Pool and hot tub glossary

The terms on your test kit and equipment labels, explained in a sentence or two, with links to the tool that uses each one.

Baking soda: Also called: sodium bicarbonate, alkalinity increaser; Raises total alkalinity with only a small effect on pH. The same chemical as kitchen baking soda.; Alkalinity calculator
British thermal unit(BTU): The heat needed to raise one pound of water by 1 °F. Pool heaters are rated in BTU per hour (BTU/h).; Heat pump sizing calculator
Calcium hardness(CH): The amount of dissolved calcium in the water, in ppm as calcium carbonate. Plaster and concrete pools need enough to avoid etching (often 250–450 ppm); vinyl and fiberglass pools can run lower. Too much leads to scale.; Pool Water Chemistry for Beginners: What Each Test Number Means
Coefficient of performance(COP): A heat pump's efficiency: heat delivered divided by electricity used. A COP of 5 means 5 units of heat for every 1 unit of electricity. COP drops as the outdoor air gets colder.; Heat pump sizing calculator
Combined chlorine(CC): Also called: chloramines; Chlorine that has already reacted with ammonia and nitrogen compounds from sweat, urine and debris, forming chloramines. It sanitizes poorly and causes the "chlorine smell." Calculated as total chlorine minus free chlorine; ideally 0.5 ppm or less.; Pool shock calculator
Cyanuric acid(CYA): Also called: stabilizer, conditioner; A chemical that shields chlorine from being destroyed by sunlight. It also binds part of the chlorine, so the more CYA you have, the more free chlorine you need for the same germ-killing power. Outdoor pools typically run 30–50 ppm; salt pools often a bit higher.; CYA (stabilizer) calculator Chlorine calculator Cyanuric Acid (Pool Stabilizer): How Much You Need and Why It Matters
Free chlorine(FC): Also called: free available chlorine, FAC; The chlorine in your water that is still available to kill germs and oxidize contaminants (hypochlorous acid plus hypochlorite ion). This is the chlorine number that matters for sanitation. CDC recommends at least 1 ppm in pools and 3 ppm in hot tubs.; Chlorine calculator Pool shock calculator
Gallons per minute(GPM): The flow rate through your pump and filter. Best measured with a flow meter; pump curves give an estimate from your plumbing's resistance.; Pump run time calculator
Langelier Saturation Index(LSI): Also called: CSI, calcium saturation index; A number that predicts whether water will dissolve calcium (negative, which can etch plaster) or deposit it as scale (positive). It combines pH, temperature, calcium hardness, alkalinity and dissolved solids. Near zero is balanced.; Pool Water Chemistry for Beginners: What Each Test Number Means
Muriatic acid: Hydrochloric acid sold for pools, usually at 31.45% (full strength) or about 14.5% (half strength). Lowers both pH and total alkalinity. Always add acid to water, never the reverse.; pH calculator Alkalinity calculator How to Lower (or Raise) Pool pH Safely
Parts per million(ppm): The unit most pool tests use. In water, 1 ppm is 1 milligram of a substance per liter, or about 1 ounce in 7,500 gallons.; Pool Water Chemistry for Beginners: What Each Test Number Means
pH: How acidic or basic the water is, on a 0–14 scale. CDC recommends 7.2–7.8 for pools and hot tubs. Chlorine works better at the lower end of that range; above 8.0 it loses much of its strength, and high pH favors scale.; pH calculator How to Lower (or Raise) Pool pH Safely
Salt chlorine generator(SWG): Also called: salt cell, saltwater chlorinator; Equipment that makes chlorine from dissolved salt using electricity. A "saltwater pool" is still a chlorine pool; the cell just produces the chlorine for you. Salt is typically kept around 2,700–3,400 ppm; your manual gives the exact range.; Salt calculator
Shocking: Also called: superchlorination, breakpoint chlorination; Raising free chlorine well above normal to destroy algae, combined chlorine or organic contamination, then holding it there until the problem clears. The right shock level depends on your CYA.; Pool shock calculator How to Shock a Pool (and How Much Shock You Actually Need)
Soda ash: Also called: sodium carbonate, pH up, pH increaser; A base that raises pH strongly and total alkalinity slightly.; pH calculator
Standby power (hot tubs): The average electricity a covered hot tub uses just to stay hot and filtered when nobody is in it. California requires new portable spas to be tested and labeled with this number.; Hot tub electricity cost
Total alkalinity(TA): The water's ability to resist pH changes (its buffer), reported in ppm as calcium carbonate. Too low and pH swings; too high and pH keeps drifting up. Many pools do well between 60 and 120 ppm depending on the sanitizer and surface.; Alkalinity calculator pH calculator How to Lower (or Raise) Pool pH Safely
Total chlorine(TC): Free chlorine plus combined chlorine. Test kits that read only "total" can't tell you whether the chlorine is still working.
Turnover: The time it takes the pump to move a volume of water equal to the whole pool through the filter. A 20,000-gallon pool with 40 gallons per minute of flow has a turnover of about 8.3 hours.; Pump run time calculator
Variable-speed pump: Also called: VS pump; A pool pump whose motor speed can be set. Because power drops roughly with the cube of speed, running slower for longer moves the same water for much less electricity.; Pump energy cost calculator
Winterizing: Also called: closing a pool; Preparing a pool for the off-season: balancing and chlorinating the water, cleaning, lowering the water level as needed, protecting plumbing and equipment from freezing, and covering.; Pool closing planner How to Close an Inground Pool for Winter