What size salt cell do I need?
Cells are marketed by pool size — "rated for 40,000 gallons" — and that number is close to meaningless on its own, because it assumes the cell runs 24 hours a day at 100% output. Your pump runs eight. The honest version of the rating is pounds of chlorine per day, and the sizing question is whether your cell can make what your pool burns within the hours your pump actually turns:
| Marketed as | Real output | Covers (8 h/day, 3 ppm demand) |
|---|---|---|
| 15,000 gal | 0.53 lb/day | ≈ 5,000 gal |
| 25,000 gal | 0.98 lb/day | ≈ 9,000 gal |
| 40,000 gal | 1.45 lb/day | ≈ 14,000 gal |
| 60,000 gal | 2.0 lb/day | ≈ 19,000 gal |
Third column: the pool each cell comfortably keeps up with on an 8-hour pump schedule at 70% duty. That's roughly a third of the number on the box — which is why so many salt pools can't hold FC in July.
A "40,000 gallon" cell makes about 1.45 lb of chlorine in a full day of running. On an 8-hour schedule it gets a third of that day, so it delivers about 0.48 lb — enough for roughly 14,000 gallons at typical demand. Nothing is wrong with the cell; the rating just quietly assumed a pump schedule nobody runs. Every manufacturer labels this way, so at least the comparison between brands stays fair.
Two levers, not one
If the cell can't keep up, you have two ways out and they're interchangeable: a bigger cell, or more pump hours. The calculator above shows both — the size to buy, and the runtime that would make a smaller one work. With a variable-speed pump the hours are nearly free, since running longer at low RPM costs little (the savings calculator has that math), so extending runtime is often the cheaper fix than upsizing.
What doesn't work is turning the output percentage up and hoping. If a cell is already at 100% and FC still falls, it is physically making all it can — more output isn't available. That's a sizing or runtime problem, not a settings one.
Why oversizing pays for itself
Cells wear by chlorine produced, not by calendar time — the plates have a finite output in them and then they're done. So a cell loafing at 50% lasts roughly twice as long as the same cell flogged at 100%. That turns a bigger cell from an indulgence into a cheaper running cost:
| On a 14,000 gal pool, 8 h/day | 25k cell | 40k cell |
|---|---|---|
| Duty it must run at | ~107% (can't keep up) | ~72% |
| Typical replacement cost | ≈ $500 | ≈ $650 |
| Expected life at that duty | — | ≈ 5–7 seasons |
The honest limits on that advice: go one tier up, not three. Cells also die of age, scale and physical damage regardless of how gently they've been run, so past a certain point you're pre-paying for life the cell won't live to deliver. And an enormous cell on a tiny pool spends its days at 10% output, which is fine but wasteful of money you could have spent on a better pump.
The plates are the hottest, highest-pH spot in your entire system, so calcium comes out of solution there first — long before you'd see it at the waterline. A scaled cell reads low salt, produces poorly, and gets replaced when it only needed cleaning and a water-balance fix. Before you buy anything, check the plates for white crust, and check your saturation index is not sitting on the scaling side. Keeping CSI slightly negative in a salt pool is the single cheapest thing you can do for cell life.
What else affects the number
CYA. Salt pools want 60–80 ppm, well above a manually chlorinated pool. Stabilizer is sunscreen for the chlorine your cell just made, and running it low means the cell works far harder for the same FC — the most common reason an adequately sized cell appears undersized. Water temperature. Cold water reduces output; most systems cut off below about 50–60 °F, which is normal, not a fault. Salt level. Low salt reduces production too — but if salt is in range and FC still lags, adding more won't help. That's the salt calculator's job, and it's a different knob entirely.