dGH and dKH to ppm converter

Convert between dGH and ppm for both general hardness (GH) and carbonate hardness (KH). The same conversion applies to both, and one drop on an API test kit equals one degree, so you can enter your drop count directly.

Updated 20 October 2026

dGH to ppm conversion table

One degree of general hardness is 17.86 ppm, so converting either way is a single multiplication. These tables cover the range aquarium test kits report; use the calculator above for anything in between.

The bands are general hardness (GH) categories, which is what determines whether a species will thrive. Carbonate hardness (KH) converts identically but is read against different functional ranges, because it describes pH stability rather than fish suitability. Switch the calculator to KH for those bands.

dKH to ppm: carbonate hardness

Carbonate hardness converts on exactly the same factor, so 1 dKH is also 17.86 ppm. What changes is what the number means. KH is not a species-suitability figure, it is a measure of how strongly your water resists pH change, so it is read against different bands.

Below about 2 dKH your pH is held very loosely and can swing or crash; 4 to 8 dKH is the comfortable range for most community tanks. A low KH is worth acting on even when your GH is fine, and our guide to adjusting water parameters covers how to raise it.

Reading an API test kit in drops

If you use the API Freshwater Master Test Kit or API's separate GH and KH kit, your result is a number of drops rather than a number in any unit. The kits are titrations: you add reagent one drop at a time until the sample changes colour, and the count is the reading.

The conversion is as simple as it gets. One drop is one degree. API's own instruction leaflet says the GH value is determined by the number of drops of reagent needed to turn the sample green, and the KH value by the number of drops needed to turn it bright yellow, then gives a conversion chart running from 1 drop (1 degree, 17.9 ppm) to 12 drops (12 degrees, 214.8 ppm).

Why TDS is not the same as GH

A TDS meter is cheap, instant, and the single most common source of confusion about hardness, because people reasonably assume its ppm reading is the same ppm the hardness tables use. It is not, and the two cannot be converted into each other.

GH measures calcium and magnesium specifically. It is a chemical test for those two ions, which are the ones that matter biologically to fish, shrimp and plants.

TDS measures everything dissolved, by proxy. A TDS meter actually measures electrical conductivity and converts it to an estimated total of all dissolved solids: calcium and magnesium, yes, but also sodium, potassium, chloride, sulphate, nitrate and whatever else is in the water.

So a tank at 250 ppm TDS might have a GH of 8 degrees or of 2, depending entirely on what those dissolved solids are. Remineralised RO water dosed for shrimp and hard tap water cut with rainwater can read the same TDS and behave completely differently. There is no conversion factor, and any chart offering one is guessing about your water's composition.

TDS is still useful, just for a different job: tracking change. A steady TDS that climbs between water changes tells you something is accumulating. For a number you can match against a species' requirements, you need an actual GH test.

What dGH and dKH mean, and how the conversion works

dGH is degrees of general hardness and dKH is degrees of carbonate hardness, the two scales European test kits use. The lower-case d is simply "degrees", and you will also see them written °dGH and °dKH, or as German degrees, since that is where the scale originates. American and Australian kits more often report the same measurements in ppm.

The conversion factor is 1 dGH = 17.86 ppm. This is a fixed chemistry-based ratio: degrees of hardness measure the same dissolved minerals as ppm, just on a different scale.

Where a result is written as mg/L, or as ppm as CaCO₃, which is how laboratory and water utility reports usually express it, that is the same number as the ppm used here. Hardness in ppm is conventionally stated as the equivalent concentration of calcium carbonate, and mg/L and ppm are interchangeable at these concentrations.

The same conversion applies to both general hardness (GH, which measures all dissolved minerals primarily calcium and magnesium) and carbonate hardness (KH, which measures only the carbonate and bicarbonate fraction). Test kits typically report results in one unit or the other depending on where they were made. European kits favour degrees, while American and Australian kits favour ppm.

What the numbers mean for fishkeeping

GH and KH affect different aspects of your tank:

GH (general hardness) determines which fish species will thrive. Soft-water species like neon tetras and discus struggle in hard water; hard-water species like guppies and snails struggle in soft water. See our water parameters guide for the full breakdown, or our best freshwater fish for Australian tap water guide for region-by-region picks.

KH (carbonate hardness) determines how stable your tank's pH is. Low KH tanks are prone to pH swings and crashes; high KH tanks resist change. KH is functionally a pH stability measurement rather than a fish-welfare measurement.

For most freshwater community tanks, you want GH in the moderate range (140 to 210 ppm / 8 to 12 dGH) and KH at moderate or higher (70+ ppm / 4+ dGH) for parameter stability.

Why categorisation matters more than exact values

A test reading of "180 ppm GH" is functionally identical to "10 dGH GH": both are moderate-hardness water suitable for most community species. Don't get hung up on whether your test kit reads in degrees or ppm; convert it once, identify the category, and choose species suited to that category.

For browsing species suited to your specific water profile, use the species directory's hardness filter after converting your test reading.