Pool words explained
Pool test kits, chemical labels and equipment manuals use a vocabulary of their own. If a word on a label or a calculator stops you, the meaning is probably below: one or two lines each, in plain English, grouped by topic, with a link to where the word matters.
Water chemistry
| Word | What it means |
|---|---|
| ppm (parts per million) | How much of something is dissolved in the water. 1 ppm is 1 milligram per litre, or about 1.3 oz of a chemical in 10,000 gallons. Every test result and dose on this site is in ppm. |
| pH | How acidic or basic the water is, on a 0–14 scale with 7 as neutral. The CDC range for pools and hot tubs is 7.0–7.8; above 8, chlorine works much less well. |
| Total alkalinity (TA) | The water’s buffer against pH swings, mostly bicarbonate, reported in ppm. Higher TA holds pH steadier, but pH tends to drift up when TA is high. |
| Carbonate alkalinity | TA with the stabilizer’s share taken out. The MAHC annex corrects for it by subtracting CYA ÷ 3. Used in the saturation index. |
| Calcium hardness (CH) | Dissolved calcium, reported as ppm of calcium carbonate. Too little and water can etch plaster; too much and it can scale. |
| Cyanuric acid (CYA), stabilizer, conditioner | Three names for the same thing: a chemical that shields chlorine from sunlight. It also slows chlorine down, so more CYA needs more chlorine. None in hot tubs, says the CDC. |
| Saturation index (LSI, CSI) | One number that says whether water will dissolve calcium (negative, corrosive) or deposit it (positive, scaling). Built from pH, temperature, calcium, alkalinity and dissolved solids. |
| TDS (total dissolved solids) | Everything dissolved in the water added up. Salt pools have high TDS by design; it nudges the saturation index slightly. |
| Balanced water | Water whose saturation index is near zero — neither eating the pool surface nor leaving scale on it. |
| Scale | Chalky white calcium deposits on tile, in heaters and in salt cells, from water with a high saturation index. |
| Muriatic acid | Hydrochloric acid, usually 31.45%, used to lower pH and alkalinity. Lower-fume versions are around 14.5%. |
| Dry acid | Sodium bisulfate granules, a solid alternative to muriatic acid. |
| Soda ash | Sodium carbonate. Raises pH strongly and alkalinity a little. |
| Baking soda | Sodium bicarbonate. Raises alkalinity with only a small change in pH. |
| Calcium chloride | Raises calcium hardness. Sold as 94–97% pellets or 83–87% flakes; heats up as it dissolves. |
Three of these have worked examples that show the numbers in practice: how much muriatic acid to lower pH in 10,000 gallons, how much baking soda to raise alkalinity and how much stabilizer for a 10,000 gallon pool. The saturation index has its own LSI calculator, and every target range is in the pool chemistry levels chart.
Chlorine and sanitizers
| Word | What it means |
|---|---|
| Free chlorine (FC) | Chlorine that is still available to sanitize. This is the number you dose to and the one that matters most. |
| Combined chlorine (CC), chloramines | Chlorine that has already reacted with sweat, urine and other waste. It sanitizes poorly and causes the “chlorine smell”. The MAHC calls for action above 0.4 ppm. |
| Total chlorine (TC) | Free plus combined. Combined chlorine = total − free. |
| Available chlorine | The percentage on a chlorine product’s label: how much chlorine it can deliver, by weight (granules, tablets) or per 100 ml (liquids). Trichlor is 90%, cal-hypo 65–73%. |
| Hypochlorous acid | The active form of free chlorine that does most of the killing. pH and stabilizer decide how much of the free chlorine is in this form. |
| Shock / superchlorinate | Raising chlorine well above the daily level for a while, to clear waste or algae. “Shock” is a pesticide claim on a label, regulated by the EPA. |
| SLAM | Shock Level And Maintain: a hobbyist method (Trouble Free Pool) of raising free chlorine to 40% of the CYA and holding it there until algae is gone. |
| Liquid chlorine | Sodium hypochlorite solution, usually 10% or 12.5%. Same chemical as household bleach, stronger. |
| Cal-hypo | Calcium hypochlorite, a granular shock (65–73%) that adds calcium. |
| Dichlor | Sodium dichloroisocyanurate, a fast-dissolving granular chlorine (about 56%) that adds stabilizer. |
| Trichlor | Trichloroisocyanuric acid, usually 3 in tablets (90%). Slow-dissolving and acidic; adds stabilizer. |
| Stabilized chlorine | Dichlor or trichlor: chlorine products that carry cyanuric acid with them. |
| Salt chlorine generator (SWG, salt cell) | A device that makes chlorine from dissolved salt kept at a few thousand ppm. The pool still contains chlorine; you just don’t buy it in jugs. |
| Bromine | An alternative sanitizer, mostly used in hot tubs. The CDC range for hot tubs is 4–8 ppm. |
| Non-chlorine shock | Potassium monopersulfate, an oxidizer that burns off waste without raising chlorine. It does not sanitize. |
| FAS-DPD test | A drop test that counts drops to read free and combined chlorine precisely, even at shock levels. |
| Chlorine demand | How much chlorine the water uses up — from sun, swimmers and debris — before a level holds. |
The stabilizer–chlorine link catches most people out. The chlorine and CYA calculator gives the free chlorine your CYA calls for, and what CYA and chlorine a salt water pool needs covers salt pools. For a shock, see how much liquid chlorine to SLAM a pool; for a daily top-up, how much liquid chlorine to raise 1 ppm in 10,000 gallons. Each product is compared in chlorine types compared.
