Bath Salt Calculator: How Much Salt for Your Tub
Cups, grams and cost per soak for Epsom salt, Dead Sea salt, magnesium flakes and baking soda — by tub size and the kind of night you're having.
Most people use a third of the bath salt that does anything. Pick your tub, what you are adding and what kind of night it is, and this tells you the number in cups and grams, what it costs per soak, and how long to stay in.
Use
How the numbers are worked out
The baseline is 2 cups of Epsom salt in a standard 40-gallon tub — the amount we landed on in how much bath salt to use per bath, and the low end of what the bag itself prints. Dead Sea salt is dosed at half that because it is a stronger mineral salt with a heavier grain (see Epsom vs. Dead Sea salt); magnesium flakes sit in between; baking soda is not a salt at all and half a cup is plenty. A small or half-filled tub gets 60% of the dose, a deep soaking tub 150%, and a sore-muscle night gets 50% more than an everyday soak. Costs use typical bulk-bag prices: about $1.40/lb for Epsom, $3.20/lb for Dead Sea, $5/lb for magnesium flakes, $0.90/lb for baking soda — your bag will differ a little.
Bath salt ratio chart
| Salt | Small tub | Standard tub | Deep tub | Sore-muscle night (standard tub) |
|---|---|---|---|---|
| Epsom salt | 1¼ cups | 2 cups | 3 cups | 3 cups |
| Dead Sea salt | ½–¾ cup | 1 cup | 1½ cups | 1½ cups |
| Epsom + Dead Sea mix (2:1) | 1¼ cups | 2 cups | 3 cups | 3 cups |
| Magnesium chloride flakes | 1 cup | 1½ cups | 2¼ cups | 2¼ cups |
| Baking soda | ¼ cup | ½ cup | ¾ cup | ¾ cup |
Measure with a real measuring cup the first few times; a "handful" is about a third of a cup, which is how everyone ends up under-dosing. Add the salt under the running tap so it dissolves before you get in, and if you want to make your own blend, the DIY bath salts recipe uses the same 2 : ½ : ¼ base.
What this calculator does not do
It does not make medical claims. Two cups of Epsom salt will make the water feel softer and the soak feel like something; it will not detox anything, and the evidence on magnesium absorption through skin is thin. If you are pregnant, have a heart condition or open skin, ask your doctor before changing what goes in the bath. If your skin runs dry, use the light setting and read the dry-skin soak first.
Mami says: The number is two cups. If you only remember one thing from this page, remember two cups, under the tap, then get in.
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