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.
Mix 453 g of bicarbonate of soda with 226 g of citric acid — one part acid to two parts bicarb by weight — plus cornflour, epsom salt and a little clay. Add your oils and fragrance, spritz with witch hazel until the mix holds like damp sand, overfill both halves of the mould, press them together hard, and leave it overnight before you unmould.
A bath bomb is two dry powders that ignore each other completely until water arrives. Citric acid and bicarbonate of soda sit in the same mould for months doing nothing; drop them in a bath and the acid gives up its protons, the bicarbonate gives up its carbon dioxide, and the whole thing comes apart in a cloud of fizz. Everything else in the recipe is there to make that reaction slower, prettier, kinder to skin, or possible to post.
The ratio is one part acid to two parts bicarbonate by weight, and it is worth knowing why rather than taking it on faith. One gram of citric acid can only consume 1.31 g of bicarbonate, so 1:2 runs the bicarbonate about 52% over what the reaction can use. That surplus is deliberate: it guarantees no acid survives into the bathwater, and it gives the bomb enough body to hold a shape.
The recipe below makes 6 standard bombs from 906 g of mix, or 12 from 1812 g. If your mould is a different size, the bath bomb recipe builder restates all of it — and the recipe scaler will do the same to a recipe you found somewhere else.
The recipe
Shares are of the finished batch, which is the only way of quoting them that stays true when you change the ratio. A 65 mm sphere holds 144 ml, which is about 151 g of mix packed firm.
How fast a fizz do you want
Everything happens in the first minute. Dramatic in a video, over before the bath is run. That is the bicarbonate weight on a 6-bomb batch; the acid takes up the rest of the base.
What nearly every published recipe means. A full fizz that lasts, with bicarbonate to spare. Start here. It is what every recipe you find means, and it is the one to move away from once you know what you want changed.
A softer, longer fizz and a milder bath. The one to move towards for sensitive skin. Worth trying if a standard bomb leaves your skin feeling tight.
What decides whether it works
Water starts the reaction, so every drop you add is fizz you are spending in the bowl rather than in the bath. Witch hazel is the standard answer because it is mostly alcohol and flashes off faster than water does. About 25 ml per kilo is roughly where damp sand happens, but it is a feel job: spritz, squeeze a handful, and stop the moment it holds its shape without crumbling.
The same 65 mm mould takes 137 g packed lightly and 180 g out of a press — a spread of about 32%. Overfill both halves, heap them, and press hard enough that the excess squeezes out of the seam. A bomb that falls apart at the equator was not packed hard enough.
Butters and oils feel like the luxurious part and are the commonest reason a bomb never hardens. 3% of the batch is plenty and about 6% is the ceiling — fat coats the powder and keeps the particles from locking together. If yours is still faintly greasy the next morning, it is the butter, not the drying time.
Bicarbonate and citric acid both pull moisture out of the air, so a humid day starts the reaction for you. The same recipe that behaves in February can rise and crack in August. Work in the driest room you have, and give a batch 48 hours rather than 24 when the air is damp.
Cornflour (cornstarch) at around 10% slows the reaction so the fizz lasts more than a minute. Kaolin clay at about 3% binds the mix so the bomb survives being posted. Epsom salt (fine) at 8% is there for the bath rather than the bomb — and it must be the fine grade, because coarse crystals make a bomb that crumbles along the grain. There is more on all of them in the additives guide.
Making a batch
A cup of bicarbonate and a cup of citric acid do not weigh the same, so a recipe in cups is not the ratio it appears to be. A scale reading to 1 g turns bath bombs from a lottery into a repeatable thing, and it costs less than one failed batch of fragrance oil.
Sieve the powders together first and break every lump — a lump of bicarbonate is a soft spot in the finished bomb. Then add the oils and fragrance, then the witch hazel a spritz at a time with your other hand moving through the mix. Pouring liquid into one spot sets off a patch of fizz you cannot recover.
The mix is at its best for a few minutes. Have the moulds out and open before you wet anything, work one half of the batch at a time if you are making a lot, and keep the rest covered — a bowl of damp mix left standing dries into a brick.
Unmoulding early is what cracks the shell. Give it 24 hours in a dry room, tap the mould rather than prising it, and rest the bomb on a soft cloth rather than a hard tray — the flat spot forms while it is still soft.
A bath bomb left open is slowly reacting with the air in the room. Shrink wrap or a sealed bag keeps it fizzing properly for months; a shelf keeps it for a fortnight. If yours have gone soft and dusty, that is what happened.
Crumbling, cracking, doming, warty tops and bombs that never harden almost all trace back to moisture or packing. The troubleshooting guide works through each of them with the fix.
Size it to your own mould
Enter your mould, how hard you pack it and how many you want, and every ingredient above is restated for your batch — with the cost per bomb worked out at the bottom.
Open the recipe builder →Keep the ratio to hand
The bicarb-to-citric ratio, binder rates and mould weights on one printable card. Free.
Estimates, not advice. Every figure on this page is computed from published reference values and standard measures, so your own inputs, equipment and conditions will move it. Nothing here is professional advice — talk to a qualified professional before acting on it.