Bath bomb recipe and ratio
Citric acid to bicarbonate for any mould you own, with the additives, butters and scent checked against what a bomb will actually take — and what each one costs you to make.
INPUTS
Batch size
: 1
1:1.5 fast and sharp · 1:2 standard · 1:2.5 slow and foamy · 1:3 mostly bicarbonate. A gram of this acid uses 1.31 g of bicarbonate, so the rest is deliberate surplus — 52% here.
Additives, butters and scent
AdditiveShare
%
%
%
25.0% not fizzing
%
Past 6% it never hardens.
%
Up to 3%, then your own IFRA figure.
SLSA is a very fine powder and a respiratory irritant. Weigh it wearing a dust mask, in still air, and do not lean over the tub — a lungful sets off coughing that lasts. It is harmless in the bath and unpleasant on the way there. The same goes for Kaolin clay — fine powders, still air, mask on.
RESULTS
Bicarbonate of soda
452.9 g
1 : 2.0 by weight 52% more than the acid can use
Weigh out
| Ingredient | Share | Weight |
|---|---|---|
| Bicarbonate of soda | 50.0% | 452.9 g |
| Citric acid (anhydrous) | 25.0% | 226.5 g |
| Cornflour (cornstarch) | 10.0% | 90.6 g |
| Epsom salt (fine) | 8.0% | 72.5 g |
| Kaolin clay | 2.0% | 18.1 g |
| Butters and oils | 3.0% | 27.2 g |
| Fragrance | 2.0% | 18.1 g |
| Witch hazel | to feel | 23 ml |
Bombs from this batch
6
Mix per bomb
151.0 g
Batch weight
905.9 g
CO₂ it releases
155.6 g
The ratio is applied to the acid and bicarbonate only, so adding a clay does not quietly change the fizz you chose. Everything else is a share of the finished batch — which is why it still fits the mould after you have changed your mind about the epsom salt.
How this is worked out
A bath bomb is a dry acid and a dry carbonate that do nothing until water arrives. The arithmetic runs from the batch inwards, not outwards: your additives, butters and fragrance are shares of the finished batch, and the acid and bicarbonate are what is left. Doing it the other way — a base weight with extras piled on top — means the batch grows every time you add a clay, and the mould you sized it to stops fitting. The ratio is then applied to that base alone, so changing the epsom salt cannot quietly change the fizz you chose. One gram of citric acid consumes 1.31 g of bicarbonate, so the classic 1:2 runs the bicarbonate about 52% over what the reaction can use. Mould volume is worked out from the diameter it was sold at, and the fill weight from how hard you say you pack it — the difference between a light hand and a press is roughly 32%.
base = batch × (1 − additives − butters − fragrance)
acid = base ÷ (1 + bicarb:acid) 1:2 by convention, 1:1.31 exactly cancels
bicarb = base − acid
mould = π/6 × diameter³ fill = mould ml × packing density
cost = (materials × wastage + packaging + time) ÷ bombs
Questions makers ask
What is the ratio of citric acid to baking soda for bath bombs?
One part citric acid to two parts bicarbonate of soda by weight. It is not arbitrary: a gram of citric acid can only consume 1.31 g of bicarbonate, so 1:2 leaves the bicarbonate about 52% in excess. That surplus is the point — it guarantees no acid survives into the bathwater, and it is what gives the bomb its body. On the 6-bomb batch above that is 453 g of bicarbonate to 226 g of citric acid.
How much mix does one bath bomb take?
Depends on the mould and on how hard you pack it, which is why both are inputs here rather than assumptions. A 65 mm sphere holds 144 ml, which is about 151 g packed firm — 137 g if you pack lightly by hand and 180 g out of a press. That spread of roughly a fifth is the usual reason a recipe that "makes six" makes five for someone else.
How much fragrance oil goes in a bath bomb?
0.5–3% of the batch weight, with 2% the working default — 18 g on this batch, or about 3.0 g per bomb. Essential oils usually go lower, around 1%. This is well under a cold process soap rate because nothing here is reacting with the oil: a surplus simply sits on the bomb, softens it and goes rancid. Your own supplier's IFRA category 9 figure is the number that binds.
Can I put shea butter or coconut oil in a bath bomb?
A little. 3% of the batch weight is a sensible default and about 6% is the ceiling — 54 g on this batch. Fat is the thing most likely to stop a bomb setting, because it coats the powder and keeps the particles from locking together. A bomb that is still faintly greasy the next morning has too much butter in it, not too little drying time.
What does SLSA do, and do I need it?
Sodium lauryl sulfoacetate. The only thing on this list that makes real bubbles rather than fizz. Past about 10% the bath foams more than it fizzes. It is optional — a bath bomb fizzes perfectly well without it, and what SLSA adds is foam rather than fizz. SLSA is a very fine powder and a respiratory irritant. Weigh it wearing a dust mask, in still air, and do not lean over the tub — a lungful sets off coughing that lasts. It is harmless in the bath and unpleasant on the way there.
Why will my bath bombs not harden?
Almost always one of three things. Too much fat: past 6% of the batch it never sets. Too much liquid: the wetting is a feel job, and about 25 ml per kilo is roughly where damp sand happens. Or humidity — a bomb needs 24 hours to harden in a dry room and closer to 48 in a damp one, and a batch made on a wet day can start fizzing quietly in the mould.
What are cornflour, kaolin and polysorbate 80 actually for?
Cornflour (cornstarch) at around 10% slows the reaction so the fizz lasts. Kaolin clay at about 3% binds and hardens the bomb so it survives being posted — its ceiling is 8%, past which the fizz goes sluggish. Polysorbate 80 at 2% is the honest answer to a stained tub: it ties oil and colour into the water instead of letting them ring the bath.
Do I have to use citric acid?
For a proper fizz, effectively yes. Citric acid carries three acidic protons on a small molecule, and every cupboard substitute carries fewer — cream of tartar has one on a molecule almost twice the weight, so gram for gram it delivers about 34% of the gas. The tool lets you pick another acid and restates the ratio for it, because a substitute needs a different weight of bicarbonate, not the same one.
Read before your next batch
- How to make bath bombs One part acid to two parts bicarbonate, packed damp and left alone overnight. Everything else is detail — but the detail is where batches fail.
- How to make shower steamers Same fizz as a bath bomb, several times the aroma, and absolutely no oils — because a steamer sits on a floor someone is standing on.
- Where to buy citric acid Everyone sells it and almost nobody sells it in the size or the grade you want. Here is where to look and what to check before you order.
- Bath bombs without citric acid Cream of tartar is the usual answer and it fizzes far less. Here is exactly how much less, and the recipe that gets the most out of it anyway.