How do I convert between TDS and EC readings?

To obtain an approximate sodium chloride TDS value, multiply the EC reading (in milliSiemens/cm) by 1000 and divide by 2.

To get an EC value, multiply the ppm reading by 2 and divide by 1000.

Thus, if your EC is 1:
1 * 1000/2= 500 ppm.

And if your ppm is 500:
500 * 2/1000= 1 EC