Pool season · North Dakota
Bismarck Pool Season: the Year on One Bar
Last updated: July 25, 2026 · dates · water temp
Bismarck'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.
Bismarck, ND — 45 swim days a yearReading it takes ten seconds: 52 dots, one per week. Solid teal is Bismarck's swim season; the hollow teal run before it (May 14 to June 4) is when to open; the hollow amber run after is when to close, with September 19 as the deadline; gray is the covered season. Today's marker shows where you stand in that cycle right now.
The Bismarck pool calendar
Opening — target May 21
Open by May 21 — a two-week head start on the June 4 crossing keeps startup chemistry small and beats the local service rush.
Closing — deadline September 19
The window opens September 9 once water leaves the algae zone; the model deadline is September 19, a week ahead of the September 30 freeze normal.
Estimated water temperature by month
Monthly averages of the site's water model run on Bismarck'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 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 15° | 14° | 23° | 36° | 49° | 60° | 68° | 70° | 64° | 52° | 36° | 23° |
The curve peaks near 70°F — comfortable-water months here usually involve a heater or a solar cover.
Every station tells its own story: compare this curve with Fargo's pool season or jump back to the North Dakota season index to see the statewide spread.
Bismarck pool season FAQ
When should I open my pool in Bismarck, ND?
The site model recommends opening by May 21 — about two weeks before Bismarck's 7-day mean temperature crosses 61°F, while the water is still too cold for algae. The step-by-step sequence is in the Bismarck opening guide.
When should I close my pool in Bismarck?
Winterize by September 19. The model holds the deadline a week ahead of Bismarck's NOAA 50% first-freeze date, so the plumbing is protected before the first hard night. The full sequence is in the Bismarck closing guide.
How long is the swim season in Bismarck?
Bismarck averages 45 days a year with normal highs of 80°F or better — right on the statewide median. The North Dakota season index ranks all 2 covered cities by the same metric.
How warm does pool water get in Bismarck?
An unheated pool near Bismarck typically peaks around 70°F in high summer and bottoms out near 13°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 Bismarck hits the closing window
Data: NOAA 1991–2020 climate normals via Bismarck 2.4 NNW (1.9 mi); live outlook by Open-Meteo. Attribution: sources · assumptions: methodology.