How it works
RO membranes are rated at a stated test pressure, temperature and feed TDS (see your membrane's spec sheet). This calculator scales output in proportion to pressure, a simplification. Real output also falls with colder water and with higher feed TDS (osmotic pressure), so at low pressure, high TDS or cold water the estimate is optimistic. Treat the "meets your daily use" check as a rough guide.
Recovery and rejection are standard definitions: recovery is product water as a share of feed water; rejection is the share of dissolved solids removed, measured with a TDS meter.
Waste ratio and water rate defaults are example assumptions.
The formula
- Estimated output ≈ rated GPD × (your psi ÷ rated psi) — simplified linear assumption
- Recovery = 1 ÷ (1 + waste ratio)
- Rejection = 1 − (RO TDS ÷ tap TDS)
- Waste per day = RO water used × waste ratio
How to use this calculator
- Enter the membrane's rated output and test pressure.
- Enter your water pressure.
- Enter the system's waste ratio and your TDS readings.
- Enter daily use and water rate to see waste and cost.
Frequently asked questions
What is a good TDS rejection rate?
Check your membrane's specification; many list a nominal rejection rate. A falling rejection rate over time can signal a membrane needs replacing.
Does a permeate pump or booster change the result?
Yes. Enter the boosted pressure and your system's actual waste ratio.