The arithmetic, and why it needs your numbers
A product used 1.5 units at a time, three times a week, across a 22 week season, is 1.5 × 3 × 22 = 99 units. At a 25 unit container that is 3.96 containers, so four to buy, with 1 unit left on the shelf. At 89.95 a container the product actually used costs 99 divided by 25 times 89.95, which is 356.20, and the four containers cost 359.80.
Every one of those inputs is yours. How much you use is a fact about your pool that only your own records hold, and it is not the same as what a label suggests, because a label cannot see your volume, your bather load or your sun. Prices are yours because they differ by region, by supplier and by month. Container sizes are yours because they differ by brand. There is nothing left for a calculator to assume, which is the point.
Where the money usually is
Add up four products at plausible frequencies and the ranking surprises people. With the defaults on the form, the sanitiser at three applications a week is 356 dollars across the season and the stabiliser at two applications a season is 35. The alkalinity adjuster, at four units every fourth week, is 35. The pH adjuster, cheap per container but used weekly, is 81.
| Product | Per application | Per week | Season quantity | Cost per unit | Season cost |
|---|---|---|---|---|---|
| Sanitiser | 1.5 | 3 | 99 | $3.60 | $356.20 |
| pH adjuster | 2 | 1 | 44 | $1.84 | $80.96 |
| Alkalinity adjuster | 4 | 0.25 | 22 | $1.60 | $35.20 |
| Stabiliser | 4 | 0.1 | 8.8 | $3.94 | $34.65 |
That is 507 dollars, of which the sanitiser is 70 percent. Frequency is doing more work in that table than price is. The stabiliser has the highest cost per unit of anything on the list and it is the smallest line on the page, because it goes in twice a summer. Anybody trying to reduce a chemical bill by shopping for a cheaper stabiliser is optimising the wrong end.
Used against bought
The page reports two totals and they are not the same number. The cost of product used is the season quantity times the price per unit, and it is the honest running cost. The cost of whole containers is what you actually hand over, and it is higher because containers do not divide evenly into seasons.
The difference — 3.60 on the sanitiser in the example, and more on products used in small amounts — is stock rather than waste. It carries over, provided it is stored the way the label says. Products with a shelf life are the exception, and how long a given product keeps is on its label, not here.
Per 10,000 gallons
The only figure on the page that compares meaningfully against somebody else is the season cost divided by the volume in units of 10,000 gallons. A 507 dollar season on a 20,000 gallon pool is 253 dollars per 10,000 gallons. The same water chemistry on a 40,000 gallon pool costs roughly twice as much in absolute terms and the same per 10,000, which is why the normalised figure is the one worth writing down.
It is still not a benchmark. Two pools of identical size in identical climates can differ by a factor of two on chemical spend because one is shaded and one is not, or because one has a cover and one does not, or because one is swum in every day. Compare your own pool against itself, season over season, and the figure earns its keep.
What this leaves out
Everything that is not a bottle. Pump electricity across a season is frequently larger than the chemical bill and is worked out on the pump run time calculator. Heating, if there is a heater, dwarfs both. Water for filter cleanings and for evaporation makeup has a real cost that arrives on a different bill entirely. Cells, cartridges, and the labour of somebody who is not you all belong in a running cost and none of them are here.
Questions people ask
How much do pool chemicals cost for a season?
Only your own numbers answer that, and the honest reason is that usage varies by more than price does. Two 20,000 gallon pools in the same town can differ two to one because one sits in full sun and one is shaded, or because one is swum in daily. Work it from your records: quantity per application, applications per week, weeks in the season, container size and your invoice price, per product. The figure that transfers between pools is the season cost per 10,000 gallons, and even that is a comparison rather than a target.
Which pool chemical costs the most over a season?
Almost always whichever one goes in most often, not whichever one has the highest price per container. Frequency multiplies and price does not. A sanitiser at three applications a week can be 70 percent of the bill while a stabiliser with the highest unit cost on the list is under 10 percent, because it goes in twice a summer. The share column on this page is the useful one for that reason: it shows where the money actually is rather than where the expensive labels are.
Why does the calculator show two different totals?
One is the product you use, one is the containers you buy. Ninety-nine units out of 25 unit containers means four containers, and the fourth is only 40 percent used at the end of the season. The product-used figure is the honest running cost of operating the pool; the container figure is the cash that leaves your account. The difference is stock that carries into next year if the label allows it to be stored, so it is a timing difference rather than a loss.
Should I buy in bulk to save money?
The page gives you the two numbers to decide with — cost per unit at each container size, and how much would be left over — but the decision has terms it cannot see. Shelf life is one, and it belongs to the product label. Storage is another, and pool chemicals have storage requirements that are on the label and its safety data sheet for reasons that matter. Combining chemicals, including two containers of similar products, is a named lethal hazard and nothing here describes any handling procedure. Read the label first and let the arithmetic be the second consideration.
How do I work out how much I use per application?
Measure it rather than reading it off a label. Weigh or measure what actually goes in, write it down with the date, and after a month you have a figure that describes your pool instead of a generic one. If you are calculating a quantity from a reading and a target instead of pouring a habitual scoop, the dose calculator on this site does that arithmetic from your volume, your gap and the strength on your label, and the demand rate calculator gives you the per day consumption to base a schedule on. Both of them start from measurements you take.