Soap · lotion · balm · bath bombs
Recipe Batch Scaler
Paste a recipe in grams, ounces or percentages and scale it to the batch you want, with every amount rounded to what your scale can actually show.
How to use the batch scaler
Paste or type the recipe with one ingredient per line. The scaler reads the name and the amount whether the amount comes first or last, and understands g, kg, oz, lb and %. If your recipe has bare numbers, say what they are under Numbers without a unit are. A recipe must be all weights or all percentages; weights in mixed units are fine.
Enter the new batch weight and the step your scale displays. Each line is multiplied by the same factor, so the proportions never change, and then rounded to the nearest step. The Exact column keeps the unrounded figure; On your scale is what to weigh.
Small amounts suffer most from rounding. When rounding moves an ingredient by more than 5 % it is flagged, and anything that rounds to zero is marked too small to weigh: make a bigger batch, use a finer scale or pre-dilute it.
Worked example
The example soap recipe weighs 1448.8 g in total. Scaling it to 500 g multiplies every line by 500 ÷ 1448.8 = 0.345. Olive oil goes from 500 g to 172.6 g, and the sodium hydroxide from 138.2 g to 47.69 g, which a 0.1 g scale shows as 47.7 g.
Scaling soap keeps the original superfat and water only if the original lye was right. For anything with lye in it, put the scaled oils through the soap lye calculator as a check before you make it.
Why rounding matters for small batches
Halving a 1 kg soap recipe makes no difference to a kitchen scale that reads to 1 g. Scaling a lotion down to a 50 g test batch does: a preservative at 1 % becomes 0.5 g, which a 1 g scale shows as either 0 g or 1 g, half or double what the formula asks for. That is why the scaler shows how far each rounded amount is from the exact one. For ingredients used at a fraction of a per cent, a jewellery scale that reads to 0.01 g, or a bigger batch, is the practical fix.
The units are exact: one ounce is 28.349523125 g and one pound is 453.59237 g, the international avoirdupois definitions.
Sources
- Wikipedia, Pound (mass): the international avoirdupois pound is exactly 0.45359237 kg, and the ounce is one sixteenth of it.
Questions
Can I scale a soap recipe without recalculating the lye?
Mathematically yes: if every ingredient, lye included, is multiplied by the same number, the superfat and lye concentration stay the same. But scaling copies any mistake in the original, and rounding the lye on a coarse scale changes the superfat a little. Run the scaled oils through a lye calculator before you make the batch.
What does “too small to weigh” mean?
The exact amount is less than half of your scale's smallest step, so it rounds to zero. A scale that reads to 1 g cannot weigh 0.3 g of anything. Use a finer scale, make a bigger batch, or mix a larger amount of the ingredient into a carrier first and weigh that.
Why doesn't the rounded total match my target exactly?
Each line is rounded on its own, so the small differences add up. With a 0.1 g scale and ten ingredients the total can be off by up to half a gram. The exact column always adds up to the target.
Can I convert a gram recipe into percentages?
Yes. Scale it to a new batch weight of 100 g with a 0.01 g step: each new gram figure is that ingredient's percentage of the whole recipe.