Water hardness converter

Convert between dGH and ppm for both general hardness (GH) and carbonate hardness (KH). Same conversion applies to both.

Updated 6 June 2026

How the conversion works

The exact conversion factor is 1 dGH = 17.848 ppm (often rounded to 17.9 in hobby resources). This is a fixed chemistry-based ratio — degrees of hardness measure the same dissolved minerals as ppm, just on a different scale.

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, 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.

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 (140-210 ppm / 8-12 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.