Compare the cost of DI-only and RO/DI systems.
Move the sliders and watch your water use, running costs and payback change. Start with your state, or enter a measured TDS reading for your water source.
The original calculator’s state presets and cost model, with a clearer view of the assumptions. Switch to custom costs when you have current quotes and replacement records.
Your estimated comparison
DI-only annual running cost
Purchase + one annualized year:
RO/DI annual running cost
Purchase + one annualized year:
Cost over time
Purchase cost + annualized running costs
Recovering additional RO/DI purchase cost
Annualized consumption estimate, not a first-year cash-spending forecast. Actual replacements happen in whole units, and new systems may include initial filters. Payback assumes the same workload and costs continue; it excludes financing and the time value of money.
How this calculator works
How is annual water use calculated?
Gallons/minute × 60 × water-on hours/week × working weeks/year. For example, 1 gallon/minute × 60 × 5 hours × 40 weeks = 12,000 pure-water gallons/year. This is an arithmetic example, not a product performance claim. RO source-water demand also includes concentrate water.
How are running costs and payback calculated?
Annual running cost = annual pure-water gallons ÷ 1,000 × consumables cost per 1,000 gallons + other recurring annual costs.
When RO/DI costs more to buy and less to operate, simple payback in years = additional purchase cost ÷ annual running-cost reduction. Multiply by working weeks/year for operating weeks. If there is no positive reduction, no payback is shown.
What does the original model assume?
Quick estimate uses 1.5 gallons/minute, a $495 DI purchase and a $2,175 multistage purchase. These are historical October 2017 assumptions, not current quotes. All 50 state TDS presets are retained from the old page; Connecticut’s spelling is corrected.
With T = source-water TDS in ppm, original DI cost per gallon = 66.74 × T ÷ 30,000. Multistage cost per gallon = 0.0042 + 120 × T ÷ 2,000,000 + 120 × T ÷ 2,000,000. The coefficients are carried over from the old code and have not been validated against current equipment or prices. Workload defaults to 10 hours/week and 52 weeks/year. You can adjust working weeks to reflect your season.
How does measured TDS change the estimate?
In quick estimate, your measured reading replaces the state preset in those historical formulas. A state preset is not a measurement of your water, and even a measured reading does not validate the old filter-life assumptions. In custom costs mode, TDS is recorded for context; your documented cost rates determine the results.
Which system should I choose?
Costs are one part of the decision. Compare pressure requirements, transport, maintenance and support in our WaterFed cart selection guide. Review the selected model's operating instructions before deciding.
Need help with the inputs?
Bring your water readings, workload estimate and equipment shortlist to abc Window Cleaning Supply. We can help you find product information and current quotes.
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