How salt cells are rated
Salt chlorine generators are sold by the largest pool they’re meant for (“rated for 40,000 gallons”) and by how much chlorine they make in a day, usually in pounds, at 100% output with the pump running 24 hours. Very few pools run the pump all day, so the rating on the box is the best case.
A Pool & Spa News guide on sizing salt chlorinators gives a simple rule based on chlorine demand, assuming the pump runs at least 8 hours a day:
| Chlorine demand | Typical pool | Cell rating |
|---|---|---|
| Light | Screened, shaded, cool climate, light use | 1 × pool volume |
| Medium | Typical backyard pool | 1.5 × pool volume |
| Heavy | Full sun, hot climate, lots of swimmers | 2 × pool volume |
If the pump runs less than 8 hours, the cell has less time to work, so we scale the rating up by 8 ÷ your hours. The same guide also notes that a correctly sized cell running fewer hours lasts longer, so oversizing is rarely a mistake.
How much chlorine your cell makes
ppm per day = lb per day × (pump hours ÷ 24) × output % × 119,826 ÷ gallons
119,826 converts pounds per gallon into milligrams per litre (ppm). In metric: ppm per day = grams per hour × pump hours × output % × 1,000 ÷ litres.Compare the answer with how much chlorine your pool loses each day. Test free chlorine in the evening and again the next evening without changing anything; the drop is your daily demand. Sunny pools with low stabilizer can lose several ppm a day.
Worked examples
20,000 gallons, typical backyard, pump runs 8 hours
- Rating = 20,000 × 1.5 = 30,000 gallons
- If the pool is in full sun and the pump only runs 6 hours: 20,000 × 2 × 8 ÷ 6 = 53,333 gallons
What a 1.45 lb/day cell makes in 20,000 gallons
- At 50% output for 8 hours: 1.45 × (8 ÷ 24) × 0.5 × 119,826 ÷ 20,000 = 1.45 ppm a day
- At 100% for 8 hours it would make 2.9 ppm, and at 100% for 12 hours 4.34 ppm
Daily chlorine from a 1.45 lb/day cell in 20,000 gallons
Show the numbers
| Output % | 6 h | 8 h | 12 h |
|---|---|---|---|
| 20 | 0.43 | 0.58 | 0.87 |
| 40 | 0.87 | 1.16 | 1.74 |
| 60 | 1.3 | 1.74 | 2.61 |
| 80 | 1.74 | 2.32 | 3.47 |
| 100 | 2.17 | 2.9 | 4.34 |
Getting the most from a salt cell
- Keep salt in the range on the cell’s label. The salt calculator tells you how much to add.
- Keep stabilizer up. Without CYA, sunlight destroys chlorine as fast as the cell makes it. Some cell makers recommend higher CYA than chlorine pools use, so check your manual.
- Inspect the cell for white scale every few months. Scaled plates make less chlorine and read salt low. Keeping pH and the saturation index in check slows scaling.
- When the pool needs a shock, use liquid chlorine. A cell can’t raise chlorine fast enough to clear algae.
Frequently asked questions
What size salt cell do I need for a 20,000 gallon pool?
For a typical backyard pool with the pump running 8 hours or more, one rated for about 30,000 gallons. In full sun or with heavy use, about 40,000 gallons.
Is it OK to oversize a salt chlorinator?
Yes. A larger cell can run at a lower setting or for fewer hours, which tends to extend its life. Pool & Spa News advises not to be afraid to oversize.
How many hours a day should a salt cell run?
Long enough to replace the chlorine your pool loses each day. Measure the overnight-to-overnight drop in free chlorine and use the output formula to match it.
Why is my salt pool chlorine low even at 100%?
Common causes are low CYA, a scaled or worn cell, low salt, too few pump hours or a cell too small for the pool’s demand. Check them in that order.
How long does a salt cell last?
Cell life is usually rated in hours of operation, and makers quote several years of normal use. Running fewer hours at a higher setting, or using a larger cell, spreads that life over more seasons.
Can I convert a chlorine pool to salt?
Yes. Add salt to the cell’s target with the salt calculator, install the cell after the filter and any heater, and keep stabilizer in range.
Sources
- Sizing Salt Chlorinators. Bob Harper, Pool & Spa News, July 29, 2011.
- How much salt does the system require?. Hayward (Aqua Rite salt chlorinators).
- Understanding the Chlorine/CYA Chart. Trouble Free Pool.
Reviewed November 7, 2026. We check the formulas against the sources above and re-run the worked examples whenever we update this page. Found something wrong? Tell us.