Pool Chemical Calculator: Doses in the Right Order

Your pool

Pool volumegal

Pool surface

Masonry (plaster, concrete, tile)Vinyl or fiberglass

Readings: now and target

Free chlorine now

ppm

Free chlorine targetppm

pH now

Total alkalinity nowppm

Total alkalinity targetppm

Cyanuric acid nowppm

Cyanuric acid targetppm

Calcium hardness nowppm

Calcium hardness targetppm

Products

Chlorine productLiquid chlorine (sodium hypochlorite)Calcium hypochlorite (cal hypo)Dichlor (sodium dichloroisocyanurate)Trichlor (trichloroisocyanuric acid)

To raise alkalinity, useBaking soda (sodium bicarbonate)Soda ash (sodium carbonate)

To lower alkalinity, useMuriatic acid (hydrochloric acid)Dry acid (sodium bisulfate)

Label strengths

Dichlor (sodium dichloroisocyanurate): % available chlorine on your label%

Baking soda (sodium bicarbonate): % sodium bicarbonate on your label%

Muriatic acid (hydrochloric acid): % hydrogen chloride (HCl) on your label%

Muriatic acid: specific gravity from your SDS (1.16 is for 31.45% only)

The specific gravity above is from my acid's SDS

Cyanuric acid (stabilizer): % cyanuric acid on your label%

Calcium chloride: % calcium chloride on your label%

Runs in your browser; nothing is sent to us. Copy link puts your numbers in the address.

This is the hub of the site: one form for every reading and target of a single pool, and one dosing list out of it, in the order an American health department publishes. Free chlorine comes first, then total alkalinity, pH, cyanuric acid and calcium hardness. What each chlorine product carries into the water is counted before the later steps are sized, so the stabilizer and hardness doses are the ones left after step 1, not the ones you would get from the readings alone. Each product has its own % on your label field, and only the field for the product you picked is shown, so switching from liquid chlorine to cal hypo can never carry one label's percentage over to another. Every dose is worked out from the pool's volume in gallons, and the pool volume calculator can work that out from your measurements.

Worked example: stabilizer after dichlor

The readings below are the ones the calculator starts with. A 15,000-gallon masonry pool reads 1 ppm free chlorine, pH 7.6, 70 ppm total alkalinity, 20 ppm cyanuric acid and 180 ppm calcium hardness. The targets are 3, 90, 40 and 250 ppm, and the chlorine product is dichlor.

• Step 1, free chlorine 1 to 3. 7.2 oz of dichlor, for +2.0 ppm. That same dose carries 1.8 ppm of cyanuric acid in with it, and the step says so.
• Before swimming. Free chlorine lands at 3 ppm, and dichlor's own label draws its line there, so the list adds a row about waiting. It gives the short form: wait until free chlorine falls below 3 ppm, from the stricter of the two panels on the EPA Reg. No. 7124-31 label, paraphrased because this site's scan of that label is too garbled to quote word for word. The shock calculator sets out both panels in full.
• Step 2, total alkalinity 70 to 90. 67.2 oz (4.20 lb) of baking soda.
• Step 3, pH. No dose. Test, then adjust in small doses and retest.
• Step 4, cyanuric acid 20 to 40. Step 1 has already carried 1.8 ppm in, so this step asks for +18.2 ppm rather than 20, and that takes 36.4 oz (2.28 lb).
• Step 5, calcium hardness 180 to 250. +70.0 ppm, which is 201.9 oz (12.62 lb) of calcium chloride at 77%, the assay on HASA's safety data sheet for it.

Two of the readings come back flagged under the steps. Free chlorine of 1.0 ppm is low against the 2 ppm CDC gives for a home pool with cyanuric acid in the water. Calcium hardness of 180 ppm is low against 200–400 ppm for masonry, the preferred band in the MAHC Annex, guidance for public pools rather than a rule for anyone's back garden.

Worked example: calcium after cal hypo

The same pool and the same readings, but the chlorine product is calcium hypochlorite and the free chlorine target is 5 ppm instead of 3.

• Step 1 becomes 11.4 oz of cal hypo for +4.0 ppm, and it adds 2.8 ppm or more of calcium hardness.
• Step 5 counts that calcium before it doses any: hardness starts from about 182.8 ppm rather than 180, so the step asks for +67.2 ppm, which is 193.7 oz (12.11 lb) of calcium chloride. That is 8.2 oz less than the dichlor run above, off a chlorine dose of 11.4 oz.
• Free chlorine lands at 5 ppm, above the cal hypo label's own re-entry figure, so a Before swimming row appears with that label's wording and its registration number.

Neither example is advice for your pool. They are the calculator's own output for a set of readings, printed here so you can check the arithmetic against yours.

The order of addition

The order is not this site's. The Indiana Department of Health's sheet on adjusting chemical levels tells operators to add chemicals in sequence to adjust for "(1) Free Available Chlorine, (2) total alkalinity, (3) pH, (4) cyanuric acid – outdoor pools, and (5) total hardness". The calculator prints that line under the steps, so what is on screen can always be checked against where it came from.

The same sheet carries a handling rule no calculator can do for you: "Add acids to water, NEVER water to acid." It also warns that "Chlorine compounds will significantly change the pH and affect the effectiveness of the chlorine.", which is one reason the list waits until step 3 to look at pH at all.

By-products the list counts

Two of the chlorine products bring something else into the water with them, and steps 4 and 5 are sized after it.

