RO Water TDS Checker
An RO membrane should reject 90–98% of dissolved solids. If your tap is 400 PPM and the RO output is 40, that is 90% — the membrane is tired. Enter both numbers to get your rejection rate and a replace/don't-replace verdict.
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RO Water TDS Checker
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An RO membrane should reject 90–98% of dissolved solids. If your tap is 400 PPM and the RO output is 40, that is 90% — the membrane is tired. Enter both numbers to get your rejection rate and a replace/don't-replace verdict.
How to use this calculator
- Enter tap water TDS (PPM).
- Enter RO output TDS (PPM).
- Get rejection rate and membrane verdict.
The formula
Rejection % = (1 − RO_output ÷ tap_input) × 100.
What the units mean
PPM (parts per million) — one part of solute per million parts of the total mixture. The workhorse unit for trace concentrations in water, air, and food.
Worked example
Tap 350 PPM, RO output 14 PPM → (1 − 14/350) × 100 = 96% rejection — healthy membrane.
Quick reference
| Rejection | Verdict |
|---|---|
| 95%+ | Excellent |
| 90–95% | Good |
| 85–90% | Replace soon |
| <85% | Replace now |
Based on tap vs RO output TDS.
Where you will actually use this
Membranes die slowly — output creeps up for months before anyone notices. This catches it early.
- RO maintenance scheduling
- Drinking water quality
- Aquarium RO/DI checks
Mistakes that give wrong answers
- Testing right after filter changes (flush first)
- Ignoring temperature effects
Key takeaways
- Formula: Rejection rate = (1 − output ÷ input) × 100%
- Remember: Based on tap vs RO output TDS.
- Watch out for: Testing right after filter changes (flush first)
- Next step: run your own numbers in the calculator above, then verify with a measurement — calculated values are only as good as the inputs.
Frequently asked questions
What is a good RO rejection rate?
90–98%. Below 90%, replace the membrane.
What TDS should RO water be?
Typically under 20 PPM, often under 10.