Question: how do you calculate percent rejection?
Answer: So, for example, if your tap water reads 280 and your RO product water reads 15, you determine the percent rejection of the RO unit by subtracting 15 from 280 to get 265, dividing 265 by 280 to get 0.946, then multiplying by 100 to get 94.6% rejection. Your RO unit is running well.