This article is from the Aquaria Newsgroup Topics and Conversion Tables FAQ, by Todd Zebert and Erik Olson with numerous contributions by others.
1 degree hardness (GH) = 17.8 ppm CaCO3
1 degree "carbonate hardness*" (KH) = 17.8 ppm CaCO3
1 meq/L alkalinity = 2.8 degrees KH
1 ppm = 1 mg/L
Nitrate-NO3 = 4.4 * Nitrate-N
Nitrite-NO2 = 3.3 * Nitrite-N
Ammonia-NH3 = 1.1 * Ammonia-N
* Note that the term ``carbonate hardness'' is bit of a misnomer, and
most amateur chemists would prefer we use the more appropriate term
``Alkalinity''. See the BEGINNER CHEMISTRY section for more details.