SLAM Pool Calculator

How the SLAM Pool Calculator Works

A SLAM dose is based on four numbers: pool volume, cyanuric acid (CYA), current free chlorine, and liquid-chlorine strength. The calculator first sets a temporary cleanup target at approximately 40% of the measured CYA level:

SLAM target free chlorine = CYA × 0.40

It then subtracts the current free-chlorine reading:

Required increase = SLAM target − current free chlorine

Finally, it converts that increase into the volume of liquid chlorine needed for your pool. The answer is a starting dose, not a guarantee. Existing algae, sunlight, debris, water temperature, and old chlorine can change the reading you get after circulation. Add the calculated amount as the product label directs, allow it to mix, and retest.

For example, a 20,000-gallon pool with 40 ppm CYA and 2 ppm free chlorine has a 16 ppm SLAM target and needs a 14 ppm increase. With fresh 10% liquid chlorine, the calculated dose is approximately 2.8 gallons.

SLAM Chlorine Target Chart

The calculator accepts your exact CYA result. This chart shows the same 40% relationship at common readings:

These are temporary cleanup targets, not normal maintenance levels or automatic swimming limits. An outdoor SLAM is generally easier to manage around 30–40 ppm CYA. If CYA is very high, get qualified advice before replacing water; improper draining can seriously damage a pool.

How to Use the Calculator During a SLAM

1. Test and prepare first

Use a high-range FAS-DPD kit to measure free chlorine, combined chlorine, pH, and CYA. Confirm the pool volume and the sodium-hypochlorite percentage printed on the bottle. Adjust pH to approximately 7.2–7.4 before raising chlorine when that range is appropriate for your pool finish and product instructions.

Remove leaves and other debris, brush all surfaces, verify circulation, and clean or backwash the filter according to its manufacturer. Never mix chlorine with acid or another chlorine product.

2. Calculate and add the initial dose

Enter the fresh test results above. Add only the calculated amount, with the pump running and using the application method, protective equipment, and separation times on the product label.

3. Retest and calculate the top-up

At the beginning, test free chlorine about every two hours while you are available. Enter the new free-chlorine reading without changing CYA, pool volume, or product strength. The calculator will return only the amount needed to restore the target—do not repeat the entire initial dose.

At minimum, test and adjust morning and evening. Keep brushing, vacuuming, and filtering throughout the process. Read the complete how to SLAM your pool guide for the physical-cleaning steps and troubleshooting checks.

4. Stop only after all three checks pass

Finish when the water is clear with no visible algae, combined chlorine is 0.5 ppm or lower, and overnight free-chlorine loss is 1.0 ppm or less. A monitor's ORP trend can tell you when to investigate, but it does not replace the FAS-DPD readings used for dosing or the overnight chlorine-loss test.

Why CYA Changes the Target

Cyanuric acid shields outdoor-pool chlorine from sunlight, but it also binds to part of the free chlorine. As CYA rises, a higher measured free-chlorine level is needed to provide the same cleanup reserve. That is why a universal instruction such as "shock to 10 ppm" can be too little for one pool and unnecessarily high for another.

Liquid chlorine is usually the simplest product for a SLAM because it raises free chlorine without adding more CYA. Dichlor and trichlor add stabilizer, which can keep raising the target during the cleanup. Cal-hypo does not add CYA, but it adds calcium and requires product- and surface-specific handling.

Do not add stabilizer just because the calculator warns that CYA is low. Confirm the result and consider the entire treatment plan first. CYA is difficult to lower without water replacement, while chlorine can be added again after a retest.

SLAM Pool Calculator FAQs

What free-chlorine level should I use for a SLAM?

This calculator uses a temporary free-chlorine target of approximately 40% of measured CYA. That gives 12 ppm FC at 30 ppm CYA, 16 ppm at 40 ppm CYA, and 20 ppm at 50 ppm CYA.

How often should I recalculate the dose?

Retest about every two hours at the beginning while you are available, then enter the new free-chlorine reading. At minimum, test and adjust morning and evening. Each result is a top-up from the current reading to the same CYA-based target.

Can I use the calculator for a saltwater pool?

Yes. The chlorine target and liquid-chlorine dose use the same math. Follow the salt system manufacturer's instructions during treatment, and disable automatic chlorine generation for the overnight loss test while keeping normal circulation running.

Why does chlorine strength matter?

One gallon of 12.5% sodium hypochlorite contains more available chlorine than one gallon of 10%, 7.5%, or 6% product. Select the percentage on the bottle. Liquid chlorine also degrades with heat and time, so an old jug may deliver less than its label suggests.

When is the SLAM finished?

All three checks must pass: clear water with no visible algae, combined chlorine at or below 0.5 ppm, and overnight free-chlorine loss of 1.0 ppm or less. Do not stop based only on appearance, elapsed time, or an ORP graph.