What Is a Bath Bomb Ingredient Ratio Calculator?
Bath bomb recipes are built on ratios, not fixed amounts — a 1:2 citric acid to baking soda mix scales up or down depending on how big a batch you want to make. This calculator takes your target total batch weight and your chosen ratio for citric acid, baking soda, Epsom salt and cornstarch, and returns the exact weight of each ingredient to measure out.
Whether you're making a single test bath bomb or a large batch for gifts, this keeps the proportions consistent so the fizz reaction and texture come out the same every time.
How to Read Your Results
Citric Acid
The acid component that reacts with the baking soda once the bomb hits water. Too much relative to the recipe makes bombs more prone to crumbling.
Baking Soda
The base (sodium bicarbonate) that reacts with citric acid to produce the fizz. It's usually the largest single ingredient by weight in a classic recipe.
Epsom Salt
An optional filler that adds bulk and is often included for its own soothing bath benefits — set this to 0 parts if you're not using it.
Cornstarch
An optional filler that slows down the fizzing reaction slightly and helps bombs hold their molded shape — also optional, set to 0 if unused.
Real-World Example
A 500 g batch at the classic 1 part citric acid : 2 parts baking soda : 1 part Epsom salt : 1 part cornstarch ratio (5 parts total):
| Ingredient | Parts | Weight |
|---|---|---|
| Citric acid | 1 | 100 g |
| Baking soda | 2 | 200 g |
| Epsom salt | 1 | 100 g |
| Cornstarch | 1 | 100 g |
Each ingredient's share is simply its parts divided by the 5 total parts, multiplied by the 500 g batch weight — the same math scales to any batch size or ratio you choose.
Tips for Better Bath Bombs
- Work in a dry environment — humidity can trigger the citric acid/baking soda reaction early and ruin the mix.
- Add liquid binder sparingly (a light witch hazel or oil spritz) just until the mixture holds together when squeezed.
- Pack molds firmly and let bombs dry for 24–48 hours before unmolding for a solid, long-lasting fizz.
- Adjust the citric acid ratio down slightly if your bombs crumble, or up slightly if they don't fizz enough.
Related Calculators
Last updated: August 2026 · Reviewed by: Simple Calculator Tools Editorial Team