FC/CYA chart: how much chlorine do you actually need?
Most pool owners pick a chlorine target and stick with it no matter what. But the amount of free chlorine you need depends entirely on how much cyanuric acid (CYA) is in your water. Get this relationship wrong and you'll end up with algae even though your FC "looks fine."
The formula
The widely accepted minimum is:
This gives you the lowest FC level that still provides effective sanitization at your current CYA. Below this number, there isn't enough active chlorine (hypochlorous acid) to reliably kill algae and bacteria.
The chart
| CYA (ppm) | Minimum FC | Target FC | Shock Level (SLAM) |
|---|---|---|---|
| 20 | 1.5 | 2–4 | 10 |
| 30 | 2 | 3–5 | 12 |
| 40 | 3 | 4–6 | 16 |
| 50 | 4 | 5–7 | 20 |
| 60 | 5 | 5–8 | 24 |
| 70 | 5 | 6–8 | 28 |
| 80 | 6 | 7–9 | 31 |
| 90 | 7 | 7–10 | 35 |
| 100 | 8 | 8–10 | 39 |
Minimum FC is the floor. Don't let your chlorine drop below this number. Target FC is the range you should aim for during normal operation. Shock level is what you need to reach and hold during a SLAM (Shock Level And Maintain) process to kill an active algae bloom.
Why does CYA affect chlorine?
CYA (stabilizer/conditioner) protects chlorine from UV breakdown. That's its job and it does it well. But CYA also binds to chlorine molecules, reducing the amount of "active" chlorine available to sanitize.
Think of it this way: at any given moment, only a small percentage of your FC is actually active. The rest is bound to CYA, held in reserve. As CYA increases, a smaller percentage of your total FC is active. To compensate, you need more total FC.
The numbers behind it
At 30 ppm CYA, about 3% of your FC is active hypochlorous acid. At 80 ppm CYA, that drops to about 1.2%. That's why the same FC reading can mean "well sanitized" at low CYA and "basically unprotected" at high CYA.
What happens when CYA is too high?
As CYA climbs above 80-90 ppm, problems compound:
- FC minimums get impractical. At 100+ ppm CYA, you need FC of 8+ just to stay sanitized. That's expensive and hard to maintain
- Shock levels become extreme. SLAMming a pool with CYA at 100 means holding FC at 39 ppm. That burns through chlorine fast
- Test accuracy drops. Most FC test kits lose precision above 10 ppm, making it hard to verify you're at the right level
- Algae risk increases. The margin between "sanitized" and "algae bloom" gets razor thin
How CYA builds up
CYA doesn't break down, evaporate, or get filtered out. It only leaves the pool through water removal (splash-out, backwashing, or draining). The most common cause of high CYA is stabilized chlorine tablets (trichlor). Every tablet you dissolve adds more CYA to the pool.
| Chlorine Source | Adds CYA? |
|---|---|
| Trichlor tablets | Yes — significant amount |
| Dichlor granules | Yes — moderate amount |
| Liquid chlorine (bleach) | No |
| Cal-hypo (granular shock) | No |
| Salt water generator (SWG) | No |
If you use trichlor tablets as your primary chlorine source, test CYA monthly. It will creep up over the season. Many pool owners switch to liquid chlorine once CYA reaches their target to prevent further buildup.
What to do when CYA is too high
The only way to lower CYA is dilution:
- Partial drain and refill. Drain 25-30% of your pool water and refill with fresh water. This proportionally reduces CYA (and all other dissolved solids)
- Retest after refilling and let the water circulate for at least a full pump cycle before testing
- Switch to liquid chlorine going forward to prevent CYA from climbing again
SWG pool owners
Saltwater pools tend to run CYA in the 60-80 range intentionally. The salt cell produces chlorine slowly and CYA helps it last longer between generation cycles. This is fine as long as you adjust your FC target to match. At 70 CYA, target FC of 6-8.
Ideal CYA ranges
| Pool Type | Recommended CYA | Why |
|---|---|---|
| Chlorine pool (tablets) | 30–50 ppm | Good UV protection without requiring excessive FC |
| Chlorine pool (liquid) | 30–50 ppm | Add CYA separately at season start. Stays stable all year |
| Saltwater (SWG) pool | 60–80 ppm | Higher CYA helps the cell's chlorine output last longer |
| Indoor pool | 0–20 ppm | No UV exposure, so minimal CYA needed |
Quick rules
- FC minimum = CYA × 7.5%
- Target FC = CYA × 7.5% to CYA × 10%
- Shock level = CYA × 40%
- Test CYA monthly — it only goes up, never down
- Above 80-90 ppm CYA, consider a partial drain
- Switch to liquid chlorine once CYA is at your target
Let the app do the math
PoolChem Tracker automatically adjusts your chlorine target based on your CYA reading. Log your results, get the right dosing recommendation, and keep FC where it needs to be.
Keep reading
- Free Chlorine vs Total Chlorine Explained — understanding FC, TC, and combined chlorine
- Pool Chlorine Levels Chart — what your chlorine readings mean at a glance
- How to Raise Pool Chlorine Safely — methods for boosting FC when it drops
- Liquid Chlorine vs Tablets — pros and cons, including CYA impact