Pool care after rain
After heavy rain, test free chlorine (FC) and pH before you add anything: the usual fix is a chlorine top-up and a longer filter run.
Always follow your product’s label directions. Pool and spa chlorine, bromine, shock and algaecides are EPA-registered pesticides: use products labeled for pool or spa use, and don’t swim or soak until levels are within the limit on your product’s label. If this page and a label disagree, the label wins.
On this page (7 sections)
Top up chlorine to your target. Rain itself dilutes a pool only a little: 2 inches replace about 3.6% of the water in a pool 4.5 ft deep on average. The dirt, pollen, and runoff it carries are what use up chlorine.
What rain changes
- Chlorine demand. Leaves, pollen, dust, and lawn runoff (sometimes with fertilizer) all react with chlorine. This is the biggest effect, and the reason pools turn cloudy or green a day or two after a storm.
- Dilution. Rainwater has no chlorine, stabilizer, calcium, or salt, so every level drops a little. See the table below: the drop is usually smaller than people expect.
- pH. Normal rain is slightly acidic, about pH 5.6 according to the EPA, because carbon dioxide dissolves in it. In a pool with normal alkalinity, the water's buffer absorbs most of that, so pH moves only slightly. Runoff and the chlorine you add afterward matter more.
- Water level. A big storm can raise the level above the skimmer, which stops it from skimming, or overflow the pool.
Yellow or green-yellow water right after a storm is often pollen or leaf tannins rather than algae. If it doesn't brush off the walls and chlorine holds steady, filter it out. If the color keeps coming back, check which algae it could be.
What to test
Test after the storm, once debris is out and the pump has run for an hour or two. The quick check at the top of this page takes the three numbers that matter most:
- Free chlorine (FC) against the minimum for your stabilizer (cyanuric acid, CYA) level. If it's below, add chlorine first. See your chlorine target.
- pH. If it dropped below 7.2, raise it. The guide to raising pH has doses.
- Total alkalinity (TA). Mostly relevant after a lot of rain on a pool that already had low TA.
Stabilizer, calcium, and salt need a retest only after very heavy or repeated storms, or after an overflow. Salt pools: check the salt reading before assuming the cell has a problem.
Dilution: inches of rain to % water replaced
Rain that falls on the water surface raises the level by the same number of inches. How much that dilutes the pool depends on how deep the water is. The share of new water is rain ÷ (water depth + rain):
| Rain | 42 in deep (above-ground) | 54 in average (inground) | FC 3 ppm becomes (54 in) | CYA 40 ppm becomes (54 in) |
|---|---|---|---|---|
| 1 in | 2.3% | 1.8% | 2.95 ppm | 39.3 ppm |
| 2 in | 4.5% | 3.6% | 2.89 ppm | 38.6 ppm |
| 3 in | 6.7% | 5.3% | 2.84 ppm | 37.9 ppm |
| 4 in | 8.7% | 6.9% | 2.79 ppm | 37.2 ppm |
| 6 in | 12.5% | 10.0% | 2.70 ppm | 36.0 ppm |
Rain on the water surface only. Runoff from decks and lawns adds more water and more dirt. We can't estimate it without knowing your yard. Levels shown are dilution only, before any chlorine is used up.
So a 2-inch storm barely changes stabilizer. If your FC went from 3 ppm to near zero, dilution isn't the reason. Demand from debris is. Treat it as a chlorine problem, and if the water turns cloudy, follow the steps for cloudy water.
How many gallons each inch of rain adds, for common sizes: 12 ft round, 71 gal; 18 ft round, 159 gal; 24 ft round, 282 gal; 12 × 24 ft, 180 gal; 16 × 32 ft, 319 gal; 20 × 40 ft, 499 gal. This is useful if you plan to pump the excess back out.
Overflow and lowering water level
For the skimmer to work, the water line should sit around the middle of the skimmer opening. After a big storm:
- pump the excess down with a filter "waste" setting (if your filter valve has one), a submersible pump, or a hose siphon, and send it where local rules allow;
- don't go below the skimmer opening, or the pump can draw air and run dry;
- above-ground pools: keep water below the top rail so the walls aren't strained, and check around the base for washed-out soil;
- clear skimmer and pump baskets, and backwash or rinse the filter; storm debris clogs them fast.
If the pool overflowed, you have lost more than dilution shows, since the mixed water that spilled took chemistry with it. Test CYA and salt again a day later. If the water went green after the storm, start the green water recovery plan.
Swimming in rain and storms
Thunder is the signal to get out. The CDC tells people to stay away from swimming pools during thunderstorms and not to resume outdoor activities for at least 30 minutes after the storm. The National Weather Service rule is similar: when thunder roars, go indoors. Get everyone out at the first thunder or lightning, and wait the full 30 minutes after the last rumble.
Light rain without thunder isn't a chemical hazard in itself. The practical risks are poor visibility, slippery decks, and a chlorine level that may be dropping. Test before the next swim.
FAQ
Is it safe to use a swimming pool during rain?
In light rain with no thunder or lightning, the rain itself doesn't create a chemical hazard. Get out at the first sign of thunder, keep the water clear enough to see the drain, and test chlorine after the rain stops.
Is it safe to swim during a thunderstorm?
No. The CDC advises staying away from swimming pools during thunderstorms and waiting at least 30 minutes after the storm before going back outside.
Why is my pool cloudy after rain?
Usually because runoff and debris used up the chlorine and added fine dirt for the filter to catch. Test free chlorine and pH, bring chlorine back to your target, clean the filter, and run it longer.