TDS stands for total dissolved solids: everything dissolved in your water, measured in parts per million (ppm), including calcium, magnesium, sodium, sulfates, and trace minerals. The EPA’s guideline for good-tasting water is under 500 ppm. San Diego’s imported supply commonly runs on the high side of that range, which is the main reason reverse osmosis outsells every other filter type in this county.
What is TDS in water?
TDS is a single number that adds up every mineral, salt, and metal dissolved in a sample of water. It doesn’t identify what’s in the water, just how much total material is in there. A TDS meter passes a small electrical current through the water and reads how easily it conducts, since dissolved minerals carry that current. More dissolved material means higher conductivity, and higher conductivity means a higher TDS number.
TDS isn’t automatically bad. Some dissolved minerals are why tap water tastes better than distilled water. The question is where the number lands and what’s actually in the mix.
What’s a good TDS reading?
There’s a widely used classification for TDS in drinking water. It’s a general guide, not a strict cutoff.
| TDS reading (ppm) | What it means |
|---|---|
| 0-150 | Very low, often reverse osmosis or distilled water |
| 150-300 | Good, typical of lightly treated municipal supplies |
| 300-500 | Fair, at the EPA’s recommended upper limit for taste |
| 500-1,000 | Poor, noticeable taste and faster scale buildup |
| Above 1,000 | Not recommended for drinking without treatment |
The EPA’s 500 ppm figure is a secondary standard, meaning it’s about taste and appearance, not a health-based limit. Water above it is still legal to drink. It just tastes flatter, leaves more spotting on glassware, and pushes appliances toward scale faster.
TDS vs. water hardness: they’re not the same thing
TDS and hardness get confused constantly, and they measure different things. Hardness counts only calcium and magnesium, reported in grains per gallon (gpg). TDS adds up everything dissolved in the water, including the calcium and magnesium that make up hardness, plus sodium, sulfates, nitrates, and trace metals. See is San Diego water hard? for the full hardness picture by city.
A practical way to tell them apart: a water softener fixes hardness by swapping calcium and magnesium for sodium, but it doesn’t lower TDS, since it’s still trading one set of dissolved minerals for another. If your goal is a lower TDS number, softening alone won’t get you there.
Why does San Diego’s TDS run high?
San Diego imports most of its water from the Colorado River and the State Water Project, both of which travel long distances through hard rock and agricultural land before reaching a tap here. That trip picks up dissolved minerals the whole way, which is the same reason the county’s water tests hard in our hardness coverage. The utility also disinfects with chloramine rather than free chlorine, and chloramine itself adds a small amount to the TDS total.
None of that makes San Diego tap water unsafe. It means the baseline mineral load is higher than what someone moving from a soft-water region is used to, and it’s part of why filtered and treated water feels like a meaningful upgrade here.
How to test your water’s TDS
A basic TDS pen costs $10 to $20 and gives a rough number in seconds: dip it in a glass of water and read the display. It’s a fine way to spot-check filtered water or confirm a system is still working, but it doesn’t tell you what’s actually dissolved in there, just the total.
For an actual breakdown, Filter Pros San Diego runs a free in-home water test that measures your exact TDS alongside hardness, chloramine, and pH at your own tap, not a county average. That’s the number worth acting on before buying any system.
Does reverse osmosis reduce TDS?
Yes, and by a wide margin. A quality reverse osmosis membrane reduces TDS by 85 to 98%, along with hardness, chloramine, nitrates, and PFAS if present. See reverse osmosis cost in San Diego for full system pricing. A standard carbon filter or an under-sink filtration unit improves taste and smell but leaves most of the TDS number untouched, since carbon adsorbs organic compounds and chlorine rather than rejecting dissolved minerals at the membrane level.
If your free water test comes back with a TDS reading you want lower, reverse osmosis drinking water is the fix that actually moves that number, not a pitcher filter or a softener.
Frequently asked questions
What is a normal TDS level for tap water?
Most municipal tap water in the US falls between 100 and 500 ppm. Under 300 ppm is considered good, and the EPA’s recommended upper limit for taste is 500 ppm. San Diego’s imported supply commonly sits toward the higher end of that range.
Is high TDS bad for you to drink?
The EPA’s TDS guideline is a secondary standard based on taste and appearance, not a health limit, so water above 500 ppm is still legal and generally safe to drink. High TDS mainly affects flavor, appliance scale, and how fast filters load up.
Does TDS measure water hardness?
No. Hardness measures only calcium and magnesium, in grains per gallon. TDS adds up everything dissolved in the water, hardness minerals included, plus sodium, sulfates, and trace metals.
What removes TDS from water?
Reverse osmosis is the main residential option, reducing TDS by 85 to 98%. Water softeners and carbon filters do not lower TDS, since a softener swaps one dissolved mineral for another and carbon targets chlorine and organic compounds instead.
Can a TDS meter tell you if your water is safe?
Not fully. A TDS meter gives a single combined number, not a breakdown of what’s dissolved, so it can’t confirm the absence of specific contaminants like PFAS or nitrates. A lab or professional water test is the only way to see what’s actually in the mix.
Does San Diego’s water have high TDS?
San Diego’s imported supply from the Colorado River and the State Water Project tends to run on the high side of the EPA’s 500 ppm taste guideline, which is one reason reverse osmosis is common here. Your exact number depends on your city and source blend.
Call (858) 400-6508 or schedule a free in-home water test to get your actual TDS number, and we’ll size a reverse osmosis drinking water system to what’s really coming out of your tap. We test and treat water throughout San Diego and Oceanside.