Bath Bomb Recipe by Weight
Scale a bath bomb recipe by weight — 2:1 baking soda to citric acid, optional dry additions, fragrance at 3%, and the exact weight of each ingredient per batch and per bomb.
Citric acid costs several times what baking soda does, so 2.25:1 trims the bill. The fizz is slightly softer.
Fill one mold with dry base and weigh it — printed mold sizes are often off.
Each entered as a percentage of the dry base. Set to 0 to leave one out.
Optional. Fill in what you pay and the calculator adds up the ingredient cost.
Your recipe
- Baking soda66.7% of dry base30.73 oz2.56 oz
- Citric acid33.3% of dry base15.37 oz1.28 oz
- SLSA1% of dry base0.46 oz0.04 oz
- Fragrance oil3% of batch weight1.44 oz0.12 oz
- Carrier oilMeasured by volume, not weight5.8–11.5 tsp—
- Witch hazel or alcoholSpritz until the mix holds a squeezeto consistency—
- Total batch48.00 oz12 bombs
The bath bomb ratio explained
A bath bomb is a delayed chemical reaction held together in a dry ball. Baking soda is the base, citric acid is the acid, and nothing happens until water arrives and lets the two react. The standard proportion is 2 parts baking soda to 1 part citric acid by weight.
Baking soda = dry base × 2/3
Citric acid = dry base × 1/3
Example: 300 g dry base = 200 g baking soda + 100 g citric acid
2:1 is a practical recipe, not a chemical balance. By weight, each part of citric acid reacts completely with only about 1.3 parts of baking soda, so a 2:1 bomb deliberately carries extra baking soda. That keeps the cost down, since citric acid is the more expensive ingredient, and the citric acid sets how much fizz the bomb has. Use more citric acid and you get a faster, stronger fizz that costs more. Use much less and even more baking soda is left over with nothing to react with, which is what makes a bomb feel chalky in the water and leave residue in the tub.
Citric acid costs several times what baking soda does, so a lot of production recipes run 2.25:1 instead. The saving is real and the difference in the bath is small: a slightly softer, slightly shorter fizz. The calculator above has both as a toggle. Pushing much past 2.25:1 to save more starts costing you the fizz that people buy bath bombs for.
Shower steamers use the same reaction with a higher share of baking soda: 3 parts to 1 part citric acid, plus a little cornstarch. The shower steamer calculator works out that recipe, including menthol.
Why weight beats volume
Almost every bath bomb recipe that fails to repeat is a recipe written in cups. Baking soda and citric acid have different densities, so a cup of one is not the same amount as a cup of the other. Both also compact in the bag over time, so a scooped cup today can hold meaningfully more than a scooped cup a month ago. Citric acid adds another variable, since fine and granular grades pack differently.
None of that matters much in a test batch of four bombs. It matters a great deal when you scale to forty, because the ratio drift scales with the batch. A recipe written by weight gives you the same bomb every time and scales cleanly in either direction.
The base recipe by weight
A worked example, with no optional additions, to show the arithmetic the calculator is doing:
| Ingredient | Weight | How it is derived |
|---|---|---|
| Baking soda | 200 g | 2/3 of the 300 g dry base |
| Citric acid | 100 g | 1/3 of the 300 g dry base |
| Fragrance oil | ≈ 9 g | 3% of the finished batch weight |
| Carrier oil | 1–2 tsp | About 1–2 tsp per 8 oz of dry base |
| Witch hazel | To consistency | Spritzed in, never weighed |
Note that fragrance is a percentage of the finished batch, not of the dry base. That distinction is small at 3% but it is the reason the calculator works backwards from your mold size rather than simply adding percentages on top.
Carrier oil is the one ingredient here that stays a volume measure. The quantity is small enough that weighing it adds precision you cannot use, and most recipes you will find written elsewhere give it in teaspoons.
Optional ingredients and what they do
Everything past baking soda and citric acid is optional. Each one is entered in the calculator as a percentage of the dry base, so it scales with the batch.
| Addition | What it does | Notes on quantity |
|---|---|---|
| SLSA | Foam and bubbles on the water surface | About 0.5 oz per 3 lb of base, which is roughly 1% |
| Cream of tartar | Harder, denser bomb that survives shipping | Set from your own recipe — start small and test |
| Cornstarch | Slows the fizz so the bomb lasts longer in the tub | Set from your own recipe — start small and test |
| Clay or Epsom salt | Binding, color, mineral content | Set from your own recipe — start small and test |
SLSA is the one with a widely published reference point, and 1% of the base is a sensible place to start. Wear a mask when you handle it — it is a very fine powder and it will get airborne.
