What size salt chlorine generator do I need?

Match the cell's actual daily output to your pool's daily chlorine demand at YOUR pump hours, not the rated number on the box. A typical 20,000-gallon pool burns about 0.5 lb of chlorine a day; on a 12-hour pump schedule that needs a cell rated about 1.0 lb/day at 24-hour basis — meaning a Pentair IC40 or Hayward T-15, not the IC20 the rated-gallons advice would pick.

Why can't the cell rated for my pool size keep up?

Because the rating is a 24-hour figure. Pentair and Hayward both quote their lb/day numbers per 24 hours of continuous pump runtime. On a normal 12-hour pump schedule you get half — so the cell rated for your pool's gallons is sized for a 24-hour pump and falls short on a 12. Size up to the cell that meets demand at your actual pump hours.

Does running my pump longer change the cell I need?

Yes — directly. The cell only makes chlorine while the pump runs, and the cell's published output assumes 24 hours of pumping. Run the pump longer and a smaller cell suffices. The same 20,000-gallon pool that needs an IC60 at 8 hours, needs an IC40 at 12, and needs only an IC15 at 24. If you're between two cell sizes, longer pump hours can settle it without buying up.

Should I size up my salt cell?

Usually yes. The cell is a consumable (~10,000-hour rating), and a bigger cell meets your demand running fewer of those hours each day — so it lasts longer in calendar years. For a 20k pool at typical demand, the IC60 outlasts the IC40 by about 1.4 times. Both Pentair and Hayward explicitly recommend buying the next size up and running it at a lower level for that reason.

What if even the biggest cell can't keep up?

It happens — a 40,000-gallon pool on a 12-hour pump schedule needs about 1.0 lb of chlorine a day, which puts the IC60 (the largest residential cell) at zero margin. The first remedy is free: run the pump longer. An IC60 at 18 hours gains 0.5 lb/day of headroom. If you can't extend runtime, dual cells or a commercial-spec generator is the fallback.

Why isn't my cell making its rated chlorine?

Two reasons stack. First, the rating assumes 24 hours of continuous pump runtime — at 12 hours you get half. Second, the rating itself is a maximum measured at warm water and ideal salt (around 2,900 ppm in Hayward's test). Cold water or salt running low both drop real output below the rating. So short pump hours plus cold or low-salt water can stack into a dramatic shortfall.

What size for a 40,000-gallon pool?

About 1.0 lb of chlorine a day at typical demand. On a 12-hour pump that needs a cell rated 2.0 lb/day at 24-hour basis — which is the IC60 exactly, at zero margin. The honest recommendation is to either pump longer (~18 hours gives an IC60 comfortable headroom) or run dual cells. Don't accept an IC60 at zero margin as a solved problem.

Is this the same as how much salt to add?

No — these are two different decisions. This page sizes the cell (the equipment). The pool salt calculator computes the salt dose (the chemical to add). The cell needs the salt to be in range to hit its rated output; the salt calc puts the salt there. Use them together: pick the cell here, then dose the salt with the salt calculator.

Salt Chlorine Generator Calculator

Sized for your pool AND your pump hours — because the cell rating on the box assumes 24-hour pumping, and on a normal 12-hour schedule you get half. Manufacturer-sourced Pentair IntelliChlor and Hayward TurboCell ratings; matched cell, sized-up payoff, honest ceiling.

Hook

The rating on the box isn't what you'll get.

Sizing a salt chlorine generator has a trap that catches almost everyone: the chlorine output printed on the box — say, 1.4 pounds a day — assumes your pump runs all twenty-four hours. Run it a normal twelve, and you get half that.

So the cell rated for your pool size often can't actually keep up on a real schedule, and the right cell is usually a size bigger than the label suggests. Sizing for your pump hours, not the box number, is the whole game.

