To raise free chlorine by 1 ppm in 10,000 gallons:
| Product | Available chlorine | Amount | Also adds |
|---|---|---|---|
| Cal-hypo | 65–73% | ~2 oz | Calcium hardness (~0.8 ppm) |
| Dichlor | 56% | ~2.3 oz | CYA (~0.9 ppm) |
| Trichlor | 90% | ~1.5 oz | CYA (~0.6 ppm) + strongly acidic |
| Liquid chlorine (12.5%) | 12.5% | ~10.7 fl oz | Salt, raises pH slightly |
| Household bleach (6%) | 6% | ~16 fl oz | Salt, raises pH slightly |
Scale linearly with volume: a 21,000 gallon pool needs 2.1× the figures above. Dose tables by pool size and target rise do the multiplication for every combination.
The "also adds" column is the one that matters
The dose is arithmetic. What the product leaves behind is what decides whether the pool still works in September.
Cal-hypo adds calcium hardness. In hard fill water it is what scales the tile line and the heater element. Calcium never leaves except by draining.
Dichlor and trichlor add cyanuric acid. CYA also never leaves except by draining, and above roughly 50 ppm it binds chlorine so tightly that the chlorine stops killing anything — a pool that reads plenty of chlorine and still goes green.
Trichlor is strongly acidic besides. A season of tablets drives pH and alkalinity down continuously, which is why tablet pools need base added constantly.
Liquid chlorine adds only salt and a small pH nudge. It is the only product with no cumulative penalty, which is why it is what commercial pools use.
The rotation that avoids all three traps
- Liquid chlorine for routine chlorination and for shock — no CYA, no calcium
- Dichlor early in the season, deliberately, to establish 30–50 ppm CYA
- Cal-hypo only if calcium hardness is genuinely low
- Trichlor tablets for a week away, not as the year-round method
How to dose
Pump running. A dose added to still water sits in one place and does nothing except bleach whatever it is touching.
Slowly, at the deep end or through the skimmer. Granular product resting on a vinyl liner bleaches it permanently, and that mark does not come out.
Pre-dissolve granules in a bucket of pool water, and add product to water rather than water to product.
Test after four hours of circulation, not immediately. A reading taken beside the return jet is a reading of the return jet.
Target and shock
Routine free chlorine: 2–4 ppm, adjusted upward if CYA is high.
Shock: raise to roughly 10× the combined chlorine reading, or 10–12 ppm for a general seasonal shock. That is a specific arithmetic target, not "a bag".
Do it after sunset. Chlorine unprotected by CYA loses more than half its strength in a few hours of direct sun, so a shock at noon is roughly half a shock.
Never mix products
Not in a bucket, not in a feeder, not in a bag left over from last year. Cal-hypo and trichlor together can ignite. Chlorine and acid together produce chlorine gas.
One chemical at a time, hours apart, and in the right order when balancing.
If chlorine will not hold
Three causes, in order of likelihood:
- CYA too high — the pool needs dilution, not more chlorine
- CYA too low — sunlight is destroying it faster than you add it
- Something is consuming it — algae, or organic load after heavy use or a storm
All three read as "I keep adding chlorine and it disappears", and only a CYA test tells them apart. Every dose above assumes you already know the pool's volume.