Carrier route record
WNBOS→STL
Southwest Airlines · Boston to St. Louis
- Flights
- 935
- Period
- July 2025 to June 2026
- Share of route
- 58%
- Carriers
- 3
Historical statistics for July 2025 to June 2026, not live status. For today’s flights, check with Southwest Airlines.
Southwest on BOS to STL sits in the middle of the pack with a C+. About seven flights in ten arrive on time by the DOT's definition, 5 points worse than the national rate of 77.6%. The late ones land about an hour behind schedule. That rests on 935 flights, with a median arrival 1 minute early.
The national on-time rate over the same period is 77.6%, so Southwest on BOS to STL runs 5 points behind the country as a whole. Across a thousand flights that is about 52 extra late arrivals.
Key figures
On time
72.4%
Arrivals under 15 minutes late, of 923 completed flights
Avg delay
62 min
Across delayed arrivals only, not all flights
Cancelled
0.96%
National rate is 1.81%
Whole route
76.5%
Every carrier on BOS to STL, 1,611 flights
These figures cover 935 flights between July 2025 and June 2026. Beyond the headline rate, 8.9% of arrivals land a full hour or more behind schedule. That is the number that wrecks a connection, and it is not visible in an on-time percentage alone.
Carrier by carrier
Southwest against every carrier on BOS to STL
Southwest flew 935 of the 1,611 flights on BOS to STL, most of the route. Its 72.4% on time is 4 points worse than the route as a whole, which runs 76.5%. Ranked by on-time rate it is last of three carriers on the route, where the range runs from Republic at 82.3% to Southwest at 72.4%. One of those carriers flew fewer than 365 flights here, so treat their positions as rough.
Only one carrier clears this site's 365-flight floor here, so there is no comparison to draw. The table below is the full picture.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| Republic Airwaysthin sample | 258 | 82.3% | 49 min | 3.10% | B− |
| Envoy Air | 418 | 82.2% | 74 min | 1.20% | B |
| Southwest Airlines (this page) | 935 | 72.4% | 62 min | 0.96% | C+ |
Departure window
When Southwest runs best on this route
Departure time matters more here than the choice of carrier. The gap between the best and worst window is 46 points, against half a point between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am363 flights93.3%
- 8am–11am31 flights90.3%
- 11am–2pm140 flights74.1%
- 2pm–5pm101 flights61.4%
- 5pm–8pm286 flights47.7%
- 8pm–5am14 flights—
Scale runs 40% to 100%. Rows with fewer than 30 flights show a dash.
The 5pm–8pm window is where this operation falls apart: 47.7% on time, against 93.3% for the 5am–8am slot. By evening the aircraft flying these rotations have absorbed a full day of small delays and there is no slack left to recover in. The 5am–8am window is the busiest, with 363 flights scheduled in it.
Across the year
Southwest on BOS to STL through the year
Delays peak in September, when the on-time rate falls to 60.0%. February is the strongest month at 92.9%. Moving a flexible trip across that boundary buys 33 points. July was the busiest month here, with 92 flights.
Across the week, Saturday is the strongest day at 80.3% on time and Sunday the weakest at 62.0%. 18 points is a smaller lever than departure hour, but it is free.
Delay causes
Why Southwest runs late on this route
One category carries most of the weight here. Late-arriving aircraft accounts for 62% of the 16,000 delay minutes carriers reported, and the remaining four split what is left. The split is by minutes rather than by flights, so a few very long delays count for as much as many short ones.
- 62%Late-arriving aircraft
- 22%Carrier
- 11%National Airspace System
- 5%Extreme weather
62% of delay minutes trace back to a late-arriving aircraft, meaning the plane was already behind before it got here. That is a cascade, and cascades start small and grow through the day. Across 16,000 reported delay minutes here, that single category outweighs weather by 12 to one. It is the strongest argument for booking the first departure of the morning, when the aircraft has been sitting at the gate overnight with nothing to inherit. The split is drawn from 15,782 reported delay minutes.
The practical consequence: the 5am–8am departure is the one that starts clean. Its aircraft has no earlier leg to be late from, which is exactly why that window shows 93.3% on time here.
Cancellations sit at 0.96% against a national 1.81%, which is ordinary. Diversions add another 0.32%.
Connections and stage length
What a delay here costs
8.9% of arrivals land an hour or more late, close to the national 8.1%. That is the figure that decides a connection, and it says a 60 to 75 minute layover is reasonable here while anything under 45 minutes is a bet. Most delays on this route last about an hour, which a normal layover absorbs.
The 1,047 miles between these two airports put this in the middle of the US network, around two and a half to three hours of block time. That is enough cruise to absorb a ten-minute late pushback and not enough to absorb thirty. It is also long enough that the aircraft usually flies three or four legs a day rather than six, which slows how fast delay compounds.
The return leg is a different operation and performs like one: it runs 66.3% on time across 934 flights, 6 points below this direction. Departure banks, aircraft rotations and the congestion at each end all differ, so scoring the two directions together would average away a real 6 points of difference.
One caution: this is a thin sample. 935 flights over twelve months is enough to clear this site's floor but not enough to pin the rate down tightly, and a single bad week moves it by more than a point. Treat the grade as a rough reading.
What to do with this
Booking Southwest on BOS to STL
Put together: treat all of this as provisional. 935 flights over twelve months is enough to clear this site's floor and not enough to be confident to better than a couple of points. The 72.4% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 923 completed flights over 12 months sit behind that.
Related
Where to go next
- BOS to STL, every carrier
- Southwest on STL to BOS, the return leg
- Republic Airways on BOS to STL82.3%
- Envoy Air on BOS to STL82.2%
- Southwest Airlines network-wide record
- Boston Logan International (BOS) delay statistics
- St. Louis Lambert International (STL) delay statistics
- Why the first flight of the morning is statistically safest
- The best time of day to fly, according to 7.04 million flight records
Source: US DOT Bureau of Transportation Statistics, on-time performance data, July 2025 to June 2026. Last updated 15 September 2026; recomputed monthly. How this data is collected.