Promise

This calculator sizes the generator the honest way — from your pool's daily chlorine demand AND your pump's daily runtime, matched to real manufacturer-rated cells (Pentair IntelliChlor, Hayward TurboCell). It shows the cell that actually meets your need at your pump hours, the next size up that runs less and lasts longer, and what to do if even the biggest residential cell can't keep up on a short schedule. It's honest that the rated output is a warm-water, full-salt ceiling — and that running your pump longer can let a smaller cell suffice.

Here's the deal: a salt cell makes its rated chlorine only if the pump runs all day. On a normal half-day schedule it makes half as much — so match the cell to your demand AND your pump hours, not just the gallons on the box. Get a size up from “just enough” and it runs fewer hours and lasts longer.

What you'll give us

Three inputs: pool volume, daily chlorine loss (the demand side), and your pump's daily runtime hours (the lever that decides which cell actually fits). The diagram below shows how they feed the engine to the matched cell + sized-up alternative.

Three inputs in, two cell recommendations out — matched cell that just meets your demand at your pump hours, plus the next size up with the longer-life payoff. The pump-hours input is highlighted because it's what the box rating quietly assumes is twenty-four.

The calculator

Defaults match worked example 1 (20,000 gal, 3 ppm/day loss, 12-hour pump). Change them to your pool, or run the defaults first to see what the result panel looks like.

Related decisions — salt to add (the dose that feeds the cell), chlorine target (the demand the cell has to meet), and pump run time (the runtime lever — longer pump hours let a smaller cell suffice).

Don't know your gallons? Pool volume calculator — two minutes, any shape, deep-links straight back here.

Sizing it: demand versus output

Sizing comes down to one comparison: how much chlorine your pool burns through in a day versus how much a cell can actually make in that day. Your pool loses chlorine to sun, swimmers, and heat — typically two to four parts per million daily. Multiply that by your gallons for your demand in pounds.

Then match it to a cell — but here's the catch the next section covers: a cell's rated output assumes the pump runs all day, so what it makes on your schedule is usually less. The chart below puts each sourced cell's ACTUAL output at the canonical 12-hour pump schedule against the demand line for a 20,000-gallon pool — the smallest cell that meets the line is your match.

Seven sourced cells' actual lb/day at a 12-hour pump schedule next to the canonical 0.50 lb/day demand. The bars in red fall short — including the IC20 that rated-gallons advice would pick for a 20k pool. The smallest that actually meets is the IC40.

The box rating assumes 24-hour pumping (the wedge)

Now the trap. When a cell is rated at 1.4 pounds a day, that figure assumes your pump runs all twenty-four hours — the rating is per 24 hours of continuous run. But most pumps run about twelve hours a day, and the cell only makes chlorine while the pump pushes water through it.

So on a normal twelve-hour schedule, that 1.4-pound cell actually delivers about 0.7 pounds. This is why the cell “rated for your pool size” so often can't keep up: the rating is a full-day number, and you're running half a day. Size for your actual pump hours, and you'll usually land one size above what the box implies.

The same IC40 across four pump schedules. The bar in red is the eight-hour case — short of demand for a 20,000-gallon pool, even though IC40 is “rated” well over what such a pool “needs.” The bar on the right is the 24-hour case, where the rating actually lands. Most people live in the middle.

The chart shows the same IC40 across four pump schedules versus the canonical demand line. The eight-hour bar — red — falls short, even though the IC40 is “rated” for a far bigger pool than 20k. The twenty-four-hour bar lands on the box's number, because that's what the box was measuring. Most people live somewhere in the middle, and that's where the wedge shows up.

Why bigger is cheaper (the size-up wedge)

Beyond just meeting demand, there's a money reason to go a size up. A cell is a consumable, rated for about ten thousand hours of operation. The cell only works as hard as it must — a bigger cell meets the same daily demand running fewer of its hours, so it reaches that ten-thousand-hour limit later.

For a typical pool, stepping from a just-enough cell to the next size up cuts the daily cell-hours by roughly a third and stretches replacement out by a year or more. Both Pentair and Hayward recommend exactly this: buy up, run it easier, replace it less often.

