What a part per million weighs
Parts per million is a weight ratio: one pound of something in a million pounds of water. A US gallon of water at pool temperature weighs about 8.345 pounds, so 20,000 gallons weighs 166,908 pounds, and one ppm across that pool is 0.1669 pounds of active material. Move a reading 20 ppm and you need 3.338 pounds of the actual substance dissolved into the water.
The metric version is friendlier and gives the same answer. A litre of water weighs a kilogram, and ppm is milligrams per litre, so 75,708 litres — the same pool — needs 75,708 × 20 = 1,514,160 mg, which is 1.514 kg. That is 3.338 pounds. The two routes agree because they are the same statement about a ratio.
The strength term, and why it divides
Almost nothing sold for a pool is the pure substance your test measures. The container holds a compound, a carrier, water, stabiliser, filler, or all of those, and the label states what share of the weight is the part that counts. If a product is 65 percent of the thing your test measures, then 3.338 pounds of that thing arrives inside 5.14 pounds of product, because 3.338 divided by 0.65 is 5.14.
Multiplying by 0.65 instead of dividing gives 2.17 pounds. That is not a small error and it does not look like one: 2.17 is a plausible-sounding number and there is nothing on the page to say it is wrong. Between the two answers there is a factor of 2.37, and the direction is the dangerous way round for anything where more is worse. The check is a sentence rather than a formula — the quantity of product is always larger than the quantity of active material inside it, so if the answer went down when you applied the strength, the division went the wrong way.
| Gap | Active material | At 65% strength | At 100% | At 12.5% |
|---|---|---|---|---|
| 5 ppm | 0.83 lb | 1.28 lb | 0.83 lb | 6.68 lb |
| 10 ppm | 1.67 lb | 2.57 lb | 1.67 lb | 13.35 lb |
| 20 ppm | 3.34 lb | 5.14 lb | 3.34 lb | 26.71 lb |
| 50 ppm | 8.35 lb | 12.84 lb | 8.35 lb | 66.76 lb |
Those figures are for 20,000 gallons and they scale straight with volume. A 12,000 gallon pool takes 0.6 of every number in the table; a 40,000 gallon pool takes double.
Liquids are sold by volume and dosed by weight
A liquid product is bought in gallons and measured out in a jug, but the strength on the label is a weight percentage, so the arithmetic needs the density to cross between the two. At 9.8 pounds per gallon, 5.14 pounds of product is 0.524 gallons, which is 67 fluid ounces. Get the density wrong by 10 percent and the poured volume is wrong by 10 percent, which is usually smaller than the error people make estimating the pool volume in the first place.
Where the reading is above the target instead
If the current reading is higher than the target, there is no dose. The page says so rather than printing a negative quantity, because a negative pound of product is not something that can be weighed out and a page that prints one is inviting somebody to read the minus sign as noise. For a dissolved measure that nothing consumes, the lever is water: replace some of the pool with water carrying less of it. The fraction is 1 minus target over current, not the difference over the current reading — going from 100 to 40 means replacing 60 percent of the volume, not 60 ppm of it, and if the fill water itself carries 10 of the measure the required fraction climbs to 67 percent. That is the water replacement calculator, which handles the fill water term and the staged partial exchanges.
Two errors this page cannot catch
The volume is the first. Every number here is directly proportional to it, and pool volumes are routinely quoted from memory, from the builder, or from a rectangle that ignores a deep end. A metered fill is the honest measurement; the pool volume calculator is the honest estimate.
The second is that water is not an inert bucket. Some of what goes into a pool is consumed by what is already dissolved in it before any of it shows on a test, so the reading after a dose can land short of the arithmetic without anything being wrong with the arithmetic. That is why the sequence is add, circulate for at least a turnover, then test — and why a page like this one produces a starting quantity rather than an answer.
Questions people ask
How many pounds does it take to raise a pool reading by 10 ppm?
It depends on the volume and on the product strength, and both have to come from you. The water part is fixed arithmetic: 20,000 gallons weighs 166,908 pounds, so 10 ppm is 1.67 pounds of active material. The product part is not fixed — 1.67 pounds of active inside a product that is 65 percent active means 2.57 pounds on the scale, and inside one that is 12.5 percent it means 13.35 pounds. There is no single answer to the question without the label in front of you, which is why the strength is a field rather than a constant.
Why does the calculator divide by the product strength instead of multiplying?
Because the strength describes what fraction of the container is the substance your test measures. You need a fixed amount of that substance in the water, and it arrives diluted inside the product, so you need more product than substance. Dividing 3.34 pounds by 0.65 gives 5.14 pounds of product; multiplying gives 2.17. Both look like reasonable numbers on a page, and the multiplication is wrong by a factor of 2.37 at that strength. The sanity check that always works: the product quantity has to be bigger than the active quantity inside it.
What target should I use?
Not one this page gives you, because it does not have any. The target belongs to the instructions that came with your test kit or your testing programme, and for a commercial or public pool it belongs to the health authority with jurisdiction, whose requirements differ between states, counties and cities and change over time. Targets also interact — a figure that suits one pool can be wrong for the same pool after a chemistry change. Read your kit instructions, and where the pool is not a private residential one, read the rules that apply to it.
The reading went up less than the calculator said. What happened?
Several ordinary things. Some of what went in reacted with what was already dissolved before any of it could register on the test, which is normal and pool-specific. The pool volume you entered may be optimistic, which makes every quantity light in exact proportion. The product may be older or wetter than the label strength assumes. Or the test was taken before the water had circulated a full turnover, in which case it measured the water near the return rather than the pool. Retest after a turnover before drawing conclusions, and treat the arithmetic as a starting point that your own records will correct over a season.
Can I work out a dose for two products at once?
Do them separately, and do not put them anywhere near each other. Combining pool chemicals is a named lethal hazard — chlorine products in particular have killed people when mixed, including in dry form in a bucket or a feeder — and this site gives no procedure for handling, storing or adding anything. Run the arithmetic once per product, and follow the product label and its safety data sheet for everything that happens after the number.
Does this work in litres and kilograms?
Yes, the units selector switches the whole page. The metric route is arithmetically simpler because a litre of water weighs a kilogram, so ppm is just milligrams per litre: 75,708 litres at 20 ppm is 1.514 kg of active material. That is the same 3.338 pounds the US route gives for the same 20,000 gallon pool, which is a useful check on both. Liquid density switches to kilograms per litre when you change the selector.