You’ll read and hear about the use of “TDS,” Total Dissolved Solids, or Nitrogen PPM (parts per million) as control factors for feeding your plants. So how are you supposed to know what values you have?
Fertilizer labels don’t give the complete picture, as the “N-P-K” values are not reported in a straightforward manner, but in terms of N, P2O5, and K2O. This simple fertilizer calculator takes those factors into account.
Note that these figures refer to the contribution of the major components only, so any minors or trace elements in the fertilizer will add to the TDS, as will any solids dissolved in your water. For example, K-Lite is a 12-1-1 formula (actually, 12.9-1.3-1.3), yielding an N-P-K contribution of about 112 ppm N when mixed with pure water at 0.77g/L, but the true total dissolved solids content of that solution in pure water would be 770 ppm.
Note: When specifying units, the mass-based measurements (ounces/gallon and grams/liter) values are quite accurate, but the volumetric ones, using teaspoons and milliliters, are estimates only, using assumed bulk densities for the fertilizers.