How to balance pool water

The order: alkalinity, pH, calcium, stabilizer, chlorine

1. Total alkalinity (TA) first. Alkalinity is the buffer: it decides how far a given dose of acid or base moves pH. Set pH first and then fix alkalinity, and the alkalinity fix moves pH again (baking soda nudges it up; acid to lower TA drags it down). With TA settled, every pH dose that follows is predictable.
2. pH second. Once the buffer is right, bring pH into range. pH matters most for comfort and for chlorine: the CDC notes that above pH 8 chlorine loses much of its effectiveness, and recommends 7.0–7.8.
3. Calcium hardness (CH) third. Calcium barely moves pH, and it changes slowly — only by adding calcium or replacing water. It belongs before the saturation check because it is one of the four numbers in it.
4. Stabilizer (CYA) fourth. Granular stabilizer can take days to dissolve, and the CYA level sets how much chlorine the pool needs, so it comes before the chlorine target. Test kits also count part of the CYA as alkalinity (the MAHC annex corrects TA by subtracting CYA ÷ 3), so adding stabilizer nudges the TA reading up — one more reason to retest.
5. Free chlorine (FC) last. The right chlorine level depends on the CYA you just set, and chlorine is used up every day, so it is the number you keep adjusting. It also keeps acid and chlorine well apart: never add them together.

That order is about efficiency, not danger. When the water isn’t safe — chlorine at zero, or the pool turning green — sanitizing comes first: bring a high pH down under about 7.6, add chlorine, and leave alkalinity, calcium and stabilizer until the water is clear.

If the pool is green

A green pool uses the same order as the shock guide, step for step:

1. Keep everyone out and test free chlorine, pH and stabilizer (CYA). Most strips have a CYA pad; a pool store will test it free.
2. Very high CYA? Drain first. Above about 80 ppm the shock level gets impractical, so partly drain and refill before buying chlorine — see how much water to drain to lower CYA.
3. Clear the debris — skim leaves, empty the baskets — and start the pump. Leave it running until the pool is clear.
4. If pH is above about 7.6, lower it first to about 7.4 with acid, run the pump a few hours and retest: chlorine works better at lower pH. Leave alkalinity, calcium and stabilizer for later. Never add acid and chlorine together.
5. At dusk, raise chlorine to the shock (SLAM) level — 40% of your CYA — usually with liquid chlorine. How much liquid chlorine to SLAM a pool works the first dose.
6. Hold it there. Test two or three times a day and top up what was used. Strips can’t read this high — use a FAS-DPD drop test kit (a drop-count chlorine test) or a pool store’s free water test. Brush daily and clean or backwash the filter as its pressure rises.
7. Stop when it is clear and chlorine drops no more than about 1 ppm overnight (the Trouble Free Pool method’s rough guide). Let chlorine fall back to your label’s swimming level, then balance alkalinity, calcium and stabilizer in the usual order.

The saturation index: is the water balanced?

pH, alkalinity, calcium and temperature together decide whether water tends to dissolve calcium (from plaster, grout and tile) or deposit it as scale. The saturation index rolls them into one number. Taylor Technologies writes it as:

SI = pH + temperature factor + log(calcium hardness) + log(carbonate alkalinity) − constant
carbonate alkalinity ≈ total alkalinity − CYA ÷ 3

Zero is perfectly balanced; negative water is corrosive and positive water is scale-forming. Taylor says water within ±0.5 of zero needs no adjustment, and treats −0.3 to +0.5 as balanced on its 6026 Watergram. The MAHC asks public pools to check it monthly. The saturation index calculator does the arithmetic from your test results — run it after the first three steps, before you add calcium you might not need.

Because pH is in the formula directly, a pool can be “balanced” with low calcium and a higher pH, or high calcium and a lower pH. That is why there is no single correct calcium number: the index is the target, and the readings are the levers.

