Best bath bomb molds
The mold decides your batch size, and a flexing one decides whether the bombs hold together at all. Here is what each type is for.
Buy stainless steel hemispheres for spheres — they do not flex, so all your pressure goes into the mix rather than into the mould. A 65 mm pair is the standard size and takes about 151 g of mix. Keep silicone for shaped moulds and embeds, where releasing a detailed shape matters more than packing pressure does, and consider a press only once you are making the same bomb every week.
A bath bomb mould does one job that no other piece of kit does: it holds the mix still while you compress it. That is the whole trick. Everything people dislike about their bombs — crumbling seams, soft spots, halves that will not marry — comes back to whether the mould let them apply pressure or absorbed it.
It also quietly fixes your batch size. A 65 mm sphere takes about 151 g of mix, so 6 of them is 906 g and every recipe you make from then on is scaled to that. A 45 mm mini takes 50 g — which is why the same recipe that "makes six" makes 18 for someone with a smaller mould.
The table below is worked out from each mould's diameter rather than from a listing, because moulds sold as "large" vary far more than moulds sold as 65 mm. Run your own through the recipe builder and it will give you the batch to match.
What each mold size holds
Capacities computed from the diameter each mould is sold at, then through the packed firm density. Bombs per kilo is per kilo of finished mix, which is the figure to buy ingredients against.
What pressure is worth
What a flexing mould actually gets into the cavity. Lighter bombs, softer bombs, and the crumbling seam everyone complains about.
The same cavity, with a mould that does not give. Denser, harder, and it holds together in a padded envelope.
About 32% more mix in the same volume than a light hand manages, and identical bomb to bomb. It also uses more mix per bomb, which is worth costing before you buy one.
What to judge a mold on
A thin plastic hemisphere bows outwards as you press, so the force goes into the mould rather than into the mix. Stainless steel and thick moulded plastic do not. If your bombs crumble at the seam despite a correctly wet mix, this is the likeliest cause, and it is the one people never suspect because the recipe looks fine.
Silicone flexes by design, which is exactly wrong for a sphere you need to compress and exactly right for a detailed shape you need to release. Use silicone for shaped moulds, embeds and anything with an undercut; use metal for round bombs. A silicone sphere is the worst of both.
A mould is a standing decision about how much you make. 6 standard bombs is 906 g of mix, of which 226 g is citric acid — the ingredient you will reorder first. A tray of twelve minis is a smaller commitment per session and a much better way to test fragrances.
Cheap hemispheres are often slightly out of round, so the halves rock instead of seating. You end up with a ridge round the equator and a bomb that splits along it. Put two halves together empty before you buy a set of ten — a mould that does not close cleanly empty will not close cleanly full.
A press is not a beginner's purchase and it is a genuine upgrade for anyone selling. It gets about 19% more mix into the same cavity than firm hand-packing, so the bombs are harder, they survive posting, and — the real benefit — they are identical to each other. The trade-off is that each bomb uses more mix, and a hand press is slower than two hands for small runs.
A press usually makes a flat-backed puck rather than a sphere — 116 ml in the 65 mm format, about 145 g pressed. Flat cavity trays are also what shower steamers want: a 60 mm cavity takes 65 g at the denser 1.15 g per millilitre a pressed puck reaches.
Before you buy
Six matched hemispheres of one size beat a variety pack every time. A recipe scaled to one mould can be made without thinking; a drawer of mismatched sizes means every batch starts with arithmetic.
Listings say small, medium and large, and mean different things. The number is what matters — a 65 mm sphere is 144 ml wherever you buy it. If a listing does not state a diameter, that tells you something about the listing.
People buy presses to solve crumbling and are often disappointed, because the mix was the problem. Get the moisture and the ratio right by hand first — the troubleshooting guide covers it — and then buy the press for consistency and speed.
Wash and dry moulds properly, particularly stainless. A drop of water left in a hemisphere starts the reaction the moment the next batch goes in, and you get one inexplicably pitted bomb per session.
A set of moulds is a real cost, and so is a press. The recipe builder has a cost section so you can see what a bomb actually costs you including packaging and your own time, which is the number that decides whether a press pays for itself.
Size a batch to the mold you own
Pick your mould and how hard you pack it, and every ingredient is restated for the number of bombs you actually want.
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.