Two readings and a division
A filter that read 10 psi when it was last cleaned and reads 14 psi three weeks later has risen 4 psi in 21 days, which is 0.190 psi a day. If the manufacturer instructions say to clean at 8 psi over the clean baseline, the full cycle is 8 divided by 0.190, which is 42 days, and there are 21 of them left. Over a 150 day season that is four cleanings, because 150 divided by 42 is 3.57 and you cannot do part of one.
That is the whole arithmetic. Everything else on the page is the same numbers viewed from another angle: the water four cleanings uses, what that water costs, and what fraction of the pool it quietly replaces over a summer.
Why the threshold is a field
Filters differ. A cartridge filter, a sand filter and a DE filter do not load the same way, do not clean the same way, and do not carry the same instruction about when to do it. The figure belongs to the filter in front of you, in the paperwork that came with it or on the label, and a page that printed a number here would be inventing an instruction on behalf of a manufacturer it knows nothing about.
The clean baseline is worth more attention than it usually gets, for the same reason. It is not a property of the filter; it is a property of this filter, at this pump speed, with the media currently in it. It climbs across the life of the media, and a baseline that has gone from 10 to 14 psi over two seasons is a fact about the media rather than about this week.
The straight line is a simplification
Pressure across a loading filter does not rise linearly. Early in a cycle the media has plenty of open path and the pressure barely moves. Late in a cycle the remaining path is narrow and each additional gram of debris does more, so the curve steepens. Projecting from a rate measured in the first week therefore overestimates the interval, and projecting from a rate measured in the last week underestimates it.
| Reading taken | Rise so far | Rate | Projected cycle to +8 psi |
|---|---|---|---|
| Day 7 | 1.0 psi | 0.143 psi/day | 56 days |
| Day 21 | 4.0 psi | 0.190 psi/day | 42 days |
| Day 35 | 7.0 psi | 0.200 psi/day | 40 days |
Those three projections describe the same filter on the same cycle, and they differ by 16 days. None of them is a lie. The practical use is to reread the gauge as the cycle goes on and let the projection tighten, rather than to trust a day 7 number for planning in September.
Filtration rate is a separate question
The rate in gallons a minute per square foot of media is not about when to clean; it is about how the filter behaves while it is running. Sixty GPM through 48 square feet is 1.25 GPM per square foot. Whether that is the right rate for a given filter is on the filter label, not here, which is why the design rate is an optional field and the page reports the comparison against whatever you enter.
What the rate does affect is how quickly the pressure climbs, which links the two halves of this page. Push more water through the same media and it loads faster, so a schedule change on a variable-speed pump changes the cleaning interval as well as the electricity bill. That is one more reason the two gauge readings have to be taken at the same speed.
The water nobody counts
A cleaning that uses 250 gallons, four times in a season, is 1,000 gallons — five percent of a 20,000 gallon pool, replaced without anybody deciding to replace it. That water leaves carrying whatever is dissolved in the pool, and it comes back carrying whatever is dissolved in the supply. Over a season that is a real term in the mineral balance, and it is the one people are most surprised by. The makeup water and buildup calculator puts it alongside evaporation and splash-out.
Questions people ask
When should I clean or backwash a pool filter?
At the rise over the clean baseline that your filter manufacturer instructions specify, which is why that figure is a field on this page rather than a number printed on it. The arithmetic then answers the question you actually have, which is when that will be: a filter rising 0.19 psi a day reaches 8 psi over clean 42 days after a cleaning. Cleaning to a calendar instead of to the gauge means doing it too often on a quiet pool and too late on a busy one, and the whole point of the gauge is that it knows which one you have.
Why is my filter pressure going down instead of up?
Because less water is reaching the filter. Pressure at that gauge is what the filter and everything after it resist, and it needs flow to exist. A full pump basket, a blocked skimmer, a partly closed valve, a suction side air leak or a failing impeller all reduce flow, and reduced flow lowers the reading even though the situation is worse than before. Rising pressure and falling flow means a loading filter. Falling pressure and falling flow means a restriction ahead of the pump. The gauge alone cannot separate them, which is why watching the return flow matters.
How many times will I clean the filter in a season?
Season length divided by the cycle length, rounded up. A 42 day cycle across 150 days is four cleanings. What changes that number is anything that loads the filter faster — a heavy pollen fortnight, a windstorm, a run of hot weather with a lot of bathers, or a higher pump speed pushing more water through the same media. The projection is worth redoing after each cleaning, and after a couple of cycles you will have a rate that describes your pool rather than an assumption.
Do the two gauge readings have to be at the same pump speed?
Yes, and this is the mistake most likely to make the whole page wrong. Filter pressure rises with flow, so a reading at full speed and a reading at half speed differ by a large amount for reasons that have nothing to do with how loaded the filter is. On a variable-speed pump, pick the speed the pump spends most of its time at, take every reading at that speed, and note it beside the number. A rise rate assembled from mixed speeds is measuring the schedule, not the filter.
What filtration rate should my filter run at?
The one on its label or in its instructions, which vary by filter type and by model and are not something this page supplies. The field is there so the arithmetic can compare your actual rate against theirs and show what flow their figure allows on your media area, or what area your flow would want. Running faster than the design rate generally means shorter cycles and less effective filtration; running slower means longer cycles. Both are consequences rather than verdicts, and the manufacturer figure is the one that governs.