# General hardness

> General hardness (GH) is the calcium and magnesium dissolved in water, expressed as mg/L CaCO3 or German degrees (dGH); hard water supplies some Ca and Mg.

- Canonical: https://genetixbay.com/knowledge/glossary/general-hardness.html
- Also written: GH, dGH, total hardness, water hardness, hard water, permanent hardness
- Glossary category: pH, EC and water · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

General hardness (GH), also called total hardness, is the concentration of divalent cations in water, mainly calcium (Ca²⁺) and magnesium (Mg²⁺). It is reported as mg/L (ppm) of calcium carbonate equivalent or in German degrees (dGH). Common classification for drinking water (US Geological Survey): soft below 60 mg/L, moderately hard 61–120, hard 121–180, very hard above 180 mg/L as CaCO3.

## In practice

Hard water brings free calcium and magnesium. Water at 200 mg/L as CaCO3 from mostly calcium supplies roughly 60–80 mg/L of Ca. Growers on [hard tap water](https://genetixbay.com/knowledge/glossary/tap-water.md) may need little or no added [Cal-Mag](https://genetixbay.com/knowledge/glossary/cal-mag.md); growers on [RO water](https://genetixbay.com/knowledge/glossary/ro-water.md) need all of it from the nutrient. Hardness usually comes with high [alkalinity](https://genetixbay.com/knowledge/glossary/alkalinity.md), so hard water also needs acid.

Find GH in the utility's water report, test with an aquarium GH kit, or get a lab analysis that lists Ca and Mg separately.

## Units and formula

- 1 dGH = 17.86 mg/L as CaCO3 (equivalent to 10 mg/L CaO)
- Hardness (mg/L as CaCO3) = 2.497 × Ca (mg/L) + 4.118 × Mg (mg/L)

## Don't confuse with

- **Carbonate hardness (KH)**: bicarbonate and carbonate, which is alkalinity.
- **EC**: total dissolved ions, including sodium, [chloride](https://genetixbay.com/knowledge/glossary/chloride-nutrient.md) and sulfate, not just Ca and Mg.

## Myth check

Cal-Mag need is driven by water source, medium and **cultivar**, not by [LED](https://genetixbay.com/knowledge/glossary/led.md) lighting. The LED claim is a *Myth*; see [Cal-Mag](https://genetixbay.com/knowledge/nutrients/cal-mag.md).

## Related terms

[Carbonate hardness](https://genetixbay.com/knowledge/glossary/carbonate-hardness.md), [Alkalinity](https://genetixbay.com/knowledge/glossary/alkalinity.md), [Tap water](https://genetixbay.com/knowledge/glossary/tap-water.md), [RO water](https://genetixbay.com/knowledge/glossary/ro-water.md), [Cal-Mag](https://genetixbay.com/knowledge/glossary/cal-mag.md)

## Read more

- [Water quality for cannabis: RO vs tap, alkalinity, bicarbonates and chloramine](https://genetixbay.com/knowledge/ph-ec/water-quality.md)
- [Cal-Mag: what it is, when you need it and when you don't](https://genetixbay.com/knowledge/nutrients/cal-mag.md)

## Sources

1. Sonneveld C, Voogt W. 2009. Plant Nutrition of Greenhouse Crops. Springer. https://doi.org/10.1007/978-90-481-2532-6