Warm enough to swim? Seattle month by month
| Month | Daily high | Daily average | Verdict |
|---|---|---|---|
| January | 49°F | 43°F | Off-season |
| February | 51°F | 44°F | Off-season |
| March | 55°F | 47°F | Off-season |
| April | 60°F | 52°F | Heater territory |
| May | 67°F | 58°F | Heater territory |
| June | 71°F | 63°F | Brisk — fine in a warm spell |
| July | 77°F | 67°F | Brisk — fine in a warm spell |
| August | 77°F | 68°F | Brisk — fine in a warm spell |
| September | 72°F | 63°F | Brisk — fine in a warm spell |
| October | 61°F | 54°F | Heater territory |
| November | 53°F | 46°F | Off-season |
| December | 48°F | 42°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 Seattle pool year
The Seattle season splits into three acts. Opening (Jun 11 – Jun 30): 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 August. Closing (Sep 2 – Sep 18): wait for water to hold below ~60°F, balance, then cover — algae is nearly dormant by then, which is what makes spring easy. With only ~63 unheated days, a heater earns its keep faster in Seattle than almost anywhere — size one honestly with the heater calculator before the season slips by.
Frequently asked questions
When should I open my pool in Seattle?
Most Seattle owners are happy opening in the Jun 11 – Jun 30 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 Seattle?
Around Sep 2 – Sep 18, 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 Seattle?
About 63 days without a heater — from roughly Jun 30 to Sep 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 Seattle dates come from?
From 1991–2020 NOAA temperature records at the Seattle Boeing Field station, 9 km from central Seattle. 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.