Warm enough to swim? St. Louis month by month
| Month | Daily high | Daily average | Verdict |
|---|---|---|---|
| January | 41°F | 33°F | Off-season |
| February | 46°F | 37°F | Off-season |
| March | 56°F | 47°F | Off-season |
| April | 68°F | 58°F | Heater territory |
| May | 77°F | 68°F | Brisk — fine in a warm spell |
| June | 86°F | 77°F | Prime swimming |
| July | 90°F | 81°F | Prime swimming |
| August | 89°F | 79°F | Prime swimming |
| September | 82°F | 72°F | Comfortable |
| October | 70°F | 60°F | Heater territory |
| November | 56°F | 48°F | Off-season |
| December | 45°F | 37°F | Off-season |
Verdicts follow the daily average: 68°F+ is comfortable for most people, 75°F+ is prime, and below 62°F you're in heater territory. An unheated pool's water tends to sit near the recent daily average, a touch above it in spring and below in fall.
Reading the St. Louis pool year
The St. Louis season splits into three acts. Opening (Apr 28 – May 16): clear the cover, rebalance, and shock — our dosing calculator turns your first test kit reading into exact amounts. Midseason: weekly testing and small corrections, with chlorine demand peaking around July. Closing (Sep 24 – Oct 10): wait for water to hold below ~60°F, balance, then cover — algae is nearly dormant by then, which is what makes spring easy.
Frequently asked questions
When should I open my pool in St. Louis?
Most St. Louis owners are happy opening in the Apr 28 – May 16 stretch — that's when daily average air temperature climbs through the low-to-mid 60s°F, which puts the water near 70°F within a couple of weeks. Opening earlier is fine (algae actually prefers a late opening: a warm, covered pool with no chlorine is a petri dish); swimming just starts brisk.
When should I close my pool in St. Louis?
Around Sep 24 – Oct 10, as daily averages fall back through the low 60s°F. Waiting until water holds below ~60°F is smart — algae is nearly dormant there, so the pool goes into winter clean. Closing too early in warm weather is the classic recipe for a swampy spring opening.
How long is the swim season in St. Louis?
About 148 days without a heater — from roughly May 7 to Oct 2. A heat pump or gas heater typically adds 3–6 weeks on each end; a solar cover alone adds a week or two by holding overnight gains.
Where do these St. Louis dates come from?
From 1991–2020 NOAA temperature records at the St Louis Sci Ctr station, 6 km from central St. Louis. The working assumption: pools become pleasant as daily average air temperature passes 62–68°F, since water lags air by a week or two. A heater, cover, screened enclosure, or an unusually sunny yard all shift the real dates.