What does RODI water cost per gallon?

Making your own RO/DI water is cheap, but it isn't free. The cost has three parts: the tap water you send down the drain, the DI resin your TDS uses up, and the prefilters and membrane you replace on a schedule. The filled-in values are examples, so swap in your own. Your inputs stay on this device.

RODI cost calculator

RODI water you make, gallons per month Waste gallons per gallon of RODI Water + sewer rate, $ per 1,000 gallons Tap water TDS (ppm) Membrane rejection (%) DI capacity per fill (ppm × gallons) DI resin refill price ($) Prefilter set price ($, sediment + carbon) Prefilter changes per year RO membrane price ($) Membrane life (years)

Per gallon

Monthly

Yearly

Tap and sewer water:
DI resin:
Prefilters and membrane:

Formulas: water $/gal = (waste + 1) × rate ÷ 1,000. Gallons per resin fill = DI capacity ÷ (tap TDS × (1 − rejection)). Resin $/gal = refill price ÷ gallons per fill. Prefilters and membrane are spread over the gallons you make in a year.

Where the examples come from: a single-membrane system wastes about 3 gallons per gallon made, and a dual-membrane "Water Saver" about 1.5; carbon blocks are changed every 6 to 12 months; membranes last upwards of 3 years; most membranes reject upwards of 98% (BRS RO/DI FAQ). $16.83 per 1,000 gallons is EPA WaterSense's 2024 US average residential water plus wastewater rate (EPA); check your own bill. The DI capacity is the low end of a Reef2Reef estimate of 3,000 to 6,000 ppm-gallons per pound of resin, for a 1.25 lb cartridge fill (thread; real capacity varies with resin quality and CO2). Prices are Bulk Reef Supply list prices checked October 16, 2026: one-cartridge resin refill $17.99, 150 GPD membrane $62.99, carbon block $10.79, sediment filter from $5.19. Prices change, so enter what you pay.

Ways to cut RODI cost

Keep the membrane rejecting well. Resin life depends on what slips past the membrane. Going from 96% to 98% rejection halves the TDS reaching the resin, which doubles how far each refill goes. Compare TDS before and after the membrane to check it.
Hold the pressure up. BRS gives 50 to 90 PSI as the ideal range. Low pressure makes less water and a worse waste ratio, which is where a booster pump can pay off.
Divert the first few minutes. RO output is high in TDS for the first minutes after start-up. BRS suggests sending that water to the drain to reduce resin use.
Reuse the waste water. It's higher in TDS than tap water but fine for jobs like rinsing equipment or watering many plants, so less of it is pure cost.
Refill resin instead of buying sealed DI cartridges. A refillable cartridge with bulk resin is the cheap route BRS recommends.

Calculate my monthly cost Cheapest good RODI systems and filters