So how do you calculate your RO membrane's rejection rate?

You'll need two numbers to get started:
TDS of your feedwater (tap water)
TDS of your RO water (not DI water)

Plug those numbers into this formula:
1 - (RO TDS/Tap TDS)

For an example, let's say your tap water TDS is 287 ppm, and your RO water TDS is 17 ppm.
1- (17/287)=
1- (0.06)=
0.94, or 94%

This means that 94% of the TDS in the tap water DOES NOT make it through to your RO water - it is sent to the drain with your waste water.

A higher the rejection rate the better.

Russ