.
Resizing matters for more than mold fit. Small test batches save money when trying a new recipe, because a failed 500 gram test wastes half as much as a failed 1,000 gram batch. Large batches save time when a recipe sells well, because one big batch beats three small ones. Resizing also helps when an oil runs short: if you have only 400 grams of an oil that a recipe needs 600 grams of, scaling the whole recipe down keeps the percentages right, while simply using less of one oil changes the recipe and the soap. Percentages are the soul of a soap recipe, and scaling protects them.
How to use the batch resizer
Enter the original total oil weight of the recipe, then the new total oil weight you want. Then enter the original weights of your ingredients, up to three, and set any unused ingredient to zero. The tool shows the scale factor and the new weight for each ingredient. For example, scaling 1,000 grams down to 750 grams uses a factor of 0.75, so an ingredient of 400 grams becomes 300 grams. If your recipe has more than three ingredients, run the tool twice, or scale the remaining ingredients by hand using the same factor. Always check that the scaled ingredients add up to the new total, allowing for small rounding differences of a gram or two.
Work in grams, not cups or spoons, because soap is chemistry and volume measures are too rough. Weigh oils on a scale that reads to at least one gram, and write the scaled recipe down before you start, so you are not doing arithmetic with gloves on and lye nearby. Keep the original percentages visible too, because percentages let you check your work: if olive oil was 60 percent of the original, it must still be 60 percent of the scaled recipe.
The step most makers forget: recalculate the lye
Scaling oils is only half the job, and the forgotten half is dangerous. Lye and water must be recalculated for the new batch size, never copied from the original recipe. If you halve the oils but keep the original lye, the soap will have twice as much lye as it needs and will be harsh and unsafe. If you double the oils but keep the original lye, the soap will be soft and oily. Always enter the scaled oils into a dedicated lye calculator and use the new lye and water figures it gives, including any superfat percentage your recipe uses. Superfat is the extra oil left unsaponified for mildness, usually around 5 percent, and it must be set in the lye calculator, not guessed.
Fragrance and colour also scale, but with care. Fragrance oil is usually measured as a percentage of oils, so it scales with the batch, but check the fragrance maker’s maximum for your oils and never exceed it. Colourants are potent, so scale them too, but test colour in a small batch first, because doubling a colourant does not always double the colour evenly. Additives such as clays, oatmeal, or honey scale with the batch as well, but keep their percentages the same as the original recipe.
Common resizing mistakes
The most common mistake is scaling only the main oil and guessing the rest, which changes the recipe’s character completely. Another is rounding too early: rounding each ingredient to the nearest ten grams before adding them up can shift the total by enough to matter, so keep one decimal place until the end and round only the final weights. A third mistake is forgetting water: water is often listed as a percentage of oils or as a lye concentration, and both must be recalculated for the new size. Finally, never scale a recipe you have not tested at its original size, because scaling a bad recipe just makes more bad soap. Test small, confirm the recipe, then scale with confidence.
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