Can I use table salt or tap water instead?

No to table salt, which adds sodium and chloride rather than hardness or buffering. Tap water already contains minerals, so dosing on top of it stacks unknown values onto known ones. Start from distilled or reverse osmosis water so the arithmetic holds.

Does the order of mixing matter?

Dissolve each salt fully before adding the next, and give the batch a minute. Undissolved crystals sitting at the bottom of a bottle mean the water you pour off is weaker than the figure you calculated.

BrewWater

How much Epsom salt and baking soda per litre of distilled water?

Short answer

Dosing distilled water is pure arithmetic. Epsom salt is 9.86 percent magnesium by mass, so one litre needs 0.101 g for every 10 mg/L of magnesium you want. Baking soda is 72.6 percent bicarbonate, giving 0.067 g per litre to reach the 40 mg/L alkalinity target.

Water volume (litres)Target magnesium (mg/L)Target alkalinity (mg/L as CaCO₃)

Dose into 1 L of distilled or reverse osmosis water
Addition	Grams
Epsom salt (MgSO₄·7H₂O)	0.101
Baking soda (NaHCO₃)	0.067

What are the doses for common volumes?

At 10 mg/L magnesium and 40 mg/L alkalinity as CaCO₃

Volume	Epsom salt	Baking soda
1 L	0.101 g	0.067 g
2 L	0.203 g	0.134 g
5 L	0.507 g	0.336 g

Measured values on this page, with sources
Measure	Value	Unit	Source
Magnesium content of Epsom salt by mass	9.861	% by mass	IUPAC Commission on Isotopic Abundances and Atomic Weights · verified 2026-07-25
Bicarbonate content of sodium bicarbonate by mass	72.63	% by mass	IUPAC Commission on Isotopic Abundances and Atomic Weights · verified 2026-07-25
Target total alkalinity for brewing water	40	mg/L as CaCO3	Specialty Coffee Association of America · verified 2026-07-25

How is the Epsom salt dose calculated?

Magnesium sulfate heptahydrate has a molar mass of 246.466 and contains one magnesium atom at 24.305, so magnesium is 9.861 percent of the crystal by mass. To reach a target concentration, divide it by that fraction and multiply by your volume. Ten milligrams per litre of magnesium therefore needs 101.4 mg of Epsom salt per litre.

How is the baking soda dose calculated?

Alkalinity is conventionally expressed as calcium carbonate rather than as bicarbonate, which adds one step. One mole of bicarbonate buffers as half a mole of calcium carbonate, so 40 mg/L as calcium carbonate is 48.8 mg/L of bicarbonate. Bicarbonate is 72.63 percent of sodium bicarbonate by mass, giving 67.1 mg of baking soda per litre.

Why weigh instead of using spoons?

The doses are around a tenth of a gram per litre, which is one to two orders of magnitude below a level teaspoon. No spoon measure resolves that at single-litre scale, and the error from eyeballing it is larger than the quantity itself. Either weigh on a scale reading to three decimal places, or mix a larger concentrate batch and measure that by volume.

What targets should you actually enter?

The alkalinity figure has a published target and it is in the table on this page. The magnesium figure does not: no coffee brewing standard specifies magnesium separately, so the ten milligrams per litre used in the examples here is an illustrative input rather than a recommendation. The calculator exists so you can enter your own number instead of inheriting someone else’s.

Common questions

Can I use table salt or tap water instead?: No to table salt, which adds sodium and chloride rather than hardness or buffering. Tap water already contains minerals, so dosing on top of it stacks unknown values onto known ones. Start from distilled or reverse osmosis water so the arithmetic holds.
Does the order of mixing matter?: Dissolve each salt fully before adding the next, and give the batch a minute. Undissolved crystals sitting at the bottom of a bottle mean the water you pour off is weaker than the figure you calculated.