Sizing up the same 20k pool from IC40 to IC60 cuts daily cell-operating hours from 8.6 to 6.0 — stretching the ten-thousand-hour cell life from about 3.2 years of seasonal running to 4.6. The bigger cell runs easier and lasts about 1.43 times longer before replacement.

The lifespan math is genuinely elegant: once a cell can meet your demand, the hours-it-runs-per-day depends only on demand and rated output, not on your pump schedule (the pump hours mathematically cancel). So sizing up is a pure cell-life win, with the same upfront question — is the bigger cell's price worth replacing it half as often?

When even the biggest cell can't keep up

Two honest limits to close on. First, even at twenty-four-hour pumping, a cell's rating is its best case — measured in warm water at a full salt level around 2,900 parts per million. In cold water or low salt, it makes less, so keep a little margin.

Second, a big pool on a short pump schedule can outrun even the largest residential cell: a forty-thousand-gallon pool run only twelve hours a day needs the very top cell at full tilt, with nothing in reserve. The fix there is usually free — run the pump longer, and a single cell regains its headroom. If you can't extend the runtime, two cells or a commercial unit is the fallback.

Left: a 40,000-gallon pool on a 12-hour pump pins the IC60 at its ceiling. Pump 18 hours and it gains real headroom; pump 24 and the rating shows up. Right: even at 24 hours, the rating itself is a ceiling — measured at the manufacturer's warm, full-salt test conditions. Cold water or low salt makes less, by an amount the manufacturer doesn't publish.

The chart on the left is the forty-thousand-gallon case: at twelve hours the IC60 just barely meets demand, at eighteen it has real headroom, at twenty-four it's the rated number. The schematic on the right is the conditions caveat — real output drops as salinity or temperature drops away from the manufacturer's test condition. There's no published cold-water percentage, so the curve is directional only.

Eight worked examples

Every figure below runs through the same lib/dosing/swg.ts engine the calculator uses, and is locked against the research-file precomputed values by assert-swg.mjs.

E1 — Sizing a standard pool (the core case)

20,000 gal @ 3 ppm/day FC loss, 12-hr pump → demand 0.50 lb/day; required rated = 1.00 lb/day → matched: Pentair IC40 (rated 1.40, 0.70 actual at 12 hr) or Hayward T-15 (1.47 rated). Sized-up: IC60.

Takeaway: A typical 20k pool on a 12-hour schedule needs an IC40 — NOT the IC20 its gallons suggest — because the rating is a 24-hour number.

E2 — The pump-schedule wedge (the sharpest insight)

Same 20k pool; the IC40's ACTUAL output by schedule: 8 hr → 0.47 (short of 0.50 — need IC60); 12 hr → 0.70 (fits); 24 hr → 1.40.

Takeaway: The same cell and pool need a different answer depending on pump hours — run 8 hours and you need an IC60; run 24 and an IC15 covers it. Size for your hours.

E3 — The size-up payoff (lifespan)

20k @ 0.50 lb/day: IC40 runs the cell ~8.6 hr/day → ~3.2 yr to replace; IC60 ~6.0 hr/day → ~4.6 yr (~1.43× longer).

Takeaway: Sizing up from IC40 to IC60 stretches replacement from ~3 to ~4.5 years — weigh that against the higher upfront cost.

E4 — The ceiling caveat (conditions)

An IC40's outputs above are warm-water, full-salt maxima (~2,900 ppm); in cold water or low salt the real output is lower.

Takeaway: Treat every output here as a best case — keep margin, hold your salt in range.

E5 — Big pool hits the ceiling (the edge case)

40,000 gal @ 3 ppm/day, 12-hr pump → demand 1.00 lb/day; required rated = 2.00 lb/day → only the IC60 (2.00) matches, at zero margin, nothing to size up to. At 18-hr pump the required rated drops to 1.34; IC60 actual rises to 1.50 (+0.50 lb/day of comfortable head).

