Pool shock calculator
Shock is a chlorine level, not a product, and that level is set by your CYA. Enter CYA and your chlorine now: you get your SLAM level, the dose to reach it, and how to tell when you're done.
Example numbers are filled in so you can see an answer. Type your own over them.
Don't know your gallons? Work it out here →
Exact geometry at 7.48 gal/ft³. Kidney shapes, diving hoppers and salt/turnover figures live in the full Pool Volume Calculator.
SLAM level by CYA
| CYA | Everyday target | SLAM level | 12.5% liquid, 3 ppm → SLAM, 15,000 gal (56,781 L) |
|---|---|---|---|
| 30 | 4–6 | 12 | 1.12 gal (4.5 qt; 4.2 L) |
| 40 | 5–7 | 16 | 1.62 gal (6.5 qt; 6.1 L) |
| 50 | 6–8 | 20 | 2.12 gal (8.5 qt; 8.0 L) |
| 60 | 7–9 | 24 | 2.62 gal (10.5 qt; 9.9 L) |
| 70 | 8–10 | 28 | 3.12 gal (12.5 qt; 12 L) |
| 80 | 9–11 | 31 | 3.50 gal (14 qt; 13 L) |
| 90 | 10–12 | 35 | 4.00 gal (15 L) |
| 100 | 11–13 | 39 | 4.50 gal (17 L) |
Why most shock calculators get the level wrong
A shock calculator with a fixed "10 ppm" preset gives the same answer at CYA 20 and CYA 80. At CYA 80 the everyday target is already 9–11 ppm, so "10 ppm" is not a shock at all, and the algae keeps winning while you keep buying bags. Your CYA sets the level. If your CYA is very high, the cheapest fix is often a partial drain first; the CYA calculator tells you how much.
How to run a SLAM
- Test CYA, then set the calculator above.
- Raise free chlorine to the SLAM level with liquid chlorine, pump running.
- Retest and top up several times a day. Brush walls and floor daily. Run the pump around the clock.
- Stop only when all three finish tests pass: combined chlorine 0.5 or lower, overnight loss 1 ppm or less, clear water.
You need a FAS-DPD drop kit for this: strips and the little OTO/DPD block cannot read shock levels or a 1 ppm overnight loss. The full walkthrough: clearing a green pool. Mustard algae needs more: mustard algae. And why a weekly shock habit is a myth: you don't need to shock weekly.
SLAM levels and finish criteria: Trouble Free Pool (FC/CYA chart; SLAM process). Doses dimensional from label strength. Estimates; confirm with a FAS-DPD test.
FAQ
How much shock do I put in my pool?
Enough to raise free chlorine to the SLAM level for your CYA, then keep it there until the pool passes the finish tests. At CYA 40 the SLAM level is 16 ppm; at CYA 60 it is 24. In 15,000 gallons, going from 3 ppm to 16 ppm takes about 1.62 gal (6.5 qt; 6.1 L) of 12.5% liquid chlorine.
Why does my shock level depend on CYA?
CYA holds most free chlorine in reserve, so the more CYA in the water, the more chlorine it takes to reach the same killing strength. A fixed "10 ppm shock" is a real shock at CYA 20 and barely above the everyday target at CYA 60.
How do I know the shock worked?
Trouble Free Pool's three finish tests: combined chlorine is 0.5 ppm or lower, you lose 1 ppm of free chlorine or less overnight (test after sunset and before sunrise), and the water is clear. Until all three pass, keep free chlorine at the SLAM level.
Can I use shock bags instead of liquid chlorine?
You can, but each adds something that stays. Dichlor shock adds about 0.9 ppm of CYA per 1 ppm of chlorine, which raises the SLAM level you are aiming at. Cal-hypo adds about 0.8 ppm of calcium per 1 ppm. Liquid chlorine adds neither, which is why a SLAM is run with liquid.
Should I shock my pool every week?
No. Shock when there is a reason: algae, cloudy water, or combined chlorine above 0.5. If free chlorine stays in the target band for your CYA, there is usually nothing to shock.
What is the mustard algae shock level?
About 1.5 times your normal SLAM level, held for about 24 hours after a normal SLAM, while you disinfect everything that touched the water: brushes, poles, toys, floats and swimsuits. The calculator shows it when you choose mustard algae.