# Water Hardness Calculator

Convert water hardness between ppm, gpg, mmol/L, dGH, and Clark degrees. Classify water hardness as soft, moderate, hard, or very hard. Free tool.

## What this calculates

Convert water hardness measurements between common units (ppm, gpg, mmol/L, dGH, Clark degrees) and classify your water hardness on the USGS scale. Enter a value in any unit to see all conversions and the hardness classification.

## Inputs

- **Hardness Value** — min 0 — The water hardness value to convert.
- **Unit** — options: ppm (mg/L as CaCO₃), Grains per Gallon (gpg), mmol/L, German Degrees (°dH / dGH), Clark Degrees (°Clark) — Select the unit of the input hardness value.

## Outputs

- **ppm (mg/L as CaCO₃)** (ppm) — Parts per million as calcium carbonate equivalent.
- **Grains per Gallon (gpg)** (gpg) — Grains of hardness per US gallon.
- **mmol/L** (mmol/L) — Millimoles per liter of calcium carbonate.
- **German Degrees (°dH)** (°dH) — German degrees of hardness.
- **Clark Degrees** (°Clark) — English (Clark) degrees of hardness.
- **Classification** — formatted as text — Water hardness classification (USGS scale).

## Details

Water hardness is a measure of dissolved calcium and magnesium minerals, typically expressed as an equivalent concentration of calcium carbonate (CaCO₃). Hard water causes scale buildup in pipes and appliances, reduces soap effectiveness, and can affect taste.

Hardness Units

- ppm (mg/L as CaCO₃): The most universal unit. 1 ppm = 1 mg of CaCO₃ per liter.

- Grains per Gallon (gpg): Common in the US water treatment industry. 1 gpg = 17.12 ppm.

- mmol/L: SI unit. 1 mmol/L CaCO₃ = 100.09 ppm.

- German Degrees (°dH): Common in Europe. 1 °dH = 17.848 ppm = 10 mg/L CaO.

- Clark Degrees: British system. 1 °Clark = 14.254 ppm = 1 grain per imperial gallon.

USGS Classification

- Soft: 0–60 ppm (0–3.5 gpg)

- Moderately Hard: 60–120 ppm (3.5–7.0 gpg)

- Hard: 120–180 ppm (7.0–10.5 gpg)

- Very Hard: >180 ppm (>10.5 gpg)

Most municipal water supplies in the US range from 60 to 300 ppm. Well water can be significantly harder. Water softeners typically use ion exchange to replace calcium and magnesium with sodium.

## Frequently Asked Questions

**Q: What causes hard water?**

A: Hard water is caused by dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions, which are picked up as water flows through limestone, chalk, and dolomite formations. Temporary hardness (from calcium bicarbonate) can be removed by boiling. Permanent hardness (from calcium sulfate/chloride) requires a water softener.

**Q: Is hard water safe to drink?**

A: Yes, hard water is generally safe and may provide beneficial minerals (calcium and magnesium). The WHO has found no convincing evidence that hard water causes adverse health effects. Some studies suggest it may have cardiovascular benefits. The main issues are aesthetic (taste, scale) and practical (appliance damage).

**Q: What gpg level requires a water softener?**

A: Water above 7 gpg (120 ppm) is considered hard and benefits from softening. Above 10.5 gpg (180 ppm), a softener is strongly recommended to protect water heaters, dishwashers, and plumbing. Below 3.5 gpg (60 ppm), softening is generally unnecessary.

**Q: How do I test my water hardness?**

A: Options include: (1) Contact your water utility for their annual water quality report, (2) Use an inexpensive test strip kit from a hardware store, (3) Use a liquid titration test kit for more accurate results, or (4) Send a sample to a certified lab for precise analysis. Test kits typically report in ppm or gpg.

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Source: https://vastcalc.com/calculators/chemistry/water-hardness
Category: Chemistry
Last updated: 2026-04-21