Water Hardness & TDS Calculator

Water hardness measures only calcium and magnesium — the two minerals responsible for scale buildup, soap scum, and dry skin. It's typically measured in ppm, gpg, or °dH.

TDS (Total Dissolved Solids) measures everything dissolved in your water — minerals, salts, metals, organic compounds. It's a much broader measurement that includes hardness minerals plus things like sodium, chloride, sulfate, nitrates, and lead.

You can have high hardness with low TDS (mineral-heavy well water) or low hardness with high TDS (water softened by ion exchange, which trades calcium for sodium — TDS stays the same).

This matters for filter choice: a water softener fixes hardness but doesn't reduce TDS. To reduce TDS, you need reverse osmosis. If you have both problems, you may need both systems.

Hardness test strips ($10–15): Cheapest option, gives you a hardness range in seconds. Strips from Hach or LaMotte are reliable. Best for confirming what your municipal water report says.

TDS meter ($15–30): Digital pen-style meter. Stick it in a glass of water, get a TDS reading in ppm. Doesn't measure hardness specifically, but gives you a general picture.

Comprehensive water test kit ($25–40): Tests hardness, pH, chlorine, iron, lead, nitrates, copper, and bacteria. Brands like Watersafe, Health Metric, and First Alert. Best for well water or older homes with concerns about specific contaminants.

Lab tests ($50–200): Mail-in kits to a certified lab (Tap Score, MyTapScore, National Testing Labs). Most accurate option — required if you're considering whole-house treatment or have specific health concerns.

City water report: If you're on municipal water, your utility publishes a Consumer Confidence Report annually with hardness and contaminant data. Often available on your utility's website. Free.

Activated carbon (pitcher, faucet, under-sink standard): Excellent for chlorine, chloramine, taste, odor, and many organic compounds. Reduces some lead with NSF/ANSI 53 certification. Doesn't remove dissolved minerals (so won't fix hardness or high TDS).

Reverse osmosis (under-sink RO, whole-house RO): Removes 95–99% of dissolved solids — including hardness, lead, arsenic, fluoride, PFAS, nitrates, and most heavy metals. The most thorough drinking water filtration available for homes. Wastes 1–3 gallons of water per filtered gallon.

Ion exchange (water softener): Specifically removes hardness (calcium, magnesium) by swapping them for sodium ions. Doesn't reduce TDS or remove contaminants — it's a hardness solution only.

UV (whole-house add-on): Inactivates bacteria, viruses, and cysts. Critical for well water; less essential for treated municipal water. Doesn't remove anything chemical — pair with carbon or RO.

Sediment pre-filter (whole-house first stage): Removes physical particles (sand, rust, sediment). Always the first stage in a multi-stage system; protects downstream filters and appliances.

Look for NSF/ANSI certifications — Standard 42 for taste/chlorine, 53 for health contaminants, 58 for RO systems, 401 for emerging contaminants like pharmaceuticals.

Soft (0–60 ppm / 0–3.5 GPG): No treatment needed. You'll get good lather, minimal scale, and your appliances will last longer. Soap may seem to leave a slightly slippery feel — that's normal for soft water.

Moderately hard (61–120 ppm / 3.5–7 GPG): Acceptable for most uses. You may notice mild spotting on glassware and slight scale on showerheads over time. Treatment is optional — most people don't bother.

Hard (121–180 ppm / 7–10.5 GPG): Visible scale on faucets, dishes, and inside the dishwasher. Soap doesn't lather well. Skin and hair may feel dry. A water softener is worthwhile here.

Very hard (181–250 ppm / 10.5–14.5 GPG): Significant scale buildup, white deposits on appliances, dishwasher detergent fails to clean properly. Water softener strongly recommended — appliance lifespan is measurably shorter without one.

Extremely hard (>250 ppm / >14.5 GPG): Severe scale, frequent appliance issues, and ongoing skin/hair problems. Water softener is essentially required. Some users add a salt-free conditioner or RO system on top.

Most US municipal water falls in the moderate-to-hard range. Well water, especially in the Midwest and Southwest, is often in the very-hard or extreme range.