Calculator
Example data table
| Volume (gal) | Demand (ppm/day) | Run hours | Temp factor | Safety% | Aging% | Recommended (lb/day) | Recommended (g/hour) |
|---|---|---|---|---|---|---|---|
| 15,000 | 2.5 | 10 | 0.85 | 25 | 15 | 1.02 | 19.3 |
| 20,000 | 3.0 | 12 | 0.90 | 20 | 10 | 1.24 | 23.5 |
| 12,000 | 2.0 | 8 | 0.80 | 30 | 15 | 0.97 | 18.3 |
These examples illustrate how run time and margins change recommendations.
Formula used
- Chlorine needed (lb/day): lbs_day = ppm_day × gallons × 8.34 ÷ 1,000,000
- Rating needed at 100% for your run time: lbs_100 = lbs_day × (24 ÷ run_hours)
- Recommended cell rating: lbs_rec = (lbs_100 ÷ temp_factor) × (1+safety) × (1+aging)
- Convert to g/hour: g_hour = (lbs_rec × 453.59237) ÷ 24
How to use this calculator
- Enter your pool volume and choose the unit.
- Estimate daily chlorine demand in ppm per day.
- Set your typical pump run time in hours daily.
- Choose a temperature factor for your season.
- Add safety and aging margins for long-term stability.
- Submit to view sizing results above this form.
- Export results for records and service notes anytime.
Practical notes
Use 1.00 for warm water where output is strong. Use 0.75–0.90 for mild conditions. Use 0.50–0.70 for colder water where output drops.
Test free chlorine daily for a week. If chlorine drifts low, increase run time first. Then raise output percent gradually. If you stay near 100%, consider a larger cell.
Understanding daily chlorine demand
Daily demand is the amount of free chlorine your water consumes each day from sunlight, swimmers, and organics. For many residential pools, 1.5–4.0 ppm per day is common, while high bather loads or intense sun can push it higher. This calculator turns that ppm estimate into a mass of chlorine needed per day, which is the most consistent way to size equipment.
Converting volume into usable sizing numbers
Volume is converted to gallons so the same chemistry relationships apply across units. The conversion step matters because a small error in volume multiplies directly into the required chlorine mass. If you do not know your exact volume, use a measured fill time, a water meter, or a volume test to reduce uncertainty.
Matching cell output to your run schedule
Salt cells are commonly rated as a maximum chlorine production per 24 hours at full output. If your pump runs fewer hours, the cell must be rated higher to deliver the same daily chlorine. The calculator scales the required rating by 24 ÷ run hours, helping you compare options when you prefer shorter daily run times.
Accounting for temperature, aging, and headroom
Output can drop in cooler water, and production often declines as plates age or scale forms. The temperature factor reduces expected output, while safety and aging margins add capacity so you are not forced to operate near 100%. Oversizing typically improves stability, lowers percent settings, and provides flexibility during heat waves or parties.
Interpreting the recommendation and exports
The primary result is a recommended cell rating in lb/day and g/hour, making it easy to match to product specifications. If you enter an existing cell rating, the estimated output setting indicates whether the unit can keep up with your assumptions. Exporting CSV or PDF is useful for service records, seasonal rechecks, and documenting changes to demand or run schedules. Re-run the sizing after major water changes, new covers, or shifts in stabilizer that alter sunlight loss patterns.
FAQs
1) What demand value should I start with?
If you are unsure, start at 2.5 ppm per day for a typical outdoor pool. Test daily for a week, then adjust the demand input until your measured chlorine trend matches your estimate.
2) Why does fewer run hours increase the recommended rating?
Cell ratings assume 24 hours of production at full output. When you run the pump fewer hours, the cell has less time to produce, so the required rating must be higher to deliver the same daily chlorine.
3) What temperature factor should I use?
Use 1.00 for warm water and normal output. Use about 0.75–0.90 for mild conditions. Use 0.50–0.70 for colder water where production commonly drops and you want extra headroom.
4) Is it better to oversize a salt cell?
A modest oversize often improves stability because you can run a lower output percentage, leaving headroom for heat waves, swim parties, and aging. Avoid extreme oversizing if it leads to unnecessary cost or poor fit.
5) What does the estimated output setting mean?
It estimates what percent output your existing cell would need, using your run time and temperature factor. Values above 100% indicate the cell may struggle under those assumptions and you may need more run time or a larger cell.
6) How often should I recalculate sizing?
Recheck after major seasonal shifts, equipment changes, or noticeable changes in daily chlorine loss. A quick recalculation helps you confirm your run schedule and output percentage still support a steady free chlorine level.