Whole-house reverse osmosis sizing calculator
A membrane splits the feed water into permeate and a salty concentrate. How much becomes permeate — the recovery — is limited by scaling in the concentrate: calcium carbonate, gypsum, silica [1]. Enter your water test and the calculator picks a membrane type, shows what reaches the tap, how much concentrate you get and what to buy.
Updated 2026-10-24 · every regulation and price links to its source at the bottom of the page. A guide, not a construction design.
Size a membrane for your water
By default — low-pressure reverse osmosis at 70 % recovery: the middle of the 65–75 % practical range of a household system [2], with a margin for unmeasured barium, strontium and silica.
Size the treatment from your well water test →
In the app, with the same data
The result above is yours, free and without signing in. In the app the same data gives you more:
- Water treatment from your test inside your house plan — the same treatment chain
- See your house in 3D — outside with the roof, inside and as a walk-through
No retyping: the values from this page are put into a new plan.
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What you typed stays in your browser until you move it into a plan yourself.
Why permeate needs remineralization
Reverse osmosis water has almost no calcium or magnesium: the UK Drinking Water Inspectorate notes that it is generally not considered suitable for drinking without minerals added back [3], and the French agency ANSES recommends at least 8 °f of hardness and alkalinity after membranes [4]. Two ways: blend some feed water back or pass the permeate through calcite. The calculator shows both as options — nothing is blended automatically.
Frequently asked questions
How much concentrate does whole-house RO make?
At 70 % recovery — about 0.43 gallons per gallon of permeate; for four people (about 160 gallons a day) about 68 gallons a day [1].
Do I need an antiscalant?
When without it the recovery is too low because of calcium carbonate or gypsum [5]; the dose is 2–5 mg/L [6].
What does a whole-house RO system cost in the US?
Point-of-entry systems run about $1,000–4,800 or more [7]; a 1,000-gallon-a-day unit alone, without tank or installation, is about $4,100–$6,800 (October 2026; indicative, may differ by seller). The calculator adds pre-treatment, tank and pump where we found prices.
Sources
- DuPont FilmTec Reverse Osmosis Membranes Technical Manual, Form No. 45-D01504-en, Rev. 20 (August 2026), §2.4.1 General, Eq. 2–5, p. 35 (INT) · recommendation · checked 2026-10-16 · source
- Peter S. Cartwright, «Nanofiltration—When It Works for the Whole House», Water Conditioning & Purification (WC&P), 13.11.2003 (US) · common practice (not a rule) · checked 2026-10-16 · source
- Drinking Water Inspectorate — Domestic water filters and softeners, Reverse osmosis systems (UK) · recommendation · checked 2026-09-29 · source
- Avis de l'Anses, saisine n° 2024-SA-0136 (16 mai 2025) — modules de filtration membranaire (arrêté du 22 juin 2012, CSP R1321-50) (FR) · recommendation · checked 2026-09-29 · source
- DuPont FilmTec Reverse Osmosis Membranes Technical Manual, Form No. 45-D01504-en, Rev. 20 (August 2026), §2.4.2.1 Brackish Water, «Adjustments of LSIc», p. 37 (INT) · recommendation · checked 2026-10-16 · source
- Avista (Kurita) Vitec™ 3000 Technical Data Sheet, REV 12.2023, DOSING; INJECTING; DILUTION; «Certified to NSF/ANSI 60» (INT) · recommendation · checked 2026-10-16 · source
- This Old House — How Much Does a Reverse Osmosis System Cost in 2026?, System type; Maintenance; DIY vs. Professional (US) · price · checked 2026-10-24 · ✓ · source