Carrier route record
WNDEN→BOS
Southwest Airlines · Denver to Boston
- Flights
- 910
- Period
- July 2025 to June 2026
- Share of route
- 23%
- Carriers
- 4
Historical statistics for July 2025 to June 2026, not live status. For today’s flights, check with Southwest Airlines.
Southwest on DEN to BOS draws a D. Roughly one flight in two 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 910 flights. The strongest carrier on the route with a full sample, United Airlines, flew 1,911 at 77.1% on time.
United Airlines is the most reliable of the three carriers covered here at 77.1% on time across 1,911 flights.
Key figures
On time
59.7%
Arrivals under 15 minutes late, of 899 completed flights
Avg delay
66 min
Across delayed arrivals only, not all flights
Cancelled
1.10%
National rate is 1.81%
Whole route
71.4%
Every carrier on DEN to BOS, 3,903 flights
These figures cover 910 flights between July 2025 and June 2026. Beyond the headline rate, 16.0% 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 DEN to BOS
Southwest flew 910 of the 3,903 flights on DEN to BOS, about 23% of it. Its 59.7% on time is 12 points worse than the route as a whole, which runs 71.4%. Ranked by on-time rate it is last of four carriers on the route, where the range runs from United at 77.1% to Southwest at 59.7%.
The choice of carrier is not cosmetic here. United Airlines arrives on time 77.1% of the time; JetBlue Airways manages 68.6%. Sample sizes sit next to every figure, because a two-point gap across 682 flights means less than a ten-point one.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| United Airlines | 1,911 | 77.1% | 70 min | 0.84% | B+ |
| Delta Air Lines | 400 | 75.8% | 56 min | 1.50% | B |
| JetBlue Airways | 682 | 68.6% | 82 min | 2.05% | C |
| Southwest Airlines (this page) | 910 | 59.7% | 66 min | 1.10% | D |
United leads JetBlue by 9 points. Real, but small enough that a better departure time on the weaker carrier can outweigh it.
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 20 points, against 9 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am7 flights—
- 8am–11am326 flights69.9%
- 11am–2pm171 flights61.9%
- 2pm–5pm89 flights51.7%
- 5pm–8pm312 flights50.0%
- 8pm–5am5 flights—
Scale runs 40% to 75%. Rows with fewer than 30 flights show a dash.
The 5pm–8pm window is where this operation falls apart: 50.0% on time, against 69.9% for the 8am–11am 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 8am–11am window is the busiest, with 326 flights scheduled in it.
Across the year
Southwest on DEN to BOS through the year
Delays peak in September, when the on-time rate falls to 32.9%. December is the strongest month at 89.2%. Moving a flexible trip across that boundary buys 56 points. July was the busiest month here, with 88 flights.
Across the week, Saturday is the strongest day at 78.6% on time and Tuesday the weakest at 55.3%. 23 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 24,000 reported delay minutes.
- 38%Late-arriving aircraft
- 37%National Airspace System
- 24%Carrier
- 1%Extreme weather
- 0%Security
38% 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 24,000 reported delay minutes here, that single category outweighs weather by 38 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 23,737 reported delay minutes.
37% 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 8,681 minutes out of 24,000, against a national NAS share of 21%.
Cancellations sit at 1.10% against a national 1.81%, which is ordinary. Diversions add another 0.11%.
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. 16.0% 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.
At 1,754 miles this is transcontinental flying, and long routes get a quiet structural advantage. Scheduled block time on a flight this length carries more padding in absolute minutes, so a twenty-minute late departure often still becomes an on-time arrival. Read the on-time rate here as slightly flattering next to a short-haul route with the same operational quality.
The return leg is a different operation and performs like one: it runs 64.8% on time across 910 flights, 5 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 5 points of difference.
One caution: this is a thin sample. 910 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 DEN to BOS
Put together: treat all of this as provisional. 910 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 59.7% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 899 completed flights over 12 months sit behind that.
Related
Where to go next
- DEN to BOS, every carrier
- Southwest on BOS to DEN, the return leg
- United Airlines on DEN to BOS77.1%
- Delta Air Lines on DEN to BOS75.8%
- JetBlue Airways on DEN to BOS68.6%
- Southwest Airlines network-wide record
- Denver International (DEN) 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.