TOOL 09
Water Hardness
Converter.
Different kits, guides, and regions all use different hardness units. Enter a value and pick its unit — get dGH, ppm, mmol/L, mEq/L and French degrees instantly.
Conversions assume hardness expressed as CaCO3 equivalent. 1 °dH = 17.848 ppm = 0.1783 mmol/L = 0.357 mEq/L = 1.253 °fH.
ABOUT THIS TOOL
Water Hardness
Converter.
Water hardness is one of the most inconsistently reported measurements in the aquarium hobby. Depending on where you live and which product you use, you may see German degrees (dGH), parts per million (ppm), millimoles per litre (mmol/L), milliequivalents per litre (mEq/L), or French degrees (°fH). Confusing them can lead to keeping soft-water fish in hard water, or vice versa, with results that are slow and hard to diagnose.
This converter reads a single value in the unit you choose and translates it into every common unit at once. That makes it easy to compare a test strip result with a care guide written in another system, or to prepare water changes that match a target you only know in one unit.
It also places your reading in a general hardness category, from very soft to very hard. Keep in mind that general hardness (GH) and carbonate hardness (KH) are different measurements — this tool handles the calcium and magnesium-based general hardness scale, though the same units are used for both.
HOW TO USE
Using the
Converter.
- Find your source valueTake the hardness number from your test kit, water report, or care guide. Note which unit it is expressed in.
- Choose the matching unitSelect that unit in the dropdown — German degrees, ppm, mmol/L, mEq/L, or French degrees.
- Enter the valueType the number into the value field. All the other units update instantly.
- Compare across unitsRead the converted rows to match your target from any guide or piece of equipment.
- Check the categoryThe category readout places your water on the very soft to very hard scale, which is a useful starting point for species selection.
- Plan accordinglyIf your water is far from a species preference, consider mixing RO water, using peat or driftwood, or choosing fish that suit your tap water.
FAQ
Hardness
Questions.
What is the difference between GH and KH?
General hardness (GH) measures dissolved calcium and magnesium — the minerals that matter to fish and plants. Carbonate hardness (KH) measures carbonates and bicarbonates, which stabilise pH. They use the same units but describe different things, and a tank can have high GH with low KH or the reverse.
Which unit should I use?
Use whichever unit your test kit reports, then convert. German degrees (dGH) are the hobby standard in most guides. If you are unsure, ppm is the most literal measurement and converts cleanly to the others.
How accurate is a strip test?
Test strips are convenient but coarse, often accurate only to the nearest degree or two. For anything important — breeding soft-water fish, matching parameters precisely — use a liquid drop kit.
Can I lower hardness at home?
Yes. Mixing with reverse osmosis or distilled water is the most controllable method. Peat, driftwood, and some aquasoils lower hardness and pH gently but unpredictably. Avoid chemical softeners that swap hardness for sodium.
Does hardness affect pH?
KH, not GH, is what buffers pH. High KH resists pH changes and keeps water alkaline; low KH lets pH drift. This is why plant-focused tanks often run lower KH to allow a gentle pH drop from CO2.
Do plants care about hardness?
Some do. Many soft-water species grow better in lower GH, while certain hard-water plants tolerate high mineral content. For fish, hardness is usually the more critical factor.