Pool season · Illinois
Decatur Pool Season: the Year on One Bar
Last updated: July 25, 2026 · dates · water temp
Decatur'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.
Decatur, IL — 95 swim days a yearEach dot on the bar is one week of Decatur's typical year, colored by what the model says that week is for: gray weeks are off-season, hollow teal marks the opening window (April 17–May 8), solid teal is swim season, and hollow amber is the closing window ending October 12. The black marker is today — its position against the colors is the entire executive summary.
The Decatur pool calendar
Opening — target April 24
Target April 24; the practical window runs April 17–May 8, and the 7-day mean crosses the 61°F threshold around May 8. Cold-water starts are the cheap ones.
Closing — deadline October 12
Hold the cover until October 2, then winterize by October 12, a week ahead of the October 24 freeze normal. Late and cold beats early and warm.
Estimated water temperature by month
This is Decatur'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 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 28° | 28° | 37° | 48° | 59° | 69° | 75° | 75° | 71° | 61° | 48° | 36° |
The curve peaks near 76°F — comfortable-water months here usually involve a heater or a solar cover.
Every station tells its own story: compare this curve with Cicero's pool season and Elgin's — neighboring normals stations can shift the windows by a week or more. The full ranking lives on the Illinois season index.
Decatur pool season FAQ
When should I open my pool in Decatur, IL?
The site model recommends opening by April 24 — about two weeks before Decatur'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 Decatur opening guide.
When should I close my pool in Decatur?
Winterize by October 12. The model holds the deadline a week ahead of Decatur's NOAA 50% first-freeze date, so the plumbing is protected before the first hard night. The full sequence is in the Decatur closing guide.
How long is the swim season in Decatur?
Decatur averages 95 days a year with normal highs of 80°F or better — about 11 days earlier than the Illinois median. The Illinois season index ranks all 18 covered cities by the same metric.
How warm does pool water get in Decatur?
An unheated pool near Decatur typically peaks around 76°F in high summer and bottoms out near 27°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 Decatur hits the closing window
Data: NOAA 1991–2020 climate normals via Decatur Wtp (2.1 mi); live outlook by Open-Meteo. Attribution: sources · assumptions: methodology.