TDS in water stands for total dissolved solids, and PPM (parts per million) is simply the unit used to measure it. When you see “TDS 300 ppm” on a water test, it means there are 300 parts of dissolved substances for every million parts of water. In practical terms, 1 ppm equals 1 milligram per liter, so these two units are interchangeable.
TDS captures everything dissolved in your water: minerals, salts, and trace amounts of organic matter. It’s not measuring one specific contaminant. It’s a single number that reflects the overall mineral content of your water, which affects taste, appliance longevity, and whether your water falls within recommended guidelines.
What TDS Actually Measures
The dissolved solids in water are primarily inorganic salts. The main positively charged minerals are calcium, magnesium, sodium, and potassium. The main negatively charged components are carbonates, bicarbonates, chloride, sulfate, and nitrate. These all come from natural sources (water dissolving rock and soil as it flows underground) and from human activity like agricultural runoff or water treatment processes.
TDS is not the same as water hardness, though people often confuse the two. Hardness specifically measures calcium and magnesium, the minerals responsible for scale buildup and soap scum. TDS includes those minerals plus everything else that’s dissolved. All hardness contributes to TDS, but not all TDS is hardness. This is why a water softener won’t lower your TDS reading. It swaps calcium and magnesium for sodium, so the total amount of dissolved material stays roughly the same.
TDS Ranges and What They Mean
The U.S. EPA sets a secondary standard of 500 ppm for TDS in drinking water. This isn’t a legally enforceable limit because TDS at moderate levels isn’t a direct health hazard. It’s a guideline based on taste and aesthetic quality. Canada uses the same 500 ppm threshold.
Here’s how different TDS levels generally break down for drinking water:
- 50 to 250 ppm: Low mineral content. Water may taste flat or “empty” because it lacks the calcium, magnesium, and zinc that give water body.
- 300 to 500 ppm: The sweet spot for most people. Water at this range typically tastes pleasant and contains a healthy mineral profile.
- 600 to 900 ppm: Noticeable mineral taste. Filtration is worth considering at this level.
- 1,000 to 2,000 ppm: Not recommended for drinking. The taste becomes unpleasant and the mineral load is excessive.
- Above 2,000 ppm: Considered unsafe. Standard household filters can’t adequately handle this concentration.
How TDS Meters Work
Handheld TDS meters are inexpensive and widely available, but they don’t directly measure dissolved solids. They measure electrical conductivity, the ability of water to carry an electrical current, and then convert that reading to a TDS estimate using a built-in formula. Dissolved minerals carry electrical charge, so more minerals means higher conductivity.
The conversion factor between conductivity and TDS varies depending on what’s actually in your water. Tap water typically uses a factor between 0.5 and 0.7. Groundwater with more calcium and magnesium uses around 0.65 to 0.7. Seawater, dominated by sodium chloride (which conducts very efficiently), uses a factor of about 0.5. This means a TDS meter gives you a useful estimate, not a precise lab measurement. For most home purposes, that estimate is accurate enough to tell you whether your water is within a normal range.
Effects of High TDS on Your Home
When TDS runs high, the practical consequences go beyond taste. Mineral deposits collect inside pipes over time, narrowing water flow and putting extra pressure on joints. This gradually reduces water pressure at your taps and increases the risk of leaks. Water heaters work harder to heat water through a layer of scale on the heating element, raising energy costs and shortening the unit’s lifespan.
Dishwashers take a similar hit. Mineral buildup coats spray arms, filters, and heating elements, forcing the machine to run less effectively. Faucets, refrigerator dispensers, and showerheads wear out faster. In pools and hot tubs, high TDS turns water cloudy and leaves mineral deposits on walls, jets, and heaters. The common thread is greater energy costs, more frequent maintenance, and shorter appliance life.
Lowering TDS at Home
Reverse osmosis (RO) is the most effective home filtration method for reducing TDS. A standard single-pass RO system removes 80% to 85% of dissolved solids. Systems with a second pass can reach 98% to 99.5% removal. If your tap water reads 400 ppm, a single-pass RO system would bring it down to roughly 60 to 80 ppm.
Standard carbon filters (like pitcher filters) improve taste and remove chlorine, but they don’t significantly reduce TDS. Water softeners, as mentioned, swap one set of minerals for another without changing the overall TDS number. If your goal is specifically to lower TDS, reverse osmosis or distillation are your two realistic options. Distillation produces very low TDS water (often under 10 ppm) but is slower and more energy-intensive than RO for household use.
One thing worth knowing: very low TDS water (under 50 ppm) can taste flat and unappealing. Some RO systems include a remineralization stage that adds small amounts of calcium and magnesium back into the filtered water, bumping the TDS into a more palatable range while keeping unwanted contaminants out.