Carrier route record
WNSTL→BOS
Southwest Airlines · St. Louis to Boston
- Flights
- 934
- 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 STL to BOS draws a C+. About one flight in three arrives at least 15 minutes late, and the typical late arrival is about an hour behind schedule. Build slack into anything you connect to. That rests on 934 flights, with a median arrival 3 minutes late.
The national on-time rate over the same period is 77.6%, so Southwest on STL to BOS runs 11 points behind the country as a whole. Across a thousand flights that is about 113 extra late arrivals.
Key figures
On time
66.3%
Arrivals under 15 minutes late, of 925 completed flights
Avg delay
59 min
Across delayed arrivals only, not all flights
Cancelled
0.96%
National rate is 1.81%
Whole route
70.3%
Every carrier on STL to BOS, 1,610 flights
These figures cover 934 flights between July 2025 and June 2026. Beyond the headline rate, 10.6% 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 STL to BOS
Southwest flew 934 of the 1,610 flights on STL to BOS, most of the route. Its 66.3% on time is 4 points worse than the route as a whole, which runs 70.3%. Ranked by on-time rate it is last of three carriers on the route, where the range runs from Republic at 76.0% to Southwest at 66.3%. 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 | 76.0% | 88 min | 3.10% | B− |
| Envoy Air | 418 | 75.9% | 79 min | 1.67% | B |
| Southwest Airlines (this page) | 934 | 66.3% | 59 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 38 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–8am38 flights86.8%
- 8am–11am264 flights87.0%
- 11am–2pm74 flights63.5%
- 2pm–5pm203 flights48.8%
- 5pm–8pm248 flights62.6%
- 8pm–5am107 flights51.4%
Scale runs 40% to 90%. Rows with fewer than 30 flights show a dash.
Book the morning departure. Flights leaving between 8am–11am arrive on time 87.0% of the time, against 48.8% for the afternoon window. That is 38 points of difference for no extra money, and it comes from the same aircraft having had no earlier flight to fall behind on. The 8am–11am window is the busiest, with 264 flights scheduled in it.
Across the year
Southwest on STL to BOS through the year
Delays peak in October, when the on-time rate falls to 50.5%. February is the strongest month at 86.0%. Moving a flexible trip across that boundary buys 36 points. July was the busiest month here, with 92 flights.
Across the week, Tuesday is the strongest day at 71.3% on time and Sunday the weakest at 60.6%. 11 points is a smaller lever than departure hour, but it is free.
Delay causes
Why Southwest runs late on this route
Carriers report a reason for every delay minute they cause, in five DOT categories. The split below is by minutes, not by number of flights, so a handful of very long weather delays weighs as much as many short ones. It covers 18,000 reported delay minutes.
- 39%Late-arriving aircraft
- 34%National Airspace System
- 24%Carrier
- 2%Extreme weather
- 1%Security
39% 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 18,000 reported delay minutes here, that single category outweighs weather by 17 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 18,240 reported delay minutes.
34% of delay minutes are charged to the National Airspace System: air traffic volume, runway configuration, ground stops. No carrier can price its way out of that, which is why airport choice moves the number here more than carrier choice does. That is 6,216 minutes out of 18,000, against a national NAS share of 21%.
Cancellations sit at 0.96% against a national 1.81%, which is ordinary. Diversions add another 0.00%.
Connections and stage length
What a delay here costs
For anyone connecting, the number to weigh is not the on-time rate but the tail behind it. 10.6% of arrivals land a full hour or more behind schedule, against 8.1% nationally. A 45-minute layover does not survive that, and no amount of walking quickly changes it. Ninety minutes is the honest minimum here, and two hours if the onward flight is on a separate ticket.
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 72.4% on time across 935 flights, 6 points above 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. 934 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 STL to BOS
Put together: treat all of this as provisional. 934 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 66.3% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 925 completed flights over 12 months sit behind that.
Related
Where to go next
- STL to BOS, every carrier
- Southwest on BOS to STL, the return leg
- Republic Airways on STL to BOS76.0%
- Envoy Air on STL to BOS75.9%
- Southwest Airlines network-wide record
- St. Louis Lambert International (STL) delay statistics
- Boston Logan International (BOS) delay statistics
- Why the first flight of the morning is statistically safest
- How often a connection fails at the major US hubs
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.