Pool terms, from test kit to pump
- Free chlorine (FC)
- The chlorine still available to sanitize, in ppm. How much you need depends on the stabilizer level: at least 7.5 % of CYA, so 3 ppm at 40 ppm CYA. Calculator
- Combined chlorine (CC)
- Chlorine that has already reacted with sweat, urine and other waste (chloramines). It causes the "chlorine smell"; CC = total chlorine − free chlorine. Aim for 0. Calculator
- Total chlorine (TC)
- Free plus combined chlorine. A test kit reads FC and TC; the difference is CC. Calculator
- Shock / SLAM
- Raising free chlorine to a high level, about 40 % of the CYA level, and holding it until algae and combined chlorine are gone. SLAM stands for Shock Level And Maintain. Calculator
- pH
- How acidic or basic the water is, on a 0-14 scale. Pools aim for 7.4 to 7.6; low pH etches and stings, high pH weakens chlorine and forms scale. Muriatic acid lowers it, soda ash raises it. Calculator
- Total alkalinity (TA)
- The water's buffer against pH change, in ppm, mostly bicarbonate. 60 to 90 ppm is common; baking soda raises it, muriatic acid lowers it. Calculator
- Calcium hardness (CH)
- Dissolved calcium in ppm. Plaster, pebble and tile pools need enough of it, often 200 to 400 ppm, or the water dissolves calcium from the surface. Calcium chloride raises it. Calculator
- Cyanuric acid (CYA), stabilizer
- Protects chlorine from sunlight. Chlorine pools commonly run 30 to 50 ppm, salt pools 60 to 80. It leaves the pool mostly with drained or splashed-out water, so add it in small steps. Calculator
- Borates
- Boric acid or borax added at 30 to 50 ppm as an extra pH buffer. Optional. Calculator
- Langelier Saturation Index (LSI)
- One number from pH, temperature, calcium, alkalinity and dissolved solids that says whether water will dissolve (below −0.3) or deposit scale (above +0.3). Calculator
- ppm (parts per million)
- The unit of every pool test: 1 ppm is 1 part in a million by weight, the same as 1 mg per litre of water. Calculator
- Salt chlorine generator (SWG, salt cell)
- A cell that turns dissolved salt into chlorine as water flows through it. It needs the salt level printed on it, usually 2,700 to 3,400 ppm. Calculator
- Turnover
- The time the pump takes to move the pool's whole volume through the filter once: gallons ÷ gpm ÷ 60, in hours. 6 to 8 hours is a common design target. Calculator
- gpm (gallons per minute)
- Flow rate through the pump, filter and pipes. Pump size, filter area and pipe speed are all worked out from it. Calculator
- Filter area (sq ft)
- The surface of sand, cartridge or DE that water passes through. Each filter type has a maximum flow per square foot, which sets the size you need. Calculator
- BTU
- British thermal unit: the heat that warms one pound of water by 1 °F. A gallon weighs 8.34 lb, so 1 °F in 20,000 gallons takes 166,800 BTU. Heaters are rated in BTU per hour. Calculator
- COP (heat pump)
- Coefficient of performance: heat delivered per unit of electricity. A COP of 5 means 5 kWh of heat for 1 kWh used; it drops as the air gets colder. Calculator
- Average depth
- (Shallow end + deep end) ÷ 2. Used for volume, and for the wall area in resurfacing and liner prices. Calculator
The target for each test is in the pool chemistry levels chart, and the amount to add for one step in the pool chemical dosing chart.
Frequently asked questions
What do FC, CC and TC mean on a pool test?
FC is free chlorine, the chlorine still able to sanitize. TC is total chlorine. CC, combined chlorine, is the difference: chlorine already used up on waste, which causes the chlorine smell. A good test kit reads FC and TC; aim for CC of 0.
What is the difference between pH and alkalinity?
pH is how acidic the water is right now, on a 0-14 scale; pools aim for 7.4 to 7.6. Total alkalinity, in ppm, is the buffer that resists pH change. Low TA lets pH swing, high TA pushes pH up and makes it take more acid to bring down.
What does CYA mean in a pool?
Cyanuric acid, sold as stabilizer or conditioner. It shields chlorine from sunlight, which would otherwise burn most of it off in a few hours. Chlorine pools commonly run 30 to 50 ppm, salt pools 60 to 80. More CYA means you need more free chlorine.
What does SLAM mean for a pool?
Shock Level And Maintain: raise free chlorine to about 40 % of the CYA level, 16 ppm at 40 ppm CYA, and hold it there with repeated doses until the water is clear and combined chlorine is gone. It is the method for clearing algae.
What does ppm mean in pool chemistry?
Parts per million by weight. One ppm is one pound of a chemical in a million pounds of water, the same as 1 mg per litre. 10,000 gallons of water weigh about 83,400 lb, so 1 ppm in 10,000 gallons is about 0.083 lb, or 1.3 oz.
Definitions follow the formulas and target levels used in the calculators on this site; level sources are cited on the chemistry chart. Read October 23, 2026.