Pumps, filters and heaters
| Word | What it means |
|---|---|
| gpm | Gallons per minute: how fast the pump moves water. The number that matches a pump to a filter. |
| Turnover | The time the pump takes to move a volume of water equal to the whole pool. A 20,000 gallon pool on an 8 hour turnover needs about 42 gpm. |
| Variable-speed pump | A pump that can run slowly for long hours. Because power falls much faster than flow, ENERGY STAR says half speed uses about one-eighth the energy. |
| Sand filter | A tank of #20 silica sand the water is pushed through. Cleaned by backwashing. |
| Cartridge filter | A pleated fabric element in a housing. Cleaned by taking it out and hosing it off. |
| DE filter | Grids coated with diatomaceous earth powder, which traps the finest particles. Recharged with fresh powder after backwashing. |
| Backwash | Running water backwards through a sand or DE filter to flush trapped dirt out to waste. |
| Rinse | The short setting after backwash that resettles the sand bed before filtering again. |
| Multiport valve | The dial on top of a sand filter: filter, backwash, rinse, waste, recirculate, closed. |
| Laterals | The slotted arms at the bottom of a sand filter that let water out but keep sand in. |
| Filter area | The size of the sand bed (or cartridge surface) in square feet. Hayward rates its sand filters at 20 gpm per square foot. |
| Clean pressure | The gauge reading (psi) right after a backwash. Hayward says to backwash when it rises 8–10 psi above that. |
| Skimmer | The opening at the waterline that pulls in floating debris. The fill line is usually around the middle of it. |
| Return (jet) | Where filtered water comes back into the pool. Pour liquid chemicals in front of one with the pump running. |
| BTU | British thermal unit: the heat that warms 1 lb of water by 1°F. Gas heaters are rated in BTU per hour. |
| Heat pump | An electric heater that moves heat from the air into the water. DOE puts their efficiency (COP) at 3–7. |
| COP | Coefficient of performance: heat out ÷ electricity in. A COP of 5 means 5 units of heat for 1 unit of electricity. |
| Solar cover | A floating bubble sheet. DOE says covers cut heating costs 50–70% because evaporation is the largest heat loss. |
For the numbers behind these: what size pump for a 15,000 gallon pool, how much sand a Hayward S244T takes, what a pool pump costs to run 8 hours a day and how long to heat a pool 10 degrees. Every filter’s sand amount is in the pool filter sand chart.