Takeaway: A 40k pool run just 12 hours needs the biggest residential cell flat-out — run the pump longer (free, see the pump run time calculator) or step to dual cells.

E6 — Hot-climate demand

20,000 gal @ 4 ppm/day (strong sun), 12-hr pump → demand 0.67 lb/day; required rated = 1.34 lb/day → matched IC40 (1.40) at little margin.

Takeaway: Strong sun pushes demand up — the IC40 that comfortably fit at 3 ppm is near its limit at 4; consider sizing up or pumping longer.

E7 — Small pool

15,000 gal @ 2.5 ppm/day, 12-hr pump → demand 0.31 lb/day; required rated = 0.63 lb/day → matched Pentair IC20 (0.70) or Hayward T-5 (0.735).

Takeaway:A small pool's matched cell is modest — but sizing up to an IC40 still buys longer cell life.

E8 — Metric

75 m³ (~19,800 gal) @ 3 ppm/day, 12-hr pump → demand 0.50 lb/day (0.225 kg/day) → matched IC40.

Takeaway: Same logic in metric — demand and pump hours pick the cell.

Reference tables

T1 · Sourced cell-rating table (the centerpiece)

SOURCED · 8 rows manufacturer-published (Pentair install/user guides, Hayward TurboCell sell sheet). Outputs are 24-hour-continuous-pump ratings. T-3's rated-gallons ceiling is “smaller pools” on the manufacturer sheet — kept as a sourced row, omitted from matched recommendation. Released CC BY 4.0. Cite as: PoolSolver, 2026 — aggregated from pentair.com and hayward.com manufacturer specifications.

Brand	Model	Output (lb/day, 24-hr basis)	Rated pool size	Source
Pentair	IntelliChlor IC15	0.600	up to 15,000 gal	pentair.com install/user guide
Pentair	IntelliChlor IC20	0.700	up to 20,000 gal	pentair.com install/user guide
Pentair	IntelliChlor IC40	1.40	up to 40,000 gal	pentair.com install/user guide
Pentair	IntelliChlor IC60	2.00	up to 60,000 gal	pentair.com install/user guide
Hayward	TurboCell T-3	0.530	smaller pools (no published ceiling)	hayward.com TurboCell sell sheet
Hayward	TurboCell T-5	0.735	up to 15,000 gal	hayward.com TurboCell sell sheet
Hayward	TurboCell T-9	0.980	up to 25,000 gal	hayward.com / T-9 spec
Hayward	TurboCell T-15	1.47	up to 40,000 gal	hayward.com / T-15 spec

T2 · Required cell rated output by pool size × pump hours (the pump-schedule wedge)

ESTIMATE · the RATED (24-hr basis) cell output you need to meet daily chlorine demand at typical 3 ppm/day FC loss. Demand is a range; this anchors at the residential typical. Released CC BY 4.0.

Pool size	Demand (lb/day)	8-hr pump	12-hr pump	16-hr pump	24-hr pump
10,000 gal	0.25	0.75	0.50	0.38	0.25
15,000 gal	0.38	1.13	0.75	0.56	0.38
20,000 gal	0.50	1.50	1.00	0.75	0.50
25,000 gal	0.63	1.88	1.25	0.94	0.63
30,000 gal	0.75	2.25	1.50	1.13	0.75
40,000 gal	1.00	3.00	2.00	1.50	1.00
60,000 gal	1.50	4.51	3.00	2.25	1.50

T3 · Size-up payoff (matched vs sized-up)

ESTIMATE · for common pool sizes at 3 ppm/day demand: the matched cell's cell-on hours/day and estimated years to replace (at the 10,000-hour Pentair titanium-cell rating) vs the next meaningfully larger cell. Real lifespan depends on water chemistry, salinity maintenance, and operating temperature. Released CC BY 4.0.