Dichlor carries cyanuric acid. PHTA's technical bulletin on it says "When dichlor is used 0.9 ppm of cyanuric acid is added", per ppm of available chlorine added. The calculator works the figure out from the molecular weights instead, which give 0.910 ppm per ppm, and prints the result on the step-1 row.

Trichlor carries cyanuric acid too, 0.607 ppm per ppm of free chlorine from the same molecular weights. It is one of the chlorine products in the list, and picking it puts its own label row under step 1.

Cal hypo carries calcium. PHTA's fact sheet for it says "0.8 ppm of calcium hardness is added to the water for each ppm of available chlorine added". The molecular weights give 0.706 instead, the smaller of the two. That is the figure the calculator carries into step 5, and it words the step-1 row as a minimum, so the hardness it reports is never more than the chemistry accounts for.

Liquid chlorine carries neither. That is why it is the product to pick when a stabilizer or hardness target is too low for a dose of anything else to land on.

Your readings against the ranges

Under the dosing steps the calculator repeats each reading you entered with a verdict on it: inside the primary range, or low or high against the one it misses, named with the range and who sets it.

CDC's page on home pools and hot tubs is the source for the chlorine and pH figures: "CDC recommends pH 7.0–7.8 and a chlorine concentration of at least 1 ppm in pools and at least 3 ppm in hot tubs." The same page raises the pool minimum to 2 ppm once there is cyanuric acid in the water, which is the figure the first example is flagged against.

The alkalinity band of 60 to 180 ppm, the 0.4 ppm ceiling on combined chlorine, the 2,500 ppm ceiling on calcium hardness and the 90 ppm ceiling on cyanuric acid are all MAHC 2024's, a public-pool code. They are here because they are the most specific published figures this site could check, not because a home pool is inspected against them.

The MAHC Annex, guidance for public pools, gives the band a surface prefers: "Calcium hardness of 200—400 ppm (mg/L) is preferred" for masonry, and 80 to 200 ppm for a vinyl or fiberglass pool. The Pool surface field picks which of the two your reading is judged against. The table below the tool prints every one of these ranges with its source and the words it rests on.

Lowering CYA or hardness

Nothing in the dosing list takes cyanuric acid or calcium hardness back out of the water. The Toledo-Lucas County Health Department's sheet gives the only method: "To LOWER Cyanuric Acid, Total Dissolved Solids (TDS), or Calcium Hardness: Drain a portion or all of the pool". So when one of those two readings is above its target, the calculator puts a single Drain first step ahead of the whole order, and this is how it decides:

• Only a reading as you entered it can ask for a drain. If step 1's by-product is what pushes cyanuric acid or hardness past the target, there is no drain: the step says instead that step 1's chlorine leaves the reading above the target, by how much.
• There is one drain, not one each. Its share is the larger of the two shares the high readings need, and the row names which of them set it.
• Each share counts step 1's by-product, so the reading lands on its target after step 1 rather than before it. Where no share can land it, because the by-product alone carries the reading past the target, the share falls back to 1 − target ÷ reading, which brings the reading itself to the target, and a warning says the target needs a chlorine that adds none of that.
• The refill water is assumed to hold none of anything. Fill water is rarely a blank, so test yours; if it carries stabilizer or hardness, the drain will not go as far as the row says.
• Draining lowers every reading by the same share, so the row prints the free chlorine, alkalinity, cyanuric acid and hardness to expect afterwards, and says pH will change too. Drain, refill and retest, then enter the new readings before dosing.
• A drain too small to print is not shown. If the share works out under half a gallon, or under 0.05% of the pool — either would print as 0 gal or 0.0% — there is no drain step at all. Where step 1 adds none of that reading, its own step then says how far above the target it sits and that this is too little to drain for; where step 1 does add some, the step falls back to the line in the first bullet, naming the chlorine and how far above the target it leaves the reading.

The warnings the list can add

Four more rows appear only when something about the plan is worth flagging, under the steps and the readings:

• Free chlorine target above 10 ppm. Section 6.6.3.1 of the MAHC, a public-pool code, puts free chlorine above 10 ppm among its imminent health hazards. The row points at the shock calculator for each product label's own limit for swimming.
• CYA target above 90 ppm. Section 5.7.3.1.3.2 of the same public-pool code caps cyanuric acid at 90 ppm, and the row quotes it.
• CYA above 90 ppm after step 1. A different case: the target itself is fine, but dichlor's or trichlor's by-product carries the reading past 90 anyway. The row gives what step 1 adds and the level it adds it to, which is the level left after a drain when there is one.
• Calcium hardness above the MAHC Annex's preferred range after step 1. Cal hypo's calcium takes hardness past the band for your surface. This row is worded as a note rather than a warning, because the Annex is guidance for public pools rather than a limit anyone is held to.

Where pH fits

pH is third in the order and gets no dose here. The steps around it are different in kind: a weight of baking soda moves alkalinity by a figure that follows from the chemistry, while the acid or base a pool needs to move pH depends on how much the alkalinity in it resists the change.

Two of the sheets this site saved do publish a rule of thumb for pH, and they disagree. The Toledo-Lucas County Health Department's sheet puts 6 oz of soda ash per 10,000 gallons against 0.2 pH units. Michigan State University Extension's manual says one pound of soda ash "per 10,000 gallons will raise the pH 0.3." — which is 10.7 oz for the same 0.2 units, about 1.8 times as much. Neither figure accounts for the alkalinity buffering the water or for cyanuric acid, and Lucas County's own line says its figure rests on a base demand test. So this list asks you to test, add a little, and test again.

The pH and alkalinity calculator sets those two rules of thumb side by side and covers the rest of that ground, including why alkalinity is the reading to fix first.