How much chlorine does a pool need?
There is no single right FC number — it depends on your stabilizer (CYA) level. CYA holds chlorine in reserve and shields it from sunlight, so the more CYA in the water, the more total FC you need for the same sanitizing power. This is the single most misunderstood fact in pool care, and it's why a stabilized pool held at the pool-store's "1–3 ppm" can still turn green:
| CYA (ppm) | Minimum FC | Target FC | SLAM level |
|---|---|---|---|
| 30 | 2.5 | 4 | 12 |
| 40 | 3 | 4.5 | 16 |
| 50 | 4 | 6 | 20 |
| 60 | 4.5 | 7 | 24 |
| 70 | 5.5 | 8 | 28 |
| 80 | 6 | 9 | 32 |
The Trouble Free Pool FC/CYA relationship. Salt-water (SWG) pools run slightly lower targets because the cell feeds chlorine continuously. Never let FC touch the minimum — that's the algae line, not the goal.
How much liquid chlorine for a 10,000-gallon pool?
For liquid chlorine and bleach the math has a handy symmetry: one gallon of X% product raises 10,000 gallons of water by about X ppm. Everything else scales from there:
| Product | Raise 1 ppm | Raise 2 ppm | Raise 4 ppm |
|---|---|---|---|
| 12.5% liquid chlorine | 10.2 fl oz | 20.5 fl oz | 1.3 qt |
| 10% liquid chlorine | 12.8 fl oz | 25.6 fl oz | 1.6 qt |
| 6% household bleach | 21.3 fl oz | 1.3 qt | 2.7 qt |
Doses per 10,000 gallons — scale linearly for your pool, or just use the calculator above with your saved volume.
Liquid chlorine strength is trade percent: grams of available chlorine per 100 mL. That's why the gallon-per-10-ppm rule works, and why strength matters more than jug price — a $6 jug of 12.5% beats a $5 jug of 6% on cost per ppm every time. Liquid also loses strength sitting in a hot garage, so buy recent stock and store it cool.
Liquid, tablets, or powder? What each dose leaves behind
Every chlorine product except liquid carries a second chemical along, and that passenger accumulates because nothing removes it except draining water:
| Product | Per 10 ppm of FC added | Where that leads |
|---|---|---|
| Liquid / bleach | ~8 ppm salt | Harmless — takes years to matter |
| Trichlor pucks | +6 ppm CYA | CYA climbs all season → FC targets climb with it |
| Dichlor granules | +9 ppm CYA | Fastest CYA accumulation of all — fine occasionally, ruinous daily |
| Cal-hypo | +7 ppm CH | Calcium climbs → scaling risk in hard-water areas |
This is why pools chlorinated on trichlor all summer end the season needing a big drain-and-refill: a puck floater adding 2 ppm FC a day adds over 35 ppm of CYA a month. Tablets aren't bad — they're excellent for vacations and their slow feed is genuinely convenient — but they're a running loan against your water, and liquid chlorine is how you avoid taking it. If your CYA is already high, the calculator above will keep working, but the dashboard will be recommending a partial drain.
Sunlight burns off unstabilized chlorine fast — a midday dose can lose a third of its punch before sunset. Dosing at dusk gives chlorine the whole night to work, which matters most when you're fighting the beginnings of algae.
When a normal dose isn't enough
If the water is green, cloudy-green, or FC vanishes overnight, you're past routine dosing — that's an algae bloom consuming chlorine as fast as you add it. The fix is the SLAM process: raising FC to the SLAM level in the table above and holding it there until the water clears. The shock calculator handles those doses, and the dashboard will tell you when your readings point that way.