An ion-exchange water softener adds roughly 0.46 mg of sodium to each litre of water for every 1 mg/L of hardness (as CaCO₃) it removes. At 100 mg/L hardness that is about 46 mg/L of sodium. At 300 mg/L it is about 138 mg/L. The figure comes from chemistry, not from a measurement of any particular machine. If you follow a sodium-restricted diet, talk to a doctor or dietitian before you decide how to handle drinking water. This guide is general information, not medical advice.
Why a softener adds sodium
A standard softener holds resin beads loaded with sodium. Hard water passes through. The beads trade calcium and magnesium for sodium. The salt you add to the brine tank recharges the beads afterwards.
Hardness is counted in calcium carbonate equivalents (CaCO₃). One calcium ion carries two positive charges, and one sodium ion carries one. So each calcium ion that leaves is replaced by two sodium ions.
The arithmetic
We use molar masses to get the ratio.
- 1 mol of CaCO₃ is 100.09 g and carries 2 equivalents of charge.
- Removing it releases 2 mol of sodium, which is 2 × 22.99 = 45.98 g.
- The ratio is 45.98 ÷ 100.09 = 0.459 mg of sodium per mg of CaCO₃ removed.
This assumes complete exchange and no losses. It also assumes the resin does not change the sodium already in the supply. Treat the result as an estimate of the increase, then add the sodium that was in your water to begin with. Your water supplier can tell you that figure.
Softeners are often set to leave a little hardness in the water, or a bypass valve blends in some raw water. Both lower the added sodium in proportion. The table shows the upper bound: full softening.
| Raw hardness (mg/L CaCO₃) | Sodium added (mg/L), full softening | Example |
|---|---|---|
| 98.2 | 45 | Wuppertal median |
| 100 | 46 | Generic |
| 117.8 | 54 | Essen |
| 138.1 | 63 | Bremen median |
| 200 | 92 | Generic |
| 214.2 | 98 | Hannover mean |
| 281.0 | 129 | Amiens |
| 282.0 | 129 | Munich |
| 300 | 138 | Generic |
| 328.4 | 151 | Berlin median |
City hardness figures are the ones on each city page. Added sodium is our calculation (hardness × 0.459, rounded to whole mg/L), not a measured value. To run your own numbers, use the calculator.
Hardness often varies inside one city. Hamburg reports a range of 91.0 to 326.7 mg/L across sampling points, which gives about 42 to 150 mg/L of added sodium depending on where you live. Frankfurt am Main shows a similar spread. Find your own figure with the lookup page.
How this compares with guidance values
There is no single worldwide limit for sodium in tap water, and the numbers below are guidance, not rules for your kitchen.
- The World Health Organization proposes no health-based guideline for sodium in drinking water. It notes that taste can become unacceptable above about 200 mg/L.
- Health Canada sets an aesthetic objective of 200 mg/L or less, also for taste.
- The US EPA’s 2003 advisory is not enforceable. It gives a range of 30 to 60 mg/L as a taste level. It also mentions 20 mg/L as a level for people on very restricted sodium diets. Check the advisory for the exact assumptions behind that number.
Full softening of water at 100 mg/L hardness (46 mg/L added) already passes the 20 mg/L level, before counting the sodium in the supply. To reach 200 mg/L from the softener alone, raw hardness would need to be about 436 mg/L (200 ÷ 0.459). That is above the median of every city listed here. A clinician can say whether any of these numbers matter for you.
Options for a low-sodium household
Keep one unsoftened tap. This is the usual advice. Water companies such as Anglian Water recommend drinking and cooking with water from an unsoftened tap. Most installers fit the softener after a branch to the kitchen cold tap, so that tap stays on raw mains water. Ask the installer to confirm this before the job. In many homes the kitchen tap is the only one where sodium matters, because the rest of the water goes to showers, laundry and the dishwasher.
Switch to potassium chloride salt. Potassium chloride can be used in many softeners in place of sodium chloride. The resin then releases potassium instead of sodium. By the same method, 2 × 39.10 = 78.20 g of potassium per 100.09 g of CaCO₃ gives about 0.78 mg of potassium per mg of hardness removed. That is more mass than with sodium, because potassium is heavier. Two cautions apply. Some people must limit potassium, for example because of kidney disease or certain medicines, so ask a clinician first. Also check your softener manual, since not every model is approved for potassium salt.
Add a drinking-water filter. A reverse osmosis unit under the sink removes most dissolved ions, including sodium. Removal rates differ between models. Look for the manufacturer’s tested figure for sodium, and ask for independent test data. Ordinary carbon jug filters are not designed to remove sodium.
Soften less. Setting the valve to leave some hardness, or blending in raw water, cuts the sodium added in proportion. The trade-off is more scale. Our softener versus conditioner versus inhibitor guide covers systems that do not add sodium, and their limits.
Skip softening where hardness is low. In places such as Wuppertal, Clermont-Ferrand (76.0 mg/L) and Essen, a whole-house softener may not be needed. Bochum sits at 133.0 mg/L.
Where it matters most
Hard supplies give the biggest sodium increase: Berlin, Düsseldorf (267.8 mg/L), Le Mans (291.0 mg/L) and Amiens. Even there, the numbers above are an upper bound for the drinking tap and do not apply to an unsoftened line. For skin questions, see hard water and skin.
Frequently asked questions
Is softened water safe to drink? Regulators set no health-based limit for sodium in drinking water, and WHO and Health Canada mention taste at around 200 mg/L. Most households drink softened water without special steps. If you are on a sodium-restricted diet, or you prepare infant formula, ask a doctor, and consider an unsoftened drinking tap.
Does a salt-free system add sodium? Template-assisted crystallisation and similar conditioners do not exchange ions, so they add no sodium. They also do not remove hardness, so they work differently from a softener. See the comparison guide above.
Can I find the sodium in my own water? Yes. Ask your water supplier for its latest analysis, or test a sample from your softened tap in a certified laboratory. Do not rely on our calculation for a sodium-restricted diet, because it ignores the sodium already in your supply.
Sources
- World Health Organization. Sodium in drinking-water: chemical fact sheet. who.int
- US EPA. Drinking Water Advisory: Consumer Acceptability Advice and Health Effects Analysis on Sodium, 2003 (EPA 822-R-03-006). epa.gov
- Health Canada. Guidelines for Canadian Drinking Water Quality: Guideline Technical Document, Sodium. canada.ca
- Anglian Water. Water quality fact sheet: water softeners. anglianwater.co.uk
- City hardness figures: utility sources listed on each city page linked above.