Epsom salt or magnesium chloride?

Both are magnesium salts, and both dissolve into a warm bath. They differ in chemistry, in how the water feels, in how they keep and in price.

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An open case of six bath bomb tiles, each in its own matte black box, with a bare lavender tile in front — Six tiles, each pressed with 40 g of magnesium sulfate and 25 g of magnesium chloride.

MgSO₄·7H₂OEpsom salt: magnesium sulfate

MgCl₂·6H₂OBath flakes: magnesium chloride

65 g40 g Epsom salt and 25 g flakes, in one Twelve Bath tile

Side by side

Epsom saltMagnesium chloride flakes

Chemical nameMagnesium sulfate heptahydrateMagnesium chloride hexahydrate

Looks likeSmall clear crystalsFlat white flakes

Magnesium by weightAbout 10%About 12%

In the waterDissolves to clear waterDissolves fast; the water feels silkier

Keeping itKeeps well in a closed bagDraws moisture from the air; keep it sealed

Price per gramUsually the lowestUsually higher

The chemistry, briefly

Epsom salt is magnesium sulfate with seven water molecules held in each crystal (MgSO₄·7H₂O). It's named after a spring in Epsom, England. Bath flakes are magnesium chloride with six (MgCl₂·6H₂O), often taken from brine or from deep salt deposits such as the Zechstein seabed under the Netherlands.

Water is a large share of the weight of both. That's why 100 g of Epsom salt holds about 10 g of magnesium, and 100 g of flakes about 12 g. We give those numbers so you know what's in the water, not as a reason to pick one.

How each one feels in the tub

Both leave the water clear. Flakes dissolve within a minute or two and give the water a slippery, silky feel. Epsom salt feels closer to plain warm water.

Magnesium chloride holds on to moisture, so it can leave skin feeling a little tacky as it dries. A quick rinse after the bath takes care of it.

Which to choose

Epsom salt if price matters most. It's sold almost everywhere and is the cheapest per gram.
Flakes if you like a silkier feel to the water. Keep the bag closed between baths.
Both. They mix without trouble. Each Twelve Bath tile is pressed with 40 g of magnesium sulfate and 25 g of magnesium chloride flakes, and a Deep Soak sachet adds 180 g of flakes when you want the bath to hold more.

A note on claims

You'll read a lot online about what each salt does for the body. We don't make claims about what magnesium does. What we can tell you is what's in the water, to the gram. Pregnant, or have a medical condition? Ask your doctor first.