Expected production for home reverse osmosis units is usually stated in gallons per day (GPD). Many factors affect the production. These include inlet water pressure, water temperature, total dissolved solids (TDS), the condition of prefilters, etc.
Here's an easy way to determine the actual production of your RO. The only tools needed are a standard household measuring cup (or any measuring device that has a milliliter, or ml, scale) and a watch or clock with a second hand.
1. For undersink units, turn off the valve at the top of the tank to isolate the tank from the system, then lock the dispensing faucet open and let the unit produce into the sink for a minute or so. The drip or small stream you see is the actual production of the unit—how fast it is making water. (For countertop units, just start the unit and let it produce water for four or five minutes until a steady production rate is established.)
2. Using the watch and measuring cup, get an accurate measure of how much water the unit produces in milliliters into the cup in one minute.
3. Multiply the result by 0.38 to convert milliliters per minute to gallons per day. The result is how many gallons your reverse osmosis unit will produce if it runs for 24 hours.
Example: If your unit is making 50 milliliters per minutes, multiply 50 X 0.38. The result is 19. Your unit is producing water at the rate of 19 gallons per day.
(whole house & well units)