The measurement, not the estimate
Ask what a pool consumes in a day and every honest answer is the same shape: it depends, and you have to measure it. The dependencies are not small ones. Ultraviolet light destroys free chlorine in an open pool at a rate that changes with the sun angle, the cloud cover and the stabiliser chemistry. Bather load matters, and a Saturday with a dozen people in the water is a different pool from the Tuesday before it. Water temperature, organic debris, rain and what is already dissolved all move the figure.
What is measurable is the pool in front of you over a known window. Test, note the time, wait, test again with nothing added and the generator off, and the drop divided by the hours is a rate. Multiply by 24 and you have a daily figure that describes those conditions. That is a real number about your pool rather than a table entry about a hypothetical one.
Working the arithmetic
A drop from 5.0 to 3.2 ppm over 18 hours is 1.8 ppm consumed, which is 0.1 ppm an hour and 2.4 ppm a day. In a 20,000 gallon pool weighing 166,908 pounds, one ppm is 0.1669 pounds, so 2.4 ppm a day is 0.4006 pounds of active material a day. As a product that is 65 percent active, that is 0.616 pounds a day on the scale — divided by 0.65, not multiplied, for the reason the dose calculator spells out at length.
| Window | Drop | ppm/day | lb/day active in 20,000 gal |
|---|---|---|---|
| 6 hours, afternoon sun | 1.2 ppm | 4.80 | 0.801 |
| 12 hours, daylight | 1.8 ppm | 3.60 | 0.601 |
| 18 hours, day and night | 1.8 ppm | 2.40 | 0.401 |
| 24 hours, full day | 2.0 ppm | 2.00 | 0.334 |
| 12 hours, overnight | 0.5 ppm | 1.00 | 0.167 |
Those rows are not five estimates of the same thing. They are five different measurements, and the sunny afternoon figure runs nearly five times the overnight one. Which window a number came from is part of the number.
Why the reading sometimes rises instead
If the second reading is not below the first, the page refuses to produce a rate rather than printing a negative one. The usual causes are mundane. A generator was producing during the window and the ppm entered for it was low. Product went in and was forgotten. The two readings came from different depths or different parts of the pool, or from a reagent set that had aged between them. Or the first reading was taken shortly after something was added, before the water had circulated, so it measured the plume near the return rather than the pool average.
The fix in every case is to repeat the measurement with production off, nothing added, samples taken from the same spot at elbow depth, and a window long enough that the drop is comfortably larger than the resolution of the test.
What to do with the number
Two things, both of them arithmetic rather than advice. The first is that it is the demand figure the salt cell page asks for, and comparing it against what a cell actually produces at your run hours and output setting tells you whether the equipment has any headroom on a bad week. The second is that it sets the interval between tests: a pool losing 4.8 ppm a day and one losing 1.0 are not on the same schedule, and knowing which one you have is worth more than any target on any chart.
What this page will not do is tell you what to add, when to add it, or what level to hold. Those depend on your tested water and on your product, and for a commercial pool on the health authority with jurisdiction.
Questions people ask
How do I measure my own pool chlorine demand?
Test the water after it has circulated, note the reading and the time. Turn off any generator or feeder and add nothing. Wait a known number of hours — a full 24 gives an average across day and night, which is usually the most useful single figure. Test again with the same kit, from the same spot at elbow depth. The drop divided by the hours, times 24, is the daily rate. Do it twice, once on a bright day and once on a dull one, and you have the range your equipment has to cover rather than a single number that will be wrong most days.
Why is my demand different in the afternoon than overnight?
Sunlight. Ultraviolet light breaks down free chlorine in open water, and in bright conditions that route can dominate everything else. A window measured through an afternoon in full sun captures the fast end, and an overnight window captures the slow end, and the two can differ by a factor of four or five on the same pool in the same week. Neither is the wrong measurement; they answer different questions. Note which window a figure came from and do not mix them.
How much product does a 2.4 ppm a day loss represent?
The active material first: in 20,000 gallons, one ppm is 0.167 pounds, so 2.4 ppm a day is 0.401 pounds a day. Then the product: divide by its strength. At 65 percent that is 0.616 pounds a day, at 100 percent it is 0.401, and at 12.5 percent it is 3.2. Over a 150 day season the 65 percent case comes to 92 pounds. All of that scales directly with the pool volume, so an error in the volume is an error of the same percentage in every figure.
Should I use this number to decide what to add?
No. It tells you what left the water between two readings you took. What to add, how much, and to what level is a decision that depends on your current tested results, on the product in front of you and its label, and for a commercial or public pool on the health authority whose rules apply. This page carries no targets, no minimums and no recommendations, and it does not say whether the water is fit to swim in. Use the rate to understand your pool and to size equipment; use your kit instructions to decide anything else.
Does the calculator handle a generator running during the window?
Partly, and with a caveat worth reading. There is a field for what was added in ppm during the window, and the rate is worked out from the first reading plus that figure minus the second. But the added figure is your own estimate of what a cell produced, and cell output varies with temperature, output setting, plate condition and flow, so an estimate there carries straight into the rate. A measurement with production off is cleaner and takes the same amount of time.