Chart
Pool FC / CYA target chart
For your CYA level, what should your free chlorine be? Trouble Free Pool’s field-tested 7.5% minimum / 11.5% target / 40% SLAM rule, in one chart. Find your CYA row, read across.
| CYA (ppm) | Min FC | Target FC | SLAM FC |
|---|---|---|---|
| 20 | 1.5 | 2.3 | 8 |
| 30 | 2.3 | 3.5 | 12 |
| 40 | 3 | 4.6 | 16 |
| 50 | 3.8 | 5.8 | 20 |
| 60 | 4.5 | 6.9 | 24 |
| 70 | 5.3 | 8.1 | 28 |
| 80 | 6 | 9.2 | 32 |
| 90 | 6.8 | 10.4 | 36 |
| 100 | 7.5 | 11.5 | 40 |
Min FC = 7.5% × CYA (sanitizer floor — never go below). Target FC = ~11.5% × CYA (comfortable maintenance band). SLAM FC = 40% × CYA (algae-clearing shock level — only during active SLAM protocol).
Pro tip — auto-applied
The PoolAssist AI chat applies this chart automatically every time you submit a reading — if your FC is below the 7.5% × CYA floor, the recommendation includes a corrective dose to bring you back to the target band. Open the chat with your readings to see it in action.
Reading the chart
Three concepts, one chart:
- Min FC is the never-go-below floor. If your FC drops below this, your chlorine has effectively stopped sanitizing — algae and bacteria can grow even with chlorine in the water.
- Target FC is the comfortable steady-state band for normal maintenance. Aim here on routine dosing.
- SLAM FC is the “algae kill” level. Only use during active SLAM protocols — sustained high FC at this level is hard on swimwear and skin if you’re not actively battling algae.
Examples
20,000 gal outdoor pool, CYA 50: min FC 3.8, target 5-6, SLAM 20. Day-to-day you want FC between 4 and 6. If you spot algae, raise to 20 and hold there until clear.
15,000 gal SWG (saltwater) pool, CYA 70: min FC 5.3, target ~8, SLAM 28. Set your salt cell output high enough to maintain at least 5-6 ppm, ideally 7-9 for buffer.
800 gal hot tub, CYA 0 (indoor): the table doesn’t apply — indoor / spa uses bromine targets or a flat FC 3-5 ppm with no CYA needed.
FAQ
What's the FC / CYA relationship?
Cyanuric acid (CYA, sold as 'stabilizer' or 'conditioner') binds to free chlorine and protects it from UV destruction. The tradeoff: bound chlorine is much weaker as a sanitizer. The Trouble Free Pool rule — battle-tested across hundreds of thousands of pools — is that you need FC ≥ 7.5% of CYA at all times to maintain effective sanitization. CYA 40 → FC ≥ 3, CYA 80 → FC ≥ 6, etc.
What is SLAM and when do I need to do it?
SLAM = Shock Level And Maintain. It's the TFP protocol for clearing algae or persistent chloramines: raise FC to 40% of your CYA level (CYA 30 → SLAM at FC 12; CYA 60 → SLAM at FC 24) and keep it there day and night until three conditions are met (FC loss overnight < 1 ppm, CC < 0.5 ppm, water is crystal clear). Typically takes 3-7 days of frequent dosing.
Is high CYA actually a problem?
It can be. Above 100 ppm CYA you need such high FC to stay sanitized (10+ ppm continuously) that water-balance gets tricky and chemical costs balloon. CYA can only be reduced by partial draining and refilling — there's no chemical that breaks it down reliably. If you find yourself at CYA 120+, drain 30-40% and refill with fresh water.
Indoor pools — do I need CYA at all?
No. Indoor pools have no UV exposure, so CYA isn't protecting anything — it just dampens chlorine effectiveness for no benefit. Target CYA 0-20 ppm indoor. If you've been using stabilized chlorine (trichlor / dichlor) and CYA crept up, switch to liquid chlorine or cal-hypo and let CYA drop on its own (it slowly degrades over months).
Why is CYA 30 the most common 'starter' target?
It's the minimum that gives meaningful UV protection on outdoor pools while keeping the required FC manageable (FC ≥ 2-3 ppm). Below CYA 20 outdoor, you lose 50%+ of your chlorine to sun every day. Above CYA 50 the FC requirement gets demanding for casual maintenance. CYA 30-50 is the sweet spot for most residential outdoor pools.
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