Pool construction and sizes
| Word | What it means |
|---|---|
| Inground pool | A pool dug into the ground: concrete (plaster), vinyl-lined or a one-piece fiberglass shell. |
| Above-ground pool | A pool with a free-standing steel, resin or frame wall, sized by diameter and wall height (e.g. 24 ft × 52 in). |
| Wall height | The height of an above-ground pool’s wall: usually 48, 52 or 54 in. The water is a few inches less. |
| Fill line | The water level the pool is designed to run at, usually around the middle of the skimmer opening. Use it for the volume. |
| Oval (above-ground) | Usually straight sides with round ends, not a true ellipse, so it holds about 14% more water than an ellipse of the same length and width. |
| Average depth | For a pool with a shallow and deep end, roughly (shallow + deep) ÷ 2. Used to work out gallons. |
| Gallons per inch | How much water one inch of depth holds: floor area in sq ft × 0.623. Handy for topping up and leak checks. |
| Cove | The sloped fill at the bottom of an above-ground wall, so the liner doesn’t stretch into a sharp corner. Sand, earth or foam, depending on the maker. |
| Base sand / mason sand | The 2 in bed of fine sand under an above-ground liner. Not the same as filter sand. |
| Overlap liner | An above-ground liner that hangs over the top of the wall and is held by coping strips. |
| Beaded liner | A liner with a bead along its edge that snaps into a track (bead receiver) at the top of the wall. |
| J-hook liner | A liner with a J-shaped edge that hooks over the top of the wall. |
| Plaster | The cement-based finish on a concrete pool. Its chemistry is why calcium hardness and the saturation index matter. |
| Kiddie pool | A small inflatable or plastic pool. The CDC says not to add chlorine or bromine; empty it at least daily. |
Worked examples: how many gallons in a 16×32 inground pool, how many gallons in a 24 ft round pool with a 52 in wall, what size liner for a 24 ft round pool with a 52 in wall and gallons per inch in a 24 ft round pool. Common above-ground sizes are tabulated in the above-ground pool size chart.
TipWhen a label and a chart disagree on a word or a number, the label wins. Pool sanitizers are EPA-registered pesticides, and the EPA’s phrase for it is “the label is the law”.
Frequently asked questions
What is the difference between free chlorine and total chlorine?
Free chlorine is what is still available to sanitize; total chlorine adds the combined chlorine that has already reacted with waste. Total minus free is combined chlorine.
Are stabilizer, conditioner and cyanuric acid the same thing?
Yes. All three names mean cyanuric acid (CYA), which protects chlorine from sunlight in outdoor pools.
What does ppm mean on a pool test?
Parts per million: milligrams per litre. 1 ppm in a 10,000 gallon pool is about 1.3 oz of the pure chemical.
Sources
- CDC Healthy Swimming — Home pool and hot tub water treatment and testing (opens in a new tab) — pH 7.0–7.8; free chlorine at least 1 ppm in pools (2 ppm with cyanuric acid) and 3 ppm in hot tubs; no CYA in hot tubs; test twice a day.
- CDC Healthy Swimming — Staying healthy in hot tubs (opens in a new tab) — No hotter than 104°F (40°C); chlorine at least 3 ppm, bromine 4–8 ppm, pH 7.0–7.8; no children under 5.
- CDC — Model Aquatic Health Code, 5th edition (2024), code (opens in a new tab) — Free chlorine minimum 1.0 ppm (2.0 with CYA, 3.0 in spas); pH 7.0–7.8; CYA at or below 90 ppm; CYA:FC above 45:1 is an imminent hazard; TA 60–180 ppm; turnover tables.
- CDC — Model Aquatic Health Code, 5th edition (2024), annex (opens in a new tab) — CYA reduces chlorine’s effectiveness; correcting total alkalinity for CYA by subtracting CYA ÷ 3.
- Trouble Free Pool (hobbyist forum wiki) — CYA/chlorine relationship (opens in a new tab) — A widely used hobbyist convention, not an official standard: minimum FC 7.5% of CYA, SLAM (shock) level 40% of CYA, salt-water pools 5%.
- Taylor Technologies — Using the Taylor circular Watergram (opens in a new tab) — Saturation index = pH + temperature factor + log CH + log carbonate alkalinity − constant; within ±0.5 of zero needs no adjustment.
- CDC Healthy Swimming — Inflatable and plastic kiddie pools (opens in a new tab) — Empty kiddie pools at least daily; it is unsafe to add chlorine or bromine to them.
- US EPA — Introduction to pesticide labels (opens in a new tab) — Pool sanitizers are registered pesticides; the label is the law.
- US EPA — Use of the term “shock” on pool and spa products (opens in a new tab) — “Shock” is a pesticide claim on pool and spa products.
- Hayward ProSeries high-rate sand filter manual (IS210TS Rev A) (opens in a new tab) — Backwash at 8–10 psi over clean pressure; No. 20 silica sand.
- ENERGY STAR — Pool pumps (opens in a new tab) — Running a pump at half speed uses about one-eighth the energy.
- US DOE Energy Saver — Heat pump swimming pool heaters (archived) (opens in a new tab) — COP 3–7; efficient above about 45–50°F air.
- US DOE Energy Saver — Swimming pool covers (archived) (opens in a new tab) — Evaporation is the largest energy loss; covers save 50–70% of heating.