Pool season · California
Ontario Pool Season: the Year on One Bar
Last updated: July 25, 2026 · dates · water temp
Ontario's whole pool calendar, condensed: when the water warms, when it cools, and where today falls between those edges. Use the opening guide and closing guide for the step-by-step work; keep this page for the dates.
Ontario, CA — 164 swim days a yearThe bar compresses Ontario's whole pool year into one lane rope: gray floats for the off-season, hollow teal for the March 4–March 25 opening window, solid teal for swim weeks, hollow amber for the closing stretch that ends November 28. Wherever the today marker sits, the nearest color change is your next appointment.
The Ontario pool calendar
Opening — target March 11
Open by March 11 — a two-week head start on the March 25 crossing keeps startup chemistry small and beats the local service rush.
Closing — deadline November 28
Hold the cover until November 18, then winterize by November 28, a week ahead of the December 30 freeze normal. Late and cold beats early and warm.
Estimated water temperature by month
This is Ontario's typical-year water curve: the same EWMA model as the live widget, run on the 1991–2020 normals instead of this week's weather.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 56° | 56° | 59° | 62° | 66° | 70° | 77° | 80° | 80° | 74° | 66° | 58° |
Warmest water typically August–September (peaking near 80°F); heaters and covers shift any month up.
Every station tells its own story: compare this curve with Oceanside's pool season and Orange's — neighboring normals stations can shift the windows by a week or more. The full ranking lives on the California season index.
Ontario pool season FAQ
When should I open my pool in Ontario, CA?
Target March 11. That date backs off two weeks from the day Ontario's 7-day mean temperature reaches 61°F, so you start up in cold, algae-resistant water. Work the full checklist in the Ontario opening guide.
When should I close my pool in Ontario?
The closing deadline is November 28 — late enough that the water has cooled out of the algae zone, early enough to beat Ontario's first-freeze normal by about a week. See the Ontario closing guide for the winterizing steps.
How long is the swim season in Ontario?
Ontario averages 164 days a year with normal highs of 80°F or better — about 21 days earlier than the California median. The California season index ranks all 147 covered cities by the same metric.
How warm does pool water get in Ontario?
An unheated pool near Ontario typically peaks around 80°F in high summer and bottoms out near 55°F in winter, spending roughly 2 months at or above the 78°F comfort line most swimmers prefer. The month-by-month curve above shows the whole cycle; the methodology page explains the ±5°F error bars.
Email me when Ontario hits the closing window
Data: NOAA 1991–2020 climate normals via Ontario International Airport (1.2 mi); live outlook by Open-Meteo. Attribution: sources · assumptions: methodology.