Pool size	Demand	Matched	hr/day	est. yr	Sized-up	hr/day	est. yr	Longer by
15,000 gal	0.38	IC20	12.9	~2.1	IC40	6.4	~4.3	~2.00×
20,000 gal	0.50	IC40	8.6	~3.2	IC60	6.0	~4.6	~1.43×
25,000 gal	0.63	IC40	10.7	~2.6	IC60	7.5	~3.6	~1.43×
30,000 gal	0.75	IC60	9.0	~3.0	—	—	—	at ceiling

Tables released CC BY 4.0. T1 is sourced (manufacturer specs); T2 and T3 are computed from the same engine the calculator uses, labelled estimates.

Methodology & sources

The sizing formula is demand vs cell ACTUAL output. Daily chlorine demand = FC loss (2–4 ppm/day, industry/TFP sourced range; ~3 ppm/day is the residential typical) × pool volume × 8.345 lb/gal (the CRC Handbook water density at 60 °F, imported from lib/dosing/core.ts). Volume comes from the shipped pool volume calculator (any shape, deep-links here). Demand is a RANGE; the result panel shows it that way.

The key correction (the page's identity): cell ratings are per 24 hours of continuous pump runtime. Pentair's manual states this verbatim (“0.70 lbs per 24 hours of continuous pool pump run time”), and Hayward's test conditions confirm it. So actual daily output = rated × (pump-hours ÷ 24). On a typical 12-hour residential schedule, the cell delivers half its rated number. Required rated output = demand ÷ (pump-hours ÷ 24). Pump hours are therefore a PRIMARY sizing input, not optional.

The cell table (T1) is manufacturer-sourced, not computed. Pentair IntelliChlor IC15/20/40/60 = 0.60/0.70/1.40/2.00 lb/day from the Pentair install/user guides; Hayward TurboCell T-3/5/9/15 = 0.53/0.735/0.98/1.47 lb/day from the Hayward TurboCell sell sheet. Stored as LITERAL values in lib/dosing/swg.ts — assert-swg.mjs forbids any expression on the right side of outputLbPerDay: so a future refactor cannot silently replace a sourced number with a computed one. (Note: Hayward's T-15 sell sheet states 1.47; some retailer pages round to 1.45 — we cite the manufacturer figure.)

Hayward T-3 is included in T1 as a sourced fact but skipped from matched-cell recommendations. Its 0.53 lb/day output is real and citable, but the manufacturer sell sheet doesn't publish a rated-pool-size ceiling for it (“smaller pools” without a number). Verified-or-omitted applies to recommendations too: we don't extrapolate a sizing recommendation the manufacturer didn't support.

Rated output is a ceiling under ideal conditions. Hayward's test of cell output is run at approximately 2,900 ppm salinity and warm water. Real output depends on salinity AND water temperature — in cold water or with salt running low, the same cell makes meaningfully less. There is no manufacturer-published cold-water percentage, so we surface the directionality without inventing a figure.

Cell lifespan is pump-independent once feasible. Cells are consumables, rated for about 10,000 hours of operation (Pentair titanium-cell spec). The cell-on hours per day = demand × 24 ÷ rated — the pump-hours mathematically cancel because more pump hours mean a lower runtime fraction within them. So the size-up wedge (matched vs sized-up) compares purely on demand and rated, and the years-to-replacement estimate is at the ten-thousand-hour rating, assumed 365 operating days per year. Real life depends on salinity maintenance and operating temperature; labelled estimate.

The big-pool ceiling edge case (E5). A large pool on short pump hours can exceed even the IC60 (the largest residential cell). Forty thousand gallons at 3 ppm/day demand on a 12-hour pump requires rated output 2.00 lb/day — IC60 at zero margin. The recommendation flows: (1) run the pump longer (free; IC60 gains comfortable headroom around 18 hours), with a live link to the pump run time calculator; (2) dual cells or a commercial-spec generator as the fallback.

