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SLAM Pool Calculator — CYA-Based FC Target for Green Pools
Calculate your exact SLAM target FC based on your CYA level.
Get the liquid chlorine dose to reach it — and maintain it until your pool clears. No sign-ups. Instant results.
💡 Quick answer
SLAM target FC = CYA ÷ 10 × 4
Example: CYA of 40 ppm → SLAM target is 16 ppm FC
Use the calculator below for your exact dose and how much liquid chlorine to buy.
⚠️ Required: You need a FAS-DPD test kit — standard strips read zero above 5 ppm FC.
💡 Before starting SLAM: adjust pH to 7.2–7.5 and alkalinity to 80–120 ppm. You cannot adjust pH accurately once FC exceeds 10 ppm.
What you need before starting SLAM:
Example calculation
A 20,000 gallon pool with a CYA of 40 ppm and current FC of 2 ppm needs to be SLAMed:
| Input | Value |
|---|---|
| Pool volume | 20,000 gallons |
| CYA level | 40 ppm |
| Current FC | 2 ppm |
| SLAM target FC | 16 ppm (40 ÷ 10 × 4) |
| FC increase needed | 14 ppm |
| Liquid chlorine to add | ~2.1 gallons (first dose) |
| Recommended to buy | 10 gallons (3–4 days supply) |
Formula: (Pool gallons × FC increase needed) ÷ 130,000 = gallons of 10% liquid chlorine
CYA to SLAM target FC chart
Quick reference for SLAM target free chlorine at each CYA level. This is the level you must raise to and maintain throughout the process.
| CYA (ppm) | SLAM Target FC | Min FC (safe swimming) | Notes |
|---|---|---|---|
| 20 ppm | 8 ppm | 2 ppm | Low CYA — FC burns off fast in sun |
| 30 ppm | 12 ppm | 2 ppm | Ideal lower range for SLAM |
| 40 ppm | 16 ppm | 3 ppm | ✅ Ideal for outdoor SLAM |
| 50 ppm | 20 ppm | 4 ppm | Acceptable — uses more chlorine |
| 60 ppm | 24 ppm | 5 ppm | High — SLAM will be slower |
| 70 ppm | 28 ppm | 6 ppm | ⚠️ Upper limit — consider partial drain |
| 80 ppm | 32 ppm | 7 ppm | ⚠️ Difficult — large chlorine volume needed |
| 90+ ppm | 36+ ppm | 8+ ppm | 🚫 Drain & dilute before SLAMing |
SLAM target = CYA × 40% (i.e. CYA ÷ 10 × 4). Min FC = CYA × 7.5%. Use our stabilizer calculator if CYA is too high.
Step-by-step SLAM process
Before you start
1. Get the right test kit. You need a FAS-DPD chlorine test to measure accurately above 5 ppm. Standard DPD test strips bleach out and read zero at high FC. The Taylor K-2006 is the gold standard.
2. Check and adjust CYA. If CYA is above 90 ppm, do a partial drain and refill to dilute it below 70 ppm before starting. Use our stabilizer calculator for the drain math.
3. Adjust pH to 7.2–7.5 using muriatic acid or soda ash. Once FC exceeds 10 ppm, pH tests become unreliable. Get it right first.
4. Adjust total alkalinity to 80–120 ppm. Use our alkalinity calculator.
5. Remove visible debris. Skim leaves, brush walls and floor, backwash or clean your filter. Turn off any salt chlorine generators or puck chlorinators.
6. Buy liquid chlorine. Pools under 20,000 gallons: buy 10 gallons of 10–12% sodium hypochlorite. Pools over 20,000 gallons: buy 15 gallons. You can return what you don't use. Shop liquid chlorine →
During the SLAM
7. Raise FC to your SLAM target. Use the calculator above to determine how much liquid chlorine to add for the first dose. Pour directly near a return jet with the pump running.
8. Run the pump 24/7. Continuous circulation is essential. Don't turn the pump off until the SLAM is complete.
9. Test and re-dose every 2–4 hours. FC will drop as it kills algae and bacteria. Every time FC falls below your SLAM target, add more liquid chlorine to bring it back up. The faster you retest and redose, the quicker the SLAM completes.
