5-Year True-Cost Calculator

Sticker price is the worst way to compare water systems. A $250 reverse osmosis unit that eats $90 of filters a year and wastes two gallons for every gallon it makes can cost more over five years than a $600 unit with cheaper filters and a better waste ratio — and a softener’s salt and regeneration water quietly add up the same way. This calculator totals everything: purchase, installation, replacement filters, and the water each system uses and wastes, for two systems side by side.

System A — typical tank RO (defaults)

Purchase price ($)

Installation ($, one-time)

Filter / cartridge replacement cost ($)

Replace every (months)

Water used per day (gallons)

For a drinking-water system, 2–4 gallons a day is typical for a household.

Waste ratio (gallons wasted per gallon produced)

Reverse osmosis only: older tank systems waste ~2–3, efficient tankless models ~0.5. Use 0 for softeners and simple carbon filters.

System B — typical tankless RO (defaults)

Purchase price ($)

Installation ($, one-time)

Filter / cartridge replacement cost ($)

Replace every (months)

Water used per day (gallons)

For a drinking-water system, 2–4 gallons a day is typical for a household.

Waste ratio (gallons wasted per gallon produced)

Reverse osmosis only: older tank systems waste ~2–3, efficient tankless models ~0.5. Use 0 for softeners and simple carbon filters.

Your water + sewer rate ($ per 1,000 gallons)

$10 per 1,000 gallons is a common combined water-and-sewer figure; your bill shows your real rate. This only matters for systems with a waste ratio.

How we count

Replacement filters are counted when they’re actually due inside the five years: a filter changed every 12 months is bought at months 12, 24, 36 and 48 — four times, not five. (A change due exactly at month 60 belongs to the next five years.)
Water cost applies to everything the system draws, not just what you drink. An RO system with a waste ratio of 2 sends 3 gallons down the pipe for every gallon in your glass, so its water line is based on 3× your drinking use.
The defaults are typical, not specific. System A starts as a typical budget tank RO, System B as a typical tankless RO. Replace every number with the real figures for the systems you’re comparing — the filter prices are on the manufacturer’s own store page.
What we leave out: electricity (a few dollars a year for most systems), your time, and any plumber visits. They’re roughly equal between systems, so they rarely change the winner.

Where this matters most

True-cost math is the backbone of our reviews. You’ll find it applied model by model in the reverse osmosis hub and the water filter hub, using the same method as this calculator — our how we test page lays out the full methodology. Shopping for a softener instead? Size it first with the softener sizing calculator, then bring the finalists back here with their salt and regeneration figures.