Pool season · Colorado
Boulder Pool Season: the Year on One Bar
Last updated: July 25, 2026 · dates · water temp
This is the one-page view of Boulder's pool year: both model windows, the estimated water-temperature curve, and today's live position on the season bar. The detailed checklists live on the opening and closing guides; this page is the calendar you check between them.
Boulder, CO — 98 swim days a yearEach dot on the bar is one week of Boulder's typical year, colored by what the model says that week is for: gray weeks are off-season, hollow teal marks the opening window (May 7–May 28), solid teal is swim season, and hollow amber is the closing window ending September 28. The black marker is today — its position against the colors is the entire executive summary.
The Boulder pool calendar
Opening — target May 14
Target May 14; the practical window runs May 7–May 28, and the 7-day mean crosses the 61°F threshold around May 28. Cold-water starts are the cheap ones.
Closing — deadline September 28
Hold the cover until September 22, then winterize by September 28, a week ahead of the October 5 freeze normal. Late and cold beats early and warm.
Estimated water temperature by month
Monthly averages of the site's water model run on Boulder's own normals — the shape of a typical year, not a forecast. The widget above shows where the current year actually sits.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 34° | 34° | 39° | 46° | 53° | 63° | 71° | 72° | 68° | 57° | 46° | 36° |
The curve peaks near 73°F — comfortable-water months here usually involve a heater or a solar cover.
Every station tells its own story: compare this curve with Aurora's pool season and Broomfield's — neighboring normals stations can shift the windows by a week or more. The full ranking lives on the Colorado season index.
Boulder pool season FAQ
When should I open my pool in Boulder, CO?
Open by May 14 — the model places Boulder's 61°F crossing about two weeks later, and every warm day you wait pushes the water toward the 65°F algae zone. The Boulder opening guide has the 12-step sequence.
When should I close my pool in Boulder?
Close by September 28: cool water keeps its winter chemistry, but freeze damage is the hard stop. The Boulder closing guide sequences the work backward from that date.
How long is the swim season in Boulder?
Boulder averages 98 days a year with normal highs of 80°F or better — right on the statewide median. The Colorado season index ranks all 19 covered cities by the same metric.
How warm does pool water get in Boulder?
An unheated pool near Boulder typically peaks around 73°F in high summer and bottoms out near 33°F in winter, never quite reaching the 78°F comfort line without a heater or solar cover. The month-by-month curve above shows the whole cycle; the methodology page explains the ±5°F error bars.
Email me when Boulder hits the closing window
Data: NOAA 1991–2020 climate normals via Boulder (2.4 mi); live outlook by Open-Meteo. Attribution: sources · assumptions: methodology.