Calculators / Water and gutters

How much water does your reverse osmosis system waste?

Put in how much RO water you use and the system. You get the gallons it sends down the drain a year, and how WaterSense systems compare.

Updated November 2026 · How we check our numbers

Down the drain each year

3,650 gallons

Typical older system: 5 gallons go down the drain for every gallon of RO water. At 2 gallons a day that's 3,650 gallons a year, about $22 of water.

Per gallon: 5 gal16.7% efficient
RO water: 730 gala year
Waste: 3,650 gala year
Cost: $22a year

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Estimate for planning only, not professional advice. Check the manufacturer’s instructions, your local code and a licensed professional before you buy, build or install. Use at your own risk. Terms.

waste per gallon = (100 − 16.7) ÷ 16.7 = 5

a year = 2 gal × 365 × 5 = 3,650 gal

cost = 3,650 ÷ 1,000 × $6 = $21.90

Same use, other systems

2 gal a day

Typical older systemEPA: five gallons or more down the drain per gallon
3,650 gal/yr
5 gal down the drain per gallon
WaterSense labeledEPA: 2.3 gallons or less per gallon (30% efficiency)
1,703 gal/yr
2.3 gal down the drain per gallon
A.O. Smith AOW-3000Efficiency 17.91% (maker's manual)
3,346 gal/yr
4.6 gal down the drain per gallon
Culligan Aqua-Cleer RO50Efficiency 14.43% (maker's guide)
4,329 gal/yr
5.9 gal down the drain per gallon
Waterdrop G3P600 (2:1)"2 : 1 Pure to Drain Ratio" (maker)
365 gal/yr
0.5 gal down the drain per gallon
Waterdrop G3P800 (3:1)"3 : 1 Pure to Drain Ratio" (maker)
243 gal/yr
0.3 gal down the drain per gallon

Ratios vs ratings

Maker ratios like "2:1 pure to drain" and NSF/ANSI 58 efficiency ratings aren't measured the same way. The certified efficiency rating, printed in the manual or on the performance data sheet, is the one to compare.

Pressure and tank

RO wastes more at low water pressure and when the storage tank is nearly full. A permeate pump or a tankless model cuts waste.

Using the reject water

Reject water is drinkable tap water with more minerals. Some people route it to a garden or toilet; check your plumbing code first.

How we work it out

gallons wasted per gallon = (100 − efficiency %) ÷ efficiency %

An NSF/ANSI 58 efficiency rating is the share of the water going in that comes out as RO water. EPA says a typical system wastes five gallons or more per gallon; WaterSense labeled ones must be at least 30% efficient, 2.3 gallons or less.

Makers quote ratios too, like Waterdrop's "2 : 1 Pure to Drain". Those aren't measured the same way as the certified rating, so compare ratings where you can.

Where the numbers come from

Water at home

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