Pool Terms, in Plain English

Every abbreviation the pool world throws at you — defined in a sentence or two, linked to the calculator that does the math.

Free chlorine (FC)

The chlorine that's actually available to sanitize — the number that matters. Target scales with your CYA (roughly 7.5% of it). Calculator →

Combined chlorine (CC)

Used-up chlorine bound to contaminants — it smells like 'pool' and stings eyes. Total chlorine minus free chlorine; above 0.5 ppm means it's shock time. Calculator →

Total chlorine (TC)

Free + combined chlorine, which is what cheap testers report. Useful only for computing CC.

CYA / stabilizer / conditioner

Cyanuric acid — sunscreen for chlorine. Without it, sunlight destroys chlorine in hours; with too much, chlorine goes numb. 30–50 ppm typical, 60–80 for salt pools. Calculator →

Total alkalinity (TA)

The water's pH shock absorber. 80–120 ppm keeps pH from bouncing around with every rainstorm. Calculator →

pH

How acidic or basic the water is. 7.2–7.8 for comfort and equipment; drifts up in most pools, especially salt ones.

Calcium hardness (CH)

Dissolved calcium. Plaster pools need 250–450 ppm or the water eats the finish; vinyl and fiberglass are more relaxed. Calculator →

SLAM

Shock Level And Maintain — the Trouble Free Pool community's name for the algae cure: raise chlorine to ~40% of CYA and hold it until the water stops consuming it. Calculator →

OCLT

Overnight Chlorine Loss Test: measure FC at dusk and dawn. Losing more than 1 ppm overnight means something alive is eating chlorine. Calculator →

SWG / salt cell

Salt Water Generator — makes chlorine from dissolved salt via electrolysis. The cell is its consumable heart: 3–7 years, $400–900. Calculator →

LSI / CSI

Saturation index — whether water wants to deposit scale (positive) or dissolve surfaces (negative). Keep between −0.3 and +0.3. Calculator →

Breakpoint chlorination

The chlorine level (≈10× your CC) that fully burns off chloramines instead of making more. The chemistry behind 'shocking.' Calculator →

Shock

A dose, not a product: chlorine raised high enough to oxidize contaminants or kill algae. Bags labeled 'shock' are just chlorine compounds with marketing.

Liquid chlorine / sodium hypochlorite

Chlorine in a jug (10–12.5%; household bleach is the 6–8% version). Adds nothing but chlorine and a little salt — the cleanest daily sanitizer. Calculator →

Dichlor

Granular stabilized chlorine. Convenient, pH-neutral — but every pound brings CYA that never leaves. Great for spas early on; a trap as a pool's daily diet. Calculator →

Trichlor / pucks

Slow-dissolving chlorine tablets. Effortless, but acidic and CYA-stacking — the classic cause of 'my chlorine stopped working' by August.

Cal-hypo

Calcium hypochlorite, the classic shock powder. Strong and unstabilized, but raises calcium with every use — watch CH if it's your habit.

Muriatic acid

Hydrochloric acid for lowering pH and alkalinity. Effective, cheap, and deserving of gloves and respect. Calculator →

Algaecide

Supplemental algae treatment. Polyquat 60 is the safe kind; copper-based kinds can stain surfaces and turn hair green. Chlorine remains the real weapon.

Clarifier

Clumps microscopic particles so the filter can catch them. For haze, not for algae. Calculator →

Flocculant

Nuclear clarifier: sinks everything to the floor for a manual vacuum to waste. Fast, laborious, wastes water — swamp recovery tool.

Borates

An optional secondary pH buffer at 30–50 ppm. Smoother water, less pH drift; a project to install, permanent once in. Calculator →

Turnover

One full pool volume through the filter. One per day is the residential standard. Calculator →

GPM

Gallons per minute — pump flow. Most pools need less than owners think. Calculator →

Backwash

Reversing flow through a sand/DE filter to flush captured debris. Do it when pressure runs ~25% over the clean baseline, not on a calendar.

Sequestrant

Keeps dissolved metals (iron, copper) from staining surfaces. Maintenance chemical for pools with metal-bearing fill water. Calculator →

Winterizing / closing

Preparing water and plumbing for the off-season: balance, final chlorine, algaecide, lines cleared in freeze country. Calculator →