Pool Chemistry: Chlorine, pH and Alkalinity

This page starts from the pool: give it dimensions and current readings and it sizes doses for chlorine, pH and total alkalinity together. If you already know your volume and just want to know how much of the specific tub in your shed to use, the pool chemical dose by product strength page works from the percentage of active chlorine printed on the label. Both pages run the same arithmetic, so a dose worked out on one matches the other.

Swimming pool chemical dosing requires precise calculation based on your pool’s volume. Too little chemical and algae and bacteria thrive. Too much and you’ll irritate swimmers’ eyes and skin, damage equipment, and waste money.

Formula: Pool Volume (gallons) = Length × Width × Average Depth × 7.48 For circular pools: Volume = π × r² × Depth × 7.48 Chemical Amount = (Target Level − Current Level) × Pool Volume × Dose Factor

Target water chemistry ranges:

The three dose factors this calculator uses, all per 10,000 gallons:

The chlorine figure is not a lookup, it is a division. One ppm of available chlorine in 10,000 gallons is 1.34 oz of the pure stuff, and a tub of dichlor is only 56% available chlorine by weight, so you need 1.34 ÷ 0.56 = 2.4 oz of it. Read the strength off whatever you actually own and the same division works. The alkalinity figure comes out the same way: a pound of baking soda in 10,000 gallons raises alkalinity about 7 ppm, so ten ppm needs a pound and a bit.

pH is the one that resists a single number. How much acid it takes to move pH by 0.4 depends heavily on total alkalinity, because alkalinity is literally the water’s resistance to pH change. The 26 fl oz above assumes alkalinity near 100 ppm. At 150 ppm it will take noticeably more, at 60 ppm noticeably less, so treat the acid answer as a first dose rather than the whole job.

Worked example. Rectangular pool: 20 ft × 40 ft × average depth 5 ft. Volume = 20 × 40 × 5 × 7.48 = 29,920 gallons, so every figure below scales by 2.992.

Current chlorine 0.5 ppm, target 2.0 ppm, a rise of 1.5 ppm: 2.4 oz × 2.992 × 1.5 = 10.8 oz of dichlor (305 g).

pH reads 7.8, target 7.4, a drop of 0.4: 65 fl oz per pH unit × 2.992 × 0.4 = 77.8 fl oz of muriatic acid, about 0.61 gallons. In dry acid that same job is roughly 3.7 lbs.

Alkalinity reads 70 ppm, target 100 ppm, a rise of 30 ppm: 0.14 lbs × 2.992 × 30 = 12.6 lbs of baking soda (5.7 kg). That is most of a 15 lb box, which surprises people the first time.

Do alkalinity before pH. Raising alkalinity pushes pH up as a side effect, so if you correct pH first you will just have to do it again. Get alkalinity into the 80 to 120 band, let it circulate, then look at pH.

Important: Always add chemicals to water, never water to chemicals, and add them with the pump running. Test again 4 to 6 hours later, not straight away. The one that catches people out is acid: never pour it anywhere near chlorine, wet or dry, and never into a bucket that had chlorine in it earlier. Acid on hypochlorite releases chlorine gas, and a poolside is exactly the sort of place where somebody does this without thinking. Space the two doses hours apart, at opposite ends of the pool.