TDS, Hardness, Alkalinity, and pH Are Four Different Numbers
A quick reference for four water measurements: what they describe, which units they use and where the numbers stop helping.
Editorial · not medical advice
Hardness and alkalinity can both be reported in mg/L as calcium carbonate, yet they describe different properties. pH has no concentration unit at all. A report is easier to read once those distinctions are visible.
| Measure | Describes | Usually reported as |
|---|---|---|
| TDS | Combined dissolved material | mg/L; often estimated from conductivity by a handheld meter |
| Hardness | Mainly calcium and magnesium | mg/L as CaCO3 |
| Alkalinity | Acid-neutralizing capacity | Often mg/L as CaCO3 |
| pH | Acid-base condition at measurement | A logarithmic value, not mg/L |
TDS asks how much is dissolved
Total dissolved solids, or TDS, is a combined quantity. It represents the dissolved material in a water sample rather than identifying each substance separately.
A laboratory can measure TDS by filtering a sample, evaporating the filtrate and weighing the dried residue, as in EPA Method 160.1. A handheld TDS meter usually does something less direct: it measures electrical conductivity and applies a conversion factor. Because dissolved ions conduct electricity, that estimate can be useful for comparison. It is still an estimate, and the conversion depends on the composition. Hanna Instruments explains this conversion in its meter guidance.
TDS does not tell you whether the total comes mostly from calcium, magnesium, sodium, bicarbonate, sulfate or something else. It is a quantity, not a safety score and not a complete mineral profile.
Hardness asks about calcium and magnesium
Water hardness is driven mainly by dissolved calcium and magnesium. It is commonly reported in milligrams per liter as calcium carbonate, written mg/L as CaCO3. That is a shared chemical-equivalent basis for reporting, not a claim that visible calcium carbonate particles are floating in the water.
Hardness helps explain practical behavior. Harder water can form scale when heated and can require more soap to produce the same lather.
Calcium and magnesium contribute to TDS, but hardness is not TDS. A water can have substantial TDS from sodium salts while remaining relatively low in calcium and magnesium. Another can derive a larger share of its dissolved-solids total from the ions that create hardness.
Knowing the total cannot tell you which case you have.
Alkalinity asks how strongly the water resists acid
Alkalinity is the water’s acid-neutralizing capacity—its ability to resist a change in pH when acid is added. In many natural waters, bicarbonate and carbonate do most of that work.
Like hardness, alkalinity is often reported as mg/L as CaCO3. The shared reporting convention can make the two look like the same measurement. They are not. Hardness describes primarily calcium and magnesium. Alkalinity describes the combined action of compounds that neutralize added acid.
Alkalinity also is not a synonym for “alkaline water.” The word alkaline usually refers to a pH above 7. Alkalinity measures capacity, not simply the starting pH.
pH asks about the balance at that moment
pH describes how acidic or basic water is. At 25°C, a reading below 7 is acidic, 7 is neutral and above 7 is basic or alkaline. The neutral point changes with temperature, as the USGS pH field manual explains. The scale is logarithmic: a one-unit change represents a tenfold change in hydrogen-ion activity.
The starting reading does not tell you how much acid the water can neutralize. Two waters can begin at the same pH but have different alkalinities, so the same addition of acid can change their pH by different amounts.
pH also does not disclose the mineral profile. Two waters can both read 7.5 while containing different ions at different concentrations. Conversely, a water can carry substantial dissolved minerals and have a pH below 7.
Read the four as a system
For a full reading, start with the individual ions. Then use TDS to understand the total, hardness to understand calcium-and-magnesium behavior, alkalinity to understand acid-neutralizing capacity, and pH to understand the present acid-base state. Check whether values came from a laboratory method or a handheld estimate, and confirm the units before comparing two reports.
These numbers can help explain water’s behavior. None of them alone can establish that water is safe. That requires testing for the specific substances and microorganisms relevant to the source and product.
Recovery’s published production profile lists individual minerals alongside TDS and pH. It does not currently supply an alkalinity value. For comparisons that depend on acid-neutralizing capacity, ask for the actual alkalinity measurement; neither TDS nor pH can fill that gap.
Sources checked September 22, 2026.
Sources
- U.S. Geological Survey — Hardness of water
- U.S. Geological Survey — Alkalinity and water
- U.S. Geological Survey — pH and water
- U.S. Geological Survey — pH field manual
- National Environmental Methods Index — EPA Method 160.1
- Hanna Instruments — The relationship between TDS and conductivity
- Unbelievable Water — Recovery production profile
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