RO Water Check
Hydrologic Lab · Residential RO

Is Your Reverse Osmosis TDS Actually Normal?

A raw TDS meter reading alone cannot tell you if an RO membrane is healthy. A 25 ppm reading means 95% rejection on 500 ppm hard tap water, but only 58% rejection on 60 ppm soft water.

Use this calculator to compute your true rejection percentage, separate real membrane wear from TDS creep or remineralization cartridges, and follow verified next steps based on NSF/ANSI 58 and WQA maintenance benchmarks.

Scope & conditions: Designed for residential thin-film composite (TFC) under-sink and tankless RO drinking systems. Not intended for saltwater aquariums, DI resin polishing beds, or commercial high-recovery plants.

RO TDS Rejection Calculator

ppm
ppm
50% 80% 90% 100% Low Rejection (<80%: check seals / membrane) Watch (80–89%) Healthy (≥90%) 94.1%
Figure 1 — Residential TFC RO Rejection Benchmarks: Typical thin-film composite membranes achieve 90%–98% TDS rejection when measured after flushing standing permeate and before any remineralization stage.

Where You Sample Changes the Reading

Most false membrane replacements happen because homeowners measure TDS at the kitchen RO faucet without realizing a 6th-stage alkaline cartridge sits downstream of the membrane, or because the first glass of water absorbed standing ions overnight.

1–3. Pre-Filters Sediment + Carbon A 4. RO Membrane Rejects 90–98% TDS Drain carries concentrate To Drain (concentrated TDS is normal) B 5–6. Tank & Post Carbon / Optional Mineral C RO Faucet Drinking Glass Point A: Feed (~320 ppm) Point B: True Membrane (~19 ppm) Point C: +Minerals (~45–75 ppm)
Figure 2 — RO Flow Path & Sampling Points: Always compare Point A (cold tap) against Point B (pure membrane permeate). Sampling at Point C after an alkaline/remineralization stage includes added calcium/magnesium ions.

Worked Examples You Can Verify

Example 1: Standard 5-Stage RO (Healthy)

Cold tap reads 320 ppm; flushed RO faucet reads 19 ppm.

Rejection = (1 − 19 / 320) × 100 = 94.1%

Result: Well above 90%. Keep the membrane; stay on schedule for 6–12 month carbon pre-filters.

Example 2: 6-Stage Alkaline RO (False Alarm)

Cold tap reads 250 ppm; faucet reads 55 ppm (78% apparent).

Because a remineralization stage adds ~35–40 ppm of beneficial minerals, testing before the mineral stage typically reveals 12–18 ppm (>92% rejection).

Next step: See our TDS Creep & Remineralization Guide.

Next Diagnostic Steps

Observed Symptom Most Likely Cause Recommended Tool / Guide
Drain water runs continuously hours after closing faucet Low tank air charge (<7 psi), faulty ASO valve, or feed pressure <40 psi Continuous Drain & Pressure Triage
First cup TDS is high, then drops after 30–60 seconds Normal overnight osmotic diffusion (TDS creep) TDS Creep & Sampling Guide
Slow trickle from faucet, TDS still low (<10% of tap) Clogged sediment/carbon pre-filters or waterlogged tank bladder Membrane vs. Pre-Filter Matrix
Planning annual filter and membrane budget 6–12 mo pre-filters + 2–4 yr membrane schedule Filter Cost & Schedule Planner