# How do I weigh a cosmetic formula?

Turns a cosmetic or skincare formula in percent (w/w) into grams for any batch size, tops it up with water to 100% if you like, and lists the ingredients in label order.

- Page: https://www.acalculator.org/chemistry/cosmetic-formula-calculator
- JSON spec: https://www.acalculator.org/chemistry/cosmetic-formula-calculator.json
- Version: e73d959d4028

## Default answer

Example with the default inputs (Batch size 250 g, Top up with water to 100% Yes, Ingredients [Ingredient Sweet almond oil, Percent (w/w) 15%; Ingredient Emulsifying wax, Percent (w/w) 5%; Ingredient Glycerin, Percent (w/w) 3%; Ingredient Preservative, Percent (w/w) 1%; Ingredient Fragrance, Percent (w/w) 0.5%]): Weigh out: Water: 188.8 g (75.5%); Sweet almond oil: 37.5 g (15%); Emulsifying wax: 12.5 g (5%); Glycerin: 7.5 g (3%); Preservative: 2.5 g (1%); Fragrance: 1.25 g (0.5%).

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| w | Batch size | The total weight of product you want to make. |
| qs | Top up with water to 100% | Add water for whatever the listed ingredients leave below 100%. |
| items | Ingredients | One row per ingredient: its name and its percent of the batch weight (w/w). |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| summary | Batch | How many ingredients and the batch weight in grams. |
| grams | Weigh out | Each ingredient’s weight in grams, in label order, to 4 significant figures. |
| waterG | Water (g) | The water that tops the formula up to 100%. |
| waterPct | Water (%) | The water’s share of the batch. |
| listed | Listed ingredients (%) | The percents you typed, added up. |
| batchG | Batch (g) | The batch size in grams. |

## Method

weight of an ingredient = batch weight × its percent ÷ 100. With top-up water: water % = 100 − the sum of the listed percents.

## Assumptions

- Percents are by weight (w/w) of the finished batch, the usual way cosmetic formulas are written.
- Label order is descending weight, then ingredients at 1% or less in the order typed (21 CFR 701.3(f)).

## Worked examples

1. w = 0.25, qs = yes, items = {"name":"Sweet almond oil","pct":15} or {"name":"Emulsifying wax","pct":5} gives summary = 6 ingredients, 250 g in all, waterG = 188.75, waterPct = 75.5, grams = Water: 188.8 g (75.5%); Sweet almond oil: 37.5 g (15%); Emulsifying wax: 12.5 g (5%); Glycerin: 7.5 g (3%); Preservative: 2.5 g (1%); Fragrance: 1.25 g (0.5%). Source: FDA, Cosmetics Labeling Guide (ingredients in descending order of predominance; those at 1% or less in any order after them, 21 CFR 701.3(f)), https://www.fda.gov/cosmetics/cosmetics-labeling-regulations/cosmetics-labeling-guide.
2. w = 0.1, qs = no, items = {"name":"Shea butter","pct":40} or {"name":"Coconut oil","pct":60} gives listed = 100, waterG = 0, grams = Coconut oil: 60 g (60%); Shea butter: 40 g (40%). Source: FDA, Cosmetics Labeling Guide (ingredients in descending order of predominance; those at 1% or less in any order after them, 21 CFR 701.3(f)), https://www.fda.gov/cosmetics/cosmetics-labeling-regulations/cosmetics-labeling-guide.

## FAQ

### How do I convert a cosmetic formula from percent to grams?

Multiply the batch weight by each percent and divide by 100. For a 250 g lotion, 15% oil is 250 × 15 ÷ 100 = 37.5 g, and 0.5% fragrance is 1.25 g.

### What does “q.s.” or top up with water mean?

Quantum satis: add as much water as the other ingredients leave to reach 100%. If the listed ingredients add to 24.5%, water is 75.5% of the batch.

### Why use percent by weight?

Weights add up exactly and do not depend on how dense each ingredient is. A formula in percent w/w scales to any batch size with the same result.

### Why does my formula not add up to 100%?

Either turn on the water top-up, or adjust the percents. The page shows no answer when the listed percents go over 100%, or are under 100% with no top-up.

### In what order do ingredients go on a cosmetic label in the US?

In descending order of weight. Ingredients at 1% or less may follow in any order after the others (21 CFR 701.3). The page lists heavier ingredients first, then those at 1% or less in the order you typed.

### Should I weigh small amounts like preservatives more precisely?

Yes. A 1% preservative in a 100 g batch is only 1 g, so use a scale that reads to 0.01 g, and follow the supplier’s use rate for each ingredient.

## Sources

- FDA, Cosmetics Labeling Guide: ingredients declared in descending order of predominance; those at not more than 1% may follow in any order (21 CFR 701.3(f)(2)). Retrieved 2026-10-03. https://www.fda.gov/cosmetics/cosmetics-labeling-regulations/cosmetics-labeling-guide
- NIST Special Publication 811, Appendix B.8: 1 oz = 28.349523125 g; 1 lb = 453.59237 g. Retrieved 2026-10-03. https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8