Bath bomb recipe scaler
Type in a recipe as it was printed, say what you want to make instead, and every ingredient is restated for your mould — with the witch hazel worked out again rather than multiplied.
INPUTS
How is your recipe written?
Type it in as printed. The first thing this does is turn those weights into shares of the batch.
The recipe you have
IngredientWeight
g
g
g
g
g
g
As written, about 3 bombs in the mould below.
The size you want
Scale it to
Overfilled, both halves compressed hard and twisted together. What most recipes assume. The witch hazel below is recalculated from the new batch, not multiplied with it — that is the figure people scale straight through, and it is the one that sets the fizz off in the bowl.
RESULTS
Batch, rescaled
1811.8 g
3.70× the original from 490.0g
Weigh out
| Ingredient | Share | Weight |
|---|---|---|
| Bicarbonate of soda | 46.9% | 850.4 g |
| Citric acid (anhydrous) | 23.5% | 425.2 g |
| Cornflour (cornstarch) | 12.2% | 221.9 g |
| Epsom salt (fine) | 12.2% | 221.9 g |
| Butters and oils | 3.1% | 55.5 g |
| Fragrance or essential oil | 2.0% | 37.0 g |
| Witch hazel | to feel | 45 ml |
The ratio this recipe is really running
1 : 2.00
citric acid (anhydrous) to bicarbonate 52% bicarbonate to spare against 1:2 conventional
Bombs it makes
12
Mix per bomb
151.0 g
Shares are what travel between batches, not weights. Once a recipe is written as percentages it fits any mould you own — which is why the first thing this does with your weights is throw them away and keep the proportions.
Keep the ratio to hand
The bicarb-to-citric ratio, binder rates and mould weights on one printable card. Free.
Get your free download
Bath Bomb Ratio Card
How this is worked out
The recipe is converted to shares of the batch before anything else happens. That single step is what makes it portable: a printed recipe is locked to whatever mould its author owned, and its proportions are not. Once the shares exist, the new batch weight comes from wherever you asked — a weight, one cavity, or a bomb count through the mould and packing you told it about — and every ingredient is share × batch. Anything the recipe derives rather than states is then recalculated from the new batch rather than scaled with it: the witch hazel, the gas the reaction releases, and the ratio the recipe is really running. Scaling a derived figure gives the right answer only until someone edits a row, and then it is quietly wrong. The example recipe above is 490 g as printed — about 3 bombs in a 65 mm mould — and 1812 g at 12.
share = ingredient ÷ recipe total
batch = bombs × (mould ml × packing density)
new = share × batch
wetting, gas and yield: recalculated from batch, never scaled
Questions makers ask
How do I double a bath bomb recipe?
Multiply every ingredient by two — but not the witch hazel, and not anything the recipe worked out for you. Liquid is the figure people most often multiply straight through, and it is the one that sets the fizz off in the bowl: it is added by feel, roughly 25 ml per kilo, so a doubled batch needs it recalculated rather than doubled. This tool converts the recipe to percentages first and then recalculates everything derived from the new batch weight.
How much mix does my mold need?
A 65 mm sphere holds 144 ml, which is about 151 g packed firm, so 12 of them need 1812 g of mix. A 45 mm mini holds 48 ml — 50 g — which is why a recipe that made six in one person's mould makes eleven in another's.
Why does the same recipe make a different number of bombs for me?
Packing, almost always. The same 65 mm mould takes 137 g packed lightly by hand and 180 g out of a press — a spread of roughly 32%. Mould diameter is the other half of it, and moulds sold as "large" vary far more than moulds sold as 65 mm.
Can I scale a recipe written in cups?
Not reliably, and this is the honest reason to move off volume measures. A cup of bicarbonate of soda and a cup of citric acid do not weigh the same, so a recipe in cups is not the ratio it looks like. Weigh the cup measures once on a scale, type those weights in here, and the recipe becomes portable for good.
What ratio should the scaled recipe come out at?
The tool reports what your recipe is actually running rather than what it claims. The example above works out at 1:2.00 bicarbonate to citric acid — 850 g to 425 g at 12 bombs — against the conventional 1:2. Anything from about 1:1.5 to 1:3 is a real recipe; below 1:1.3 there is acid left over, which stings rather than fizzes harder.
Does scaling up change how the batch behaves?
Yes, in one way worth knowing about. A bigger batch takes longer to get into the moulds, and the mix is drying and reacting the whole time you are working. Past about a kilo most people split it into two bowls and wet the second one only when the first is packed. The proportions do not change; your working time does.
The recipe I found has a lot of butter in it. Is that a problem?
Probably. Anything past about 6% of the batch as butters and oils tends not to harden, and scaling it up simply makes more soft bombs. The warnings here are checked against the scaled percentages rather than the original weights for exactly that reason — a share that was already too high stays too high at any size.
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.