First thing to know is that NPK in the US is given as N, P2O5 and K2O. If you want to get ppm of K, you need to use a conversion factor 0.83.
Next thing you need to know is that ppm is the same as mg/L. So now you just need to do the conversion from NPK and g/gal into mg/L.
26 = 26% K2O = 0.83 * 26 = 21.58% K
We use 3.7g/gal, and 21.58% is K, so 3.7 g/gal * 0.2158 = 0.798 g/gal (ie 0.798 grams of K per gallon)
Now convert g/gal into mg/L (ie ppm)
0.798g/gal * 1000mg/g * 0.264 gal/L = 210.7 ppm
Here’s hydrobuddy:
Nice!