# Carbonate hardness

> Carbonate hardness (KH) is the bicarbonate and carbonate content of water, often given in German degrees (dKH); in practice it is a measure of alkalinity.

- Canonical: https://genetixbay.com/knowledge/glossary/carbonate-hardness.html
- Also written: KH, dKH, carbonate hardness (KH), temporary hardness, German degrees KH
- 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"

Carbonate hardness (KH, from the German *Karbonathärte*) measures the bicarbonate and carbonate in water. Despite the name, it is really a measure of [alkalinity](https://genetixbay.com/knowledge/glossary/alkalinity.md), not of calcium or magnesium. It is also called temporary hardness because boiling drives off [CO2](https://genetixbay.com/knowledge/glossary/co2-enrichment.md) and precipitates part of it as limescale. Aquarium test kits report it in degrees of carbonate hardness (dKH).

## In practice

KH is what makes [hard tap water](https://genetixbay.com/knowledge/glossary/tap-water.md) resist [pH Down](https://genetixbay.com/knowledge/glossary/ph-down.md). A grower with water at 8 dKH (about 143 mg/L as CaCO3) needs considerably more acid per litre than one at 2 dKH (about 36 mg/L), even if both waters start at [pH](https://genetixbay.com/knowledge/glossary/ph.md) 7.8. An aquarium KH drop-test kit is a cheap way to know your water. Above about 6 dKH, plan for regular acid dosing or blending with [RO water](https://genetixbay.com/knowledge/glossary/ro-water.md).

## Units and formula

- 1 dKH = 17.86 mg/L as CaCO3 ≈ 0.357 meq/L
- 1 mg/L as CaCO3 = 0.056 dKH

## Don't confuse with

- **General hardness (GH)**: calcium and magnesium content.
- **Alkalinity**: KH is a way of expressing it.
- **Potassium (K)**: the "K" in KH is German, not the element.

## Related terms

[Alkalinity](https://genetixbay.com/knowledge/glossary/alkalinity.md), [General hardness](https://genetixbay.com/knowledge/glossary/general-hardness.md), [Buffering capacity](https://genetixbay.com/knowledge/glossary/buffering-capacity.md), [Tap water](https://genetixbay.com/knowledge/glossary/tap-water.md)

## Read more

- [Water quality for cannabis: RO vs tap, alkalinity, bicarbonates and chloramine](https://genetixbay.com/knowledge/ph-ec/water-quality.md)

## Sources

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