Two kinds of water leaving
Water leaves a pool by evaporation and by being carried or drained out, and the difference between those two is the whole subject of this page. Evaporation removes water molecules and nothing else, so everything dissolved stays behind in a smaller volume. Top the pool back up and you have added more of whatever the supply carries on top of what was already there. Splash-out, backwash and a drain-down remove water at the pool concentration, taking dissolved material with them in proportion.
The weekly ledger is straightforward once the two are separated. A 512 square foot pool losing 1.5 inches a week is losing 479 gallons to evaporation, because one inch across that surface is 319 gallons. Four filter cleanings at 250 gallons across a 22 week season averages 45 gallons a week. Makeup is the sum, 524 gallons a week, which comes to 11,533 gallons across the season — 58 percent of a 20,000 gallon pool, arriving a few hundred gallons at a time without anybody noticing.
Where a reading ends up
Each week, the pool gains what the makeup water brings and loses what leaves at pool strength. Written as a recursion, the concentration next week is this week plus makeup times fill concentration minus removal times current concentration, all divided by the volume. That is a geometric series, so it has a closed form and a fixed point, and the fixed point is the interesting part.
The level it converges on is the fill water concentration multiplied by the ratio of makeup to removal. With 524 gallons of makeup a week and only 45 of it leaving at pool strength, that ratio is 11.5, so 150 ppm fill water drives toward 1,730 ppm. It does not get there in one season — the pool only exchanges about 0.2 percent of its volume a week, so it moves slowly — but the direction is set, and the destination does not depend on where the pool started.
| Fill water | Removal at pool strength | Converges toward | After 22 weeks from 300 ppm |
|---|---|---|---|
| 150 ppm | 45 gal/week | 1,730 ppm | 370 ppm |
| 150 ppm | 250 gal/week | 437 ppm | 333 ppm |
| 50 ppm | 45 gal/week | 577 ppm | 314 ppm |
| 0 ppm | 45 gal/week | 0 ppm | 285 ppm |
Two things fall out of that table. The fill water concentration sets the destination almost single-handedly, and the removal rate sets how fast the pool travels toward it. A pool on soft supply water barely moves. A pool on hard supply water that is never drained is on a slow one-way trip, and the trip is invisible week to week.
The pool with no removal at all
Set splash-out and filter cleanings to zero and the arithmetic changes character. Nothing leaves carrying dissolved material, so there is no fixed point and no ceiling: the reading rises by makeup times fill concentration divided by volume, every week, forever. At 479 gallons a week of 150 ppm supply into 20,000 gallons that is 3.6 ppm a week, or 79 ppm a season, and it never stops.
That is not a hypothetical. A cartridge filter pool with an attentive owner, a cover that limits splash, and no reason to ever drain is exactly this pool, and the only reason it does not run away is that eventually somebody replaces water. This calculator is one way of finding out when eventually needs to be.
What is not in the model
Anything that adds the measure other than the fill water. Products add dissolved material by definition — that is what a dose is — and a page tracking calcium hardness will not see calcium arriving in a product, nor salt arriving in salt. Rain dilutes, and a wet fortnight can undo a month of drift. Plaster in a new pool releases material into the water for a while. Splash-out that is already inside your measured level drop should not be entered twice.
The model also assumes the pool is well mixed and refilled to the same level each week, which a circulating pool with an autofill approximates well and a pool topped up by hand every third week approximates less well. None of that changes the direction, and the direction is the thing worth knowing.
Test the fill water
The single input that most changes the answer is the concentration in the supply, and it is the one almost nobody measures. Municipal hardness varies enormously between systems and can change when a utility switches source. Well water varies more. A pool filled from softened water is a completely different problem from one filled from the raw supply. That test costs very little and it is the difference between this page telling you something and this page telling you what you assumed.
Questions people ask
How much water does a pool lose to evaporation in a season?
It follows from the surface area and the level drop you measure, and nothing else. One inch across 512 square feet is 319 gallons, so a pool dropping 1.5 inches a week is losing 479 gallons a week to evaporation, which is 10,533 gallons over 22 weeks. That is over half the volume of a 20,000 gallon pool. Rates vary enormously with wind, humidity, water temperature and whether there is a cover, so a figure from anywhere but your own fixed mark and a tape is a guess.
Why does calcium hardness keep going up in my pool?
Because evaporation removes water and leaves the calcium behind, and the water you top up with brings more of it. Whether the reading climbs, holds or falls is set by two things: how much calcium the fill water carries, and how much water leaves the pool carrying dissolved material with it rather than evaporating. A pool that is never backwashed and never splashes has essentially no removal, so the reading only goes one way. Testing the fill water is the first step in understanding your own case; the fill figure sets the destination.
Does backwashing lower a dissolved reading?
Yes, in proportion to the fraction of the volume it removes, and the effect is smaller than people expect. A 250 gallon cleaning on a 20,000 gallon pool is 1.25 percent of the water, so it takes 1.25 percent of the dissolved material with it and replaces that volume with fill water. Four of those in a season is five percent of the pool. That slows a rise; it does not usually reverse one unless the fill water is much cleaner than the pool. The convergence figure on this page shows where the balance between makeup and removal actually points.
What happens if my fill water is as concentrated as my pool?
Then makeup water does not dilute anything and evaporation still concentrates, so the reading rises regardless of how much you replace. This is the case that catches people out when they try to fix a high reading by draining and refilling: if the supply carries the measure at or above the target, no amount of replacement gets there. The water replacement calculator on this site checks that condition explicitly before doing any arithmetic, and the answer in that situation is a different water source or a different target rather than more draining.
Should splash-out go in the level drop or in its own field?
One or the other, never both. If you measured the level drop over a normal week with the pool in use, splash-out is already inside that number, and the field should stay at zero — though the arithmetic will then treat it as evaporation and slightly understate the removal. If you measured the drop over a week with nobody swimming, or from a bucket test, the drop is closer to pure evaporation and the splash field is where the rest goes. The distinction matters because evaporation leaves dissolved material behind and splash-out takes it away.