One change at a time, then retest

Every adjustment works on the whole pool, and test kits read alkalinity and hardness in steps of about 10 ppm. Adding a full dose and adjusting three things at once is how pools overshoot. The habit that saves money:

• Add part of the dose, in front of a return with the pump running. Labels cap single additions: one muriatic acid label says no more than 16 fl oz per 10,000 gallons at a time, one soda ash label 1 lb per 10,000 gallons. In a 15,000-gallon pool that is 24 fl oz of acid or 1.5 lb of soda ash per addition.
• Let it circulate — a few hours for pH and alkalinity, longer for calcium, days for granular stabilizer.
• Retest, then decide on the rest. A dose calculator is an estimate; the test is the truth.
• Keep a log. Write down the reading, what you added and the next reading. After a season you will know how your pool responds better than any calculator.

A worked example: 15,000 gallons

Say a 15,000-gallon plaster pool at 80°F tests at free chlorine 0.5 ppm, pH 7.2, alkalinity 60 ppm, calcium 150 ppm and stabilizer 20 ppm. Everything is legal; most of it is on the low side. The targets for this example are alkalinity 90, pH 7.5, calcium 250, CYA 40 and chlorine 4 ppm.

Step 1, alkalinity. Each 10 ppm of alkalinity in 15,000 gallons takes about 2.1 lb of baking soda (sodium bicarbonate), so 30 ppm is about 6.3 lb. Add half, circulate, retest. Baking soda raises pH only slightly — here from 7.2 to about 7.35. The 10,000-gallon version is worked on how much baking soda to raise alkalinity in a 10,000 gallon pool.

Step 2, pH. With alkalinity at 90, taking pH from about 7.35 to 7.5 needs roughly 9.9 oz of soda ash (sodium carbonate). It adds a few ppm of alkalinity too, which is why it comes after the TA step rather than before. See how much soda ash to raise pH in a 10,000 gallon pool. If your pH had come in high instead, the tool is acid: how much muriatic acid to lower pH in a 10,000 gallon pool.

Step 3, calcium. A 100 ppm rise in 15,000 gallons is about 16.3 lb of 83–87% calcium chloride, the exact case on how much calcium chloride to add to a pool. It gets hot as it dissolves: add it slowly to a bucket of pool water, never water onto the flakes.

Step 4, stabilizer. Cyanuric acid is dosed one-to-one by weight: 1 ppm is 1 gram per cubic metre. Twenty ppm in 15,000 gallons is about 2.5 lb of granular stabilizer — compare how much stabilizer for a 10,000 gallon pool. Let it dissolve fully (often days), don’t backwash meanwhile, and retest CYA after.

Step 5, chlorine. At 40 ppm CYA, the Trouble Free Pool convention puts the minimum at about 3 ppm free chlorine; the CDC minimum with stabilizer is 2 ppm. Raising 15,000 gallons from 0.5 to 4 ppm takes about 53.8 fl oz (1.7 qt) of 12.5% liquid chlorine. The single-ppm rule of thumb is on how much liquid chlorine to raise 1 ppm in 10,000 gallons.

Finally, run the new readings through the saturation index calculator. If it lands between about −0.3 and +0.5, leave it alone.

When the numbers fight each other

• High alkalinity and high pH. Acid lowers both. Add acid in parts, keep pH above 7.0, and aerate between doses (point returns up, run a fountain or spa spillover): aeration raises pH without raising alkalinity, so each round takes TA down a little more. How much muriatic acid to lower alkalinity in a 10,000 gallon pool shows the amounts.
• pH keeps climbing. Often alkalinity is on the high side, or the water is aerated by waterfalls or a salt cell. Lowering TA a little usually slows the drift.
• Stabilizer or calcium too high. Neither comes out with chemicals. The only fix is replacing water with a partial drain and refill — the drain and refill calculator works out how much.
• Chlorine won’t hold. Check CYA (too low and the sun burns it off; too high and you need much more of it), then look for algae or a heavy debris load.

For target levels in one table, see the pool chemistry levels chart; for doses per 10,000 gallons, the pool chemical dose chart.