10. Brush daily. Brush all walls, floor, steps, ladder tracks, and skimmer interiors once per day. This breaks up algae biofilm and exposes it to chlorine.
11. Clean the filter. When filter pressure rises 25% above your clean baseline, backwash or clean it. A clogged filter slows the SLAM significantly.
12. Do NOT test or adjust other chemistry (pH, alkalinity, calcium, etc.) until the SLAM is complete and FC drops below 10 ppm.
Completion criteria — all three must be met
✅ Criterion 1: Crystal clear water. No visible algae or cloudiness. You must be able to clearly see the bottom of the deep end.
✅ Criterion 2: Combined chlorine (CC) below 0.5 ppm. Test with your FAS-DPD kit. High CC means chloramines are still present — keep going.
✅ Criterion 3: Pass the overnight chlorine loss test (OCLT). Test FC after dark (30+ minutes after sunset). Test again at first light before the sun touches the water. If FC dropped by less than 1 ppm overnight, the SLAM is complete.
⚠️ Don't declare victory early. Water can look clear but still fail the OCLT. Run all three tests before stopping.
Liquid chlorine dose chart by pool size
How much 10% liquid chlorine (sodium hypochlorite) to raise free chlorine by common amounts. Use this as a cross-reference alongside the calculator above.
| Pool size | +5 ppm FC | +10 ppm FC | +15 ppm FC | +20 ppm FC |
|---|---|---|---|---|
| 10,000 gal | 0.4 gal | 0.8 gal | 1.2 gal | 1.5 gal |
| 15,000 gal | 0.6 gal | 1.2 gal | 1.7 gal | 2.3 gal |
| 20,000 gal | 0.8 gal | 1.5 gal | 2.3 gal | 3.1 gal |
| 25,000 gal | 1.0 gal | 1.9 gal | 2.9 gal | 3.8 gal |
| 30,000 gal | 1.2 gal | 2.3 gal | 3.5 gal | 4.6 gal |
Based on 10% sodium hypochlorite. If using 12.5% liquid chlorine, multiply all amounts by 0.8. Use our chlorine calculator for other chlorine types.
SLAM method vs regular pool shock: what's the difference?
| Factor | Regular Shock | SLAM Method |
|---|---|---|
| Duration | One-time addition | Continuous maintenance for 1–7 days |
| FC target | Fixed (e.g., 10 ppm) | CYA-dependent (CYA × 40%) |
| Best for | Routine maintenance, mild cloudiness | Algae, green water, persistent cloudiness |
| Chlorine type | Cal-hypo granular or liquid | Liquid chlorine (recommended) |
| Completion test | None — wait and hope | 3 specific pass/fail criteria |
| Success rate | Low for established algae | Very high when followed correctly |
For routine weekly maintenance shock, use our shock calculator. For any case of green water, black algae, or a pool that won't clear despite normal shocking, SLAM is the right method.
Why liquid chlorine is the best choice for SLAM
💧 No CYA increase
Liquid chlorine (sodium hypochlorite) adds zero CYA. Dichlor and trichlor add CYA with every dose, pushing your SLAM target higher and making the process harder and more expensive.
📏 Easy to dose accurately
Liquid chlorine is measured in gallons or fluid ounces. The math is consistent and predictable. Granular shock must be pre-dissolved and can vary in concentration.
⚡ Fast-acting
Liquid chlorine enters solution instantly. Pour near a return jet and it's active within minutes. No dissolving time, no undissolved granules on the pool floor.
💰 Most economical at scale
A SLAM on a green pool can require 10–20+ gallons of liquid chlorine. At $3–6 per gallon, this is far cheaper than equivalent cal-hypo or dichlor quantities.
💡 Buy liquid chlorine at your local pool store, Home Depot, or Walmart — typically sold as pool bleach or sodium hypochlorite. Make sure it's labelled 10–12.5% available chlorine. Shop liquid chlorine on Amazon →
SLAM method calculator FAQs
What does SLAM stand for in pool care?
SLAM stands for Shock Level And Maintain. It's a structured process for raising free chlorine to a specific elevated level (determined by your CYA) and holding it there continuously until algae and bacteria are completely eliminated and the pool passes three completion tests.