For the others, the useful percentages vary enough between recipes that a default would be a guess. The calculator leaves them at zero so you can enter the figure your own recipe uses. If you are starting from nothing, add one at a time and change only that one between test batches, or you will not know which addition caused the difference.
Every dry addition displaces base. Adding 10% of extras to a fixed mold size means 10% less of the fizzing reaction in each bomb. The calculator accounts for this, which is why the baking soda figure drops when you turn an addition on.
How to scale a batch
Scaling is where by-weight recipes earn their keep, and it runs in both directions.
From your mold is the usual starting point. Pack one mold with plain dry base, tip it onto the scale, and you have your per-bomb weight. Multiply by how many bombs you want and you have the batch. Enter those two numbers in the By Mold Size tab above and every ingredient weight falls out. Printed mold capacities are frequently optimistic, so weigh rather than trusting the packaging.
From what you have works the other way. If you have 500 g of citric acid and want to know what it will make, switch to the By Base Weight tab and work from the dry base figure. At 2:1 that 500 g of citric acid pairs with 1 kg of baking soda for a 1.5 kg base.
Getting the moisture right
This is the step that decides whether the batch works, and it is the only one with no number attached to it.
Witch hazel and rubbing alcohol both do the same job: they dampen the mix enough to hold a shape and then evaporate before they can set off much of the reaction. Water technically works and is a bad idea, because it does not evaporate fast enough and will start the fizz in the bowl. Witch hazel is the more forgiving of the two and the usual recommendation for beginners.
Spritz a little, mix it through with your hands, then squeeze a handful. When it holds together like damp sand and keeps its shape after you open your hand, it is ready. If it falls apart, add another spritz. If you hear it fizzing or see it puffing up, you have gone too far.
There is no fixed amount because the right amount changes with the humidity in the room. The same recipe needs noticeably less liquid on a damp day than a dry one, which is why a recipe that specifies a tablespoon of witch hazel will fail half the time. Work to the feel of the mix instead.
Pack the mold firmly, overfill both halves slightly, and press them together hard. Under-packing is the other half of the crumbling problem and it gets blamed on the recipe far more often than it deserves.
Pricing what you make
The cost fields in the calculator cover ingredients only. A bath bomb you intend to sell also carries shrink wrap or a box, a label, and your time, and those together usually outweigh the baking soda and citric acid. The method for turning a unit cost into a retail price is the same one used for any handmade product — total your real cost per unit, then apply a markup that absorbs fees, breakage and discounts. The candle pricing calculator walks through that math with labor and overhead included, and it applies to bath bombs unchanged.
Frequently Asked Questions
How much citric acid per cup of baking soda?
Half a cup gets you close, but cup measures are the reason bath bomb batches come out differently every time. A cup of baking soda weighs somewhere around 7–8 oz depending on how tightly it has settled, and citric acid weighs differently again depending on grain size. By weight the answer is fixed: 2 parts baking soda to 1 part citric acid — 200 g to 100 g, or 8 oz to 4 oz.
Can I make bath bombs without citric acid?
You can, but you are building a different product rather than swapping one ingredient. Citric acid is the acid half of the reaction, and common substitutes such as cream of tartar are much weaker acids, so the fizz is gentler and shorter. The 2:1 ratio does not carry across, because the substitute reacts at a different rate. Treat a citric-acid-free recipe as something to develop and test from scratch.
Why weigh instead of measuring by volume?
Baking soda and citric acid have different densities, so equal volumes are not equal amounts. Both compact in storage, so a scooped cup today can hold more than a scooped cup last month. Those errors compound as you scale — a small ratio drift in a batch of four becomes a batch of forty that will not hold together. Weighing removes the variable.
How much fragrance oil goes in bath bombs?
Around 3% of the total batch weight. For a 48 oz batch that is roughly 1.4 oz. Bath bombs dissolve into water that sits against skin, so the safe usage rate for a bath product is not the same as the rate for a candle. Check the IFRA certificate that came with your oil for the bath and body category, and use that limit if it is lower than 3%.
Why did my bath bombs crumble or expand out of the mold?
Both come from the binding liquid. Crumbling means the mix was too dry, or it was not packed into the mold firmly enough. Expanding, doming or cracking means it was too wet and the reaction started in the bowl. Spritz witch hazel in small amounts and stop as soon as a squeezed handful holds its shape without fizzing. Humidity changes how much you need from one batch to the next.
What is the best ratio for bath bombs?
2 parts baking soda to 1 part citric acid by weight is the standard and what most published recipes use. Some makers run 2.25:1 to cut cost, since citric acid is several times more expensive than baking soda; the trade is a slightly softer, shorter fizz. Going much beyond 2.25:1 leaves enough unreacted baking soda that the bomb feels chalky in the water.