Epsom salt contains about 9.86% elemental magnesium by weight, or roughly 495 mg per level teaspoon.
Bath crystals and laxative powders look simple, yet the numbers behind them trip people up. The household product is magnesium sulfate heptahydrate. Only part of that crystal is elemental magnesium. This guide shows the exact percentage, an easy spoon-by-spoon breakdown, and clear conversions you can use without a calculator.
Magnesium Content In Epsom Salt — By Weight And By Spoon
Epsom salt is the common name for magnesium sulfate heptahydrate, written as MgSO4·7H2O. Using standard atomic weights and the heptahydrate formula mass of about 246.47 g/mol, magnesium’s share (24.305 g per mole) works out to ~9.86% of the crystal by weight. Pharmacy labels also print the elemental amount per spoon; a typical label shows about 495 mg of magnesium in a level teaspoon. The table below brings both views together so you can size amounts by volume or by weight.
| Measure | Estimated Crystal Weight | Elemental Magnesium |
|---|---|---|
| 1 teaspoon (level) | ≈ 5.0 g | ≈ 495 mg |
| 1 tablespoon | ≈ 15 g | ≈ 1,485 mg |
| 1/4 cup | ≈ 60 g | ≈ 5,940 mg |
| 1/2 cup | ≈ 120 g | ≈ 11,880 mg |
| 1 cup | ≈ 240 g | ≈ 23,760 mg |
Where do those weights come from? If a level teaspoon lists about 495 mg of elemental magnesium and magnesium is 9.86% of the crystal, then that spoon weighs near 5.0 g (0.495 ÷ 0.0986). From there, tablespoons and cups scale in a straight line. Brands pack crystals a little differently, so gram figures are estimates. The elemental value printed on your package per teaspoon is the anchor to trust for that brand.
How The Percentage Was Calculated
First, pin down the chemical form: the household product is the heptahydrate (MgSO4·7H2O) with a formula mass of about 246.47 g/mol. Magnesium’s standard atomic weight is 24.305. Divide magnesium’s mass by the full formula mass to get the fraction that is elemental magnesium, then multiply by any weighed amount of crystal. The spoon reference cross-checks the math: many over-the-counter labels print ~495 mg magnesium per level teaspoon.
Why The Percent Matters
“Grams of magnesium sulfate” and “milligrams of magnesium” are not the same thing. The salt includes sulfur, oxygen, and water molecules. Only the magnesium atom counts toward the elemental number on nutrition panels and supplement facts. Using the percent keeps units straight and helps when comparing different salts, since each form has a different elemental share.
Label-Backed Reference Points
A national pharmacy label reads: each teaspoon contains about 495 mg of elemental magnesium. You can see an example on the DailyMed label. For the math inputs, the standard atomic weights table lists magnesium at 24.305, which pairs with the 246.47 g/mol heptahydrate value to yield the 9.86% figure; see the IUPAC atomic weights.
How Much Fits With Typical Daily Intake
For context, adult men generally aim near 400–420 mg of magnesium per day from all sources, and adult women near 310–320 mg. One level teaspoon of crystals already sits in that range. More than a teaspoon goes well beyond a day’s usual intake unless a clinician has set a plan. The laxative panel on the package gives separate directions for occasional constipation and is not a daily mineral routine.
Bath Use Versus Oral Use
Packaging also allows use as a soaking aid. A warm bath with two cups of crystals puts a large total amount of magnesium into the tub water, but that is not the same as swallowing that amount. Skin exposure is a different route. Many people enjoy the soak; still, daily intake targets come from food and, when needed, oral products used as directed.
