How long does DI resin last?
As many gallons as its ion capacity divided by the TDS reaching it — and what reaches it is your tap water times whatever fraction the membrane lets through. A standard 10-inch mixed-bed cartridge holds about 2,000 ppm-gallons (SpectraPure's published ~100 gallons at 20 ppm feed). Behind a typical 95 % membrane, 200 ppm tap water arrives at the resin as 10 ppm, so the cartridge makes 200 gallons — at 25 gallons a month, 243 days. Move the same system to 50 ppm municipal water and it is 800 gallons; to 500 ppm well water, 80. The RODI guide makes the case that this is why "resin lasts about N months" is never a quotable fact; this page runs the arithmetic for your reading and then does the part the guide stopped short of — adding up the rest of the bill.
What a gallon of RO/DI really costs
Resin is the line that varies tenfold, but it is not the only line. A system also drinks tap water — at a typical 4:1 waste ratio, 5 gallons in for every gallon out — and it wears through prefilters and a membrane on a calendar rather than a gallon count. At 25 gallons a month, a $20 cartridge, $11 per thousand gallons of tap water and $45 a year of consumables:
| Tap TDS | Reaching the DI | Gal / cartridge | Days / cartridge | Resin / yr | Tap water / yr | All-in per gal | Resin's share |
|---|---|---|---|---|---|---|---|
| 50 ppm | 2.5 ppm | 800 | 973 | $7.50 | $17 | 23¢ | 11 % |
| 100 ppm | 5 ppm | 400 | 487 | $15 | $17 | 26¢ | 20 % |
| 200 ppm | 10 ppm | 200 | 243 | $30 | $17 | 31¢ | 33 % |
| 300 ppm | 15 ppm | 133 | 162 | $45 | $17 | 36¢ | 42 % |
| 500 ppm | 25 ppm | 80 | 97 | $75 | $17 | 46¢ | 55 % |
Computed by this page's engine at 95 % rejection, 4:1 waste, 25 gal/month, with $45/yr of membrane and prefilters in every row. Every price is an editable default in the tool.
Two things the guide's resin-only table hides. The 10-fold spread in resin cost becomes a 2-fold spread all-in, because the tap water and the consumables are the same in every row. And on soft water, resin is only 11 % of the bill — the fixed lines dominate, so the "cost per gallon" a soft-water reefer quotes on a forum is mostly their membrane amortization and their water rate, not the resin at all. On 500 ppm well water the picture inverts: resin is 55 % and everything else is noise.
The membrane is the lever — but only on hard water
Going from a typical 95 % membrane to a healthy 98 % one makes resin last exactly 2.5× longer at any tap TDS — (1 − 0.95) ÷ (1 − 0.98), and the tap figure cancels. What that is worth does not cancel. On 50 ppm water it saves $4.50 a year in resin, so a $40 membrane replaced early takes 9 years to pay for itself; on 500 ppm water it saves $45 a year and pays back in 11 months. The tool's "better membrane" line does this comparison for your water, which is how to decide whether a tired membrane is a problem or just a number.
To know which membrane you actually have, measure it: the TDS after the membrane divided by the TDS before is the fraction it lets through, and the "Measured" option takes those two readings directly. Anything over about 20 ppm reaching the DI means the membrane, not the resin, is where the money is going — fix that first, because buying resin behind a failing membrane is pouring it down the drain at 4:1.
Is it cheaper to just buy RO/DI water?
Fish stores sell it at about 49¢–75¢ a gallon. At 25 gallons a month on 200 ppm water the system above makes it for 31¢ all-in, so a $150 system pays for itself in about 19 months against the middle of that range, and every month after is money kept. The honest flip side: at 5 gallons a month — a nano with a lid — the consumables barely amortize and a gallon costs 91¢ to make, more than the store charges: buying water is cheaper, and the tool will say so. Enter your own usage and system price and the tool prints the payback or the word "never".
The DI calculators you find first are industrial: resin by the cubic foot at a nominal 12,000 grains, run to exhaustion. A standard 10-inch cartridge holds about 0.028 ft³ of resin, which at that rating is 5,689 ppm-gallons in theory — roughly 2.8× the 2,000 used here. The gap is not an error in either number. Industry runs a bed until it is spent; a reef keeper retires a cartridge at the first ppm of leakage, because the ions a tired bed lets go of first are the ones the tank least wants. The practical anchor is the one to budget with, and it comes from a manufacturer's own published figure for a hobby cartridge — one product line, so read it as a budget rather than a guarantee.
When is the cartridge actually spent?
When the meter after it reads 1. Not "a few" — one. Mixed-bed resin does not fade evenly; it holds everything until a section of the bed is exhausted and then begins letting ions through, and the first to slip are frequently the weakly-held ones — silicate, and the ammonium that tap chloramine leaves behind — that a reef least wants. Color-change resin turns as the cation bed exhausts, which is not the same moment, and is often later. So the number this page gives you is a calendar reminder rather than a replacement date: at 25 gallons a month on 200 ppm water it is 243 days, so put a note in the calendar at about day 195 to start checking the meter every fill. The cartridge is the cheap part; the water it quietly stops cleaning is not.
Are high-capacity cartridges worth it?
Only if the price is under the capacity multiple. A high-capacity cartridge claims about 3.5× the standard figure, so against a $20 standard cartridge it breaks even at $70; at $60 it is a modest win per gallon, and at anything over the break-even it is a convenience you pay for. And the win scales with your tap water like everything else here. On 50 ppm water the switch saves $1.07 a year; on 500 ppm well water it saves $11, and the cartridge changes drop from 3.8 a year to 1.1. The "Custom" option takes any cartridge's rated capacity as gallons-at-a-stated-feed times that feed's ppm, which is how to compare two products that quote different test conditions.
Winter, pressure, and the "Measured" option
A membrane's rejection is not a fixed property either. Cold feed water is rejected less efficiently, low pressure is rejected less efficiently, and a carbon block that has let chlorine through damages the membrane permanently — so the same system reads 98 % in July and 90 % in January in an unheated garage, and the resin bill follows. That is why the tool's "Measured" setting exists: the TDS before the membrane and the TDS after it are two readings on an inline meter, and their ratio is the rejection today rather than the figure on the box. Take them at the start of each season and enter the worse one, because that is the one the resin will see.
Freshwater tanks: RO without the DI
The DI stage exists for 0 TDS, which is a reef requirement, not an aquarium one. A planted tank, a discus tank or a shrimp tank that wants soft water is perfectly served by the membrane's permeate at 10–25 ppm, remineralized to the GH and KH the fish want with the GH/KH calculator. Take the resin line out of the budget and a gallon of RO alone at the example usage costs about 21¢ — tap water and consumables only, and the same whatever your tap TDS. The RODI guide covers which freshwater setups gain from it at all; most community tanks on moderate tap water do not.
The water you never see
At 4:1, the 25 gallons a month above is 125 gallons of tap water, of which 100 goes down the drain — about $17 a year at a typical combined water and sewer rate, which is why it sits on the bill here and in no other calculator. A 2:1 system halves it; a permeate pump or booster gets closer to 1:1. How many gallons your tank actually uses is the other half of this budget, and the evaporation and ATO calculator estimates it from your tank rather than a rule of thumb.