Chicago Marathon Start Waves: What the Gap Is Worth in Heat
Chicago's first and last waves start an hour apart. Our forecast prices that gap at about 1 to 3 seconds per mile. Here is the math.
Chicago sends its runners off in three waves, and the first and last start an hour apart on the clock. Most advice treats the wave as a logistics question. It is also a weather question, because the morning warms while the field is still on the course. This preview asks what the gap is worth in heat, using our race-day forecast.
Three starts, one hour apart
The official race timeline lists the men's and women's professional start at 7:30 AM and the High Performance program at 7:32. The mass waves follow: Wave 1 at 7:35 AM, Wave 2 at 8:00 AM and Wave 3 at 8:35 AM. Corrals close 15 minutes before each wave, at 7:20, 7:45 and 8:10. Wheelchair and handcycle starts begin earlier, from 7:20.
Plan around 7:35 for Wave 1. The 7:30 time often quoted online is the professional start.
What the forecast says the morning does
Our race-day forecast starts cool and warms through the start window. The hourly air temperature is 61.3°F at 7 AM, 62.5°F at 8 and 63.7°F at 9, with dew points in the mid 50s. The wind comes from the south at about 11 mph across those hours. Measured at each wave's gun, Wave 3 starts about 1.3°F warmer than Wave 1 (61.9°F against 63.2°F in the race-day grid). This is the forecast as it stood on October 4. It is not an observation of what the morning will be.
What the wave gap is worth
We priced each wave with the shared race-average engine, the same code the race page uses. For each start time, it averages the hours a runner is on course, weighting each hour by the time spent in it, then turns the heat penalty into seconds per mile at the goal pace. Here is Wave 1 against Wave 3 at three goals.
- 3:30 goal: Wave 1 loses 6.5 minutes (14.9 sec/mi) to heat. Wave 3 loses 7.2 minutes (16.6 sec/mi). The gap is 1.7 sec/mi.
- 4:15 goal: Wave 1 loses 9.1 minutes (20.8 sec/mi). Wave 3 loses 10.1 minutes (23.1 sec/mi). The gap is 2.3 sec/mi.
- 5:00 goal: Wave 1 loses 11.1 minutes (25.5 sec/mi). Wave 3 loses 12.3 minutes (28.1 sec/mi). The gap is 2.6 sec/mi.
Over 26.22 miles, 1.7 to 2.6 seconds per mile comes to 45 to 70 seconds, or about 0.7 to 1.1 minutes. The minute figures above are rounded, so they can differ by a tenth from the per-mile gap.
The goal matters more than the wave
Hold the wave fixed and the goal does most of the work. A Wave 1 runner pays 6.5 minutes of heat at a 3:30 and 11.1 minutes at a 5:00. That 4.6 minute spread is about four times the wave gap at the slower goal. Slower runners spend more hours on course, so they meet more of the warm late morning.
How to use this on race week
Run the wave you are given. In this forecast, moving from the last wave to the first is worth about a minute at any goal. That is too small to change a race plan by itself. The bigger choice is how you run the first hour, since the forecast warms about 1.2°F an hour through the start window and the effort you hold early costs more by mile 20.
Plan the first hour by effort. Our race pacing guide covers running by effort when the clock and the conditions disagree.
Check the forecast again the day before. The forecast moves, and the race page's forecast section follows the goal and wave you set.
These figures are the engine's price for the race-day forecast last updated October 4, 2026. They are not observed weather. We will check them against the conditions on October 11 and post what we find.
In this forecast the wave is worth about a minute. Know your clock, plan the first hour around the day you will actually run, and look at the forecast once more before you leave for Grant Park.