Practical Conversions You Can Use
The table below turns common elemental targets into weighed crystals and household spoons. It uses the same 9.86% fraction and the teaspoon reference of ~495 mg magnesium.
| Target Elemental Mg | Approx. Crystal Weight | Approx. Teaspoons |
|---|---|---|
| 100 mg | ≈ 1.02 g | ≈ 0.20 tsp |
| 200 mg | ≈ 2.03 g | ≈ 0.40 tsp |
| 350 mg | ≈ 3.55 g | ≈ 0.71 tsp |
| 420 mg | ≈ 4.26 g | ≈ 0.85 tsp |
| 500 mg | ≈ 5.07 g | ≈ 1.01 tsp |
How To Measure Accurately At Home
Level Your Spoon
Use a dry measuring spoon and scrape the top flat with a knife or card. A heaping spoon can double the amount without you noticing, so get that surface flat.
Weigh When It Matters
A pocket scale removes guesswork. If a plan calls for 2 g of crystal, weigh 2 g and you’re set. To estimate elemental magnesium from a weighed amount, multiply grams of crystal by 0.0986. A 3 g portion holds ~296 mg magnesium (3 × 0.0986).
Dissolve Fully
For oral use, dissolve the measured amount in a full glass of water and follow the package directions. Stir until no grains remain. For soaks, sprinkle crystals into warm water and swirl by hand until clear.
Mind Brand Differences
Spoon weights vary a bit with crystal size and humidity. The elemental value printed on your label per teaspoon is the figure to follow for that brand. If your label lists a slightly different number per spoon than 495 mg, use your label’s number in place of the one shown here.
Why Labels Show Elemental Magnesium
Magnesium salts come in many forms. Each form carries a different fraction of elemental magnesium. Oxide, citrate, glycinate, chloride, and sulfate all yield different numbers. Elemental counts let you compare them apples-to-apples. A product with 100 mg elemental magnesium gives 100 mg of the mineral regardless of the salt it came with.
What About Absorption?
Absorption depends on form, dose, and the person taking it. Sulfate salts draw water into the bowel at higher amounts, which is why the laxative panel exists. That effect can appear before the body absorbs much magnesium. If your goal is daily nutrition from food and standard supplements, check the elemental number and follow directions made for that use case.
Dosing Notes, Safety, And Sensible Use
Follow your package panel. Oral crystals are for short-term relief of occasional constipation. People with kidney disease, on certain medicines, or who are pregnant or nursing should talk with a healthcare professional before taking oral doses. Loose stools are a known effect at higher amounts. If you track daily intake for nutrition, count only the elemental number, not total grams of crystals.
Worked Examples
Example 1: You Want 200 mg Of Elemental Magnesium
Check the conversion table. Two hundred milligrams lines up with about 0.40 of a teaspoon. If you prefer a scale, weigh ~2.03 g of crystal.
Example 2: You Only Have A Tablespoon
One tablespoon equals about three level teaspoons. Using the label figure, that spoon holds ~1,485 mg of elemental magnesium. That is far beyond a day’s usual intake target and fits the product’s role as a laxative when used as directed.
Example 3: You’re Mixing A Bath
Two cups of crystals weigh near 480 g and hold ~47 g of elemental magnesium in the water. That looks large on paper, but soaking is a separate use and the dissolved amount in the tub does not equal bodily intake.
Storage, Handling, And Quality Checks
Keep the bag or carton tightly sealed to prevent clumping. Store in a dry place. If clumps form, press them out before measuring. Check that the front panel lists magnesium sulfate USP 100% and that the inner seal is intact. For oral use, read every line of the drug facts panel and match your dose to the spoon or cup you are using.
Method Notes
The 9.86% figure comes from standard atomic weights for magnesium (24.305) and the heptahydrate formula mass (246.47 g/mol). Elemental per spoon uses the pharmacy label that lists ~495 mg magnesium per level teaspoon. Conversions assume level, not heaping, spoons. If your label prints a different elemental number per spoon, swap that value into the same math and your results will line up with your brand.
Bottom Line For Readers Who Need The Numbers
Elemental magnesium makes up ~9.86% of magnesium sulfate heptahydrate. A level teaspoon delivers ~495 mg of magnesium, a tablespoon ~1,485 mg, and a quarter cup ~5,940 mg. For daily nutrition, adults land near 400 mg from all sources. Food covers a large share; oral crystals are for short spurts as labeled.