How do I calculate my SLAM target FC?
SLAM target FC = CYA ÷ 10 × 4. For CYA of 30 ppm → SLAM target is 12 ppm. For CYA of 40 ppm → 16 ppm. For CYA of 50 ppm → 20 ppm. Use the calculator at the top of this page for your exact dose of liquid chlorine.
Can I SLAM with granular shock instead of liquid chlorine?
You can, but liquid chlorine is strongly preferred. Granular cal-hypo raises calcium hardness with every dose. Dichlor and trichlor add CYA, which raises your SLAM target mid-process. Liquid chlorine (sodium hypochlorite) adds only free chlorine with no unwanted side effects. For a multi-day SLAM, the practical and chemical advantages of liquid chlorine are significant.
What if my CYA is 0 or unmeasurable?
If CYA is below 20 ppm or undetectable, use a SLAM target of 10 ppm. At very low CYA, FC is highly active but burns off rapidly in sunlight. You'll need to test and re-dose very frequently (every 1–2 hours in direct sun) and consider adding stabilizer to raise CYA to 30–40 ppm after the SLAM is complete, before resuming normal chlorination.
My pool turned green overnight — do I need to SLAM?
Yes. Green water means algae has taken hold — this can happen in as little as 24–48 hours when chlorine drops. A standard shock dose will not be sufficient. Calculate your SLAM target, prepare for 2–5 days of continuous dosing and testing, and follow the checklist above. Most pools clear within 3–5 days if the process is followed correctly.
Do I need to drain my pool to fix green water?
In most cases, no. SLAM clears green water without draining as long as CYA is below 90 ppm and the pool structure is sound. Draining is only necessary when CYA is excessively high (above 90–100 ppm), when there is heavy metal staining that won't respond to chemistry, or when the pool has been neglected for multiple seasons with extreme organic load. A proper SLAM is almost always the first and best solution.
How do I know when the SLAM is really done?
The SLAM is complete only when all three criteria are met simultaneously: (1) water is crystal clear with no visible algae, (2) combined chlorine (CC) is below 0.5 ppm on a FAS-DPD test, and (3) the overnight chlorine loss test (OCLT) shows less than 1 ppm drop from after-sunset to pre-sunrise. All three must pass on the same night. Missing any one means the SLAM must continue.
What happens after SLAM is complete?
Allow FC to drop naturally to the normal target range for your CYA level (typically 3–7 ppm for most outdoor pools). Test and balance pH, alkalinity, and calcium hardness — these may have drifted during the SLAM. Deep-clean your filter. Resume your normal chlorination routine. Use our chlorine calculator for regular dosing going forward.
How much liquid chlorine do I need to SLAM my pool?
For pools under 20,000 gallons, start with 10 gallons of liquid chlorine (10–12% sodium hypochlorite). For pools over 20,000 gallons, start with 15 gallons. You will need to top up multiple times per day — buy enough for 3–4 days and return unused jugs. Use the calculator above for your exact first-dose amount.
How long does the SLAM process take?
Light algae (green tint, slightly cloudy): 1–3 days. Moderate algae (clearly green): 3–5 days. Heavy algae (dark green or black-green): 5–10 days. The SLAM is not finished until you pass all three completion tests: crystal clear water, combined chlorine (CC) below 0.5 ppm, and overnight chlorine loss (OCLT) under 1 ppm. Do not stop early.
Can I swim during a SLAM?
Yes, with two conditions: you can clearly see the bottom of the deep end, AND your FC is between the minimum safe level for your CYA and your SLAM target. Do not swim if the water is green or cloudy at any FC level. For a 40 ppm CYA pool, swimming is possible when water is clear and FC is between 3 ppm (minimum) and 16 ppm (SLAM target).
What CYA level is best for SLAM?
SLAM is most effective when CYA is between 30–40 ppm. This keeps the SLAM target FC manageable (12–16 ppm) while still protecting chlorine from UV. Above 70 ppm CYA, the SLAM target becomes very high (28+ ppm) and requires large volumes of chlorine. If CYA is above 90 ppm, do a partial drain and dilute to below 70 ppm using our stabilizer calculator before starting.