Cross-page consistency. Imports LB_PER_GAL_WATER, volumeToGal, and Volume from lib/dosing/core.ts — the chemistry engine's single source of truth for water density and volume conversion. The page never redeclares the 8.345 constant; the assert script grep-checks this. Salt-corner spoke of the chemistry cluster, paired bidirectionally with the salt-dose calculator (same equipment, different question) and cross-linked to the chlorine target and the pump-runtime lever.

Reference tables released under CC BY 4.0. T1 cite: aggregated from pentair.com (IntelliChlor install/user guides) and hayward.com (TurboCell sell sheet).

Frequently asked questions

What size salt chlorine generator do I need?: Match the cell's actualdaily output to your pool's daily chlorine demand at your pump hours, not the rated number on the box. A typical 20,000-gallon pool burns about 0.5 lb of chlorine a day; on a 12-hour pump schedule that needs a cell rated about 1.0 lb/day at 24-hour basis — meaning a Pentair IC40 or Hayward T-15, not the IC20 the rated-gallons advice would pick.
Why can't the cell rated for my pool size keep up?: Because the rating is a 24-hour figure. Pentair and Hayward both quote their lb/day numbers per 24 hours of continuous pump runtime. On a normal 12-hour pump schedule you get half — so the cell rated for your pool's gallons is sized for a 24-hour pump and falls short on a 12. Size up to the cell that meets demand at your actual pump hours.
Does running my pump longer change the cell I need?: Yes — directly. The cell only makes chlorine while the pump runs, and the cell's published output assumes 24 hours of pumping. Run the pump longer and a smaller cell suffices. The same 20,000-gallon pool that needs an IC60 at 8 hours, needs an IC40 at 12, and needs only an IC15 at 24. If you're between two cell sizes, longer pump hours can settle it without buying up. The pump run time calculator covers what longer hours actually cost.
Should I size up my salt cell?: Usually yes. The cell is a consumable (~10,000-hour rating), and a bigger cell meets your demand running fewer of those hours each day — so it lasts longer in calendar years. For a 20k pool at typical demand, the IC60 outlasts the IC40 by about 1.4 times. Both Pentair and Hayward explicitly recommend buying the next size up and running it at a lower level for that reason.
What if even the biggest cell can't keep up?: It happens — a 40,000-gallon pool on a 12-hour pump schedule needs about 1.0 lb of chlorine a day, which puts the IC60 (the largest residential cell) at zero margin. The first remedy is free: run the pump longer. An IC60 at 18 hours gains 0.5 lb/day of headroom. If you can't extend runtime, dual cells or a commercial-spec generator is the fallback.
Why isn't my cell making its rated chlorine?: Two reasons stack. First, the rating assumes 24 hours of continuous pump runtime — at 12 hours you get half. Second, the rating itself is a maximum measured at warm water and ideal salt (around 2,900 ppm in Hayward's test). Cold water or salt running low both drop real output below the rating. So short pump hours plus cold or low-salt water can stack into a dramatic shortfall.
What size for a 40,000-gallon pool?: About 1.0 lb of chlorine a day at typical demand. On a 12-hour pump that needs a cell rated 2.0 lb/day at 24-hour basis — which is the IC60 exactly, at zero margin. The honest recommendation is to either pump longer (~18 hours gives an IC60 comfortable headroom) or run dual cells. Don't accept an IC60 at zero margin as a solved problem.
Is this the same as how much salt to add?: No — these are two different decisions. This page sizes the cell (the equipment). The pool salt calculator computes the salt dose (the chemical to add). The cell needs the salt to be in range to hit its rated output; the salt calc puts the salt there. Use them together: pick the cell here, then dose the salt with the salt calculator.

Related calculators

Next in Pool Chemistry: Saltwater Pool Conversion Cost Calculator.

More in Pool Chemistry: Pool Salt Calculator, Pool Chlorine Calculator, Pool Volume Calculator.

Related across clusters: Pool Pump Run Time Calculator.

All Pool Chemistry calculators: browse the hub.