Carrier route record
WNBOS→DEN
Southwest Airlines · Boston to Denver
- 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 BOS to DEN draws a D+. About one flight in three arrives at least 15 minutes late, and the typical late arrival is roughly 55 minutes 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 1,908 at 83.0% on time.
United Airlines is the most reliable of the three carriers covered here at 83.0% on time across 1,908 flights.
Key figures
On time
64.8%
Arrivals under 15 minutes late, of 898 completed flights
Avg delay
54 min
Across delayed arrivals only, not all flights
Cancelled
0.88%
National rate is 1.81%
Whole route
76.5%
Every carrier on BOS to DEN, 3,899 flights
These figures cover 910 flights between July 2025 and June 2026. Beyond the headline rate, 10.2% 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 DEN
Southwest flew 910 of the 3,899 flights on BOS to DEN, about 23% of it. Its 64.8% on time is 12 points worse than the route as a whole, which runs 76.5%. Ranked by on-time rate it is last of four carriers on the route, where the range runs from United at 83.0% to Southwest at 64.8%.
The choice of carrier is not cosmetic here. United Airlines arrives on time 83.0% of the time; JetBlue Airways manages 70.9%. Sample sizes sit next to every figure, because a two-point gap across 681 flights means less than a ten-point one.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| United Airlines | 1,908 | 83.0% | 64 min | 1.00% | B |
| Delta Air Lines | 400 | 81.4% | 46 min | 1.75% | B |
| JetBlue Airways | 681 | 70.9% | 60 min | 1.76% | C |
| Southwest Airlines (this page) | 910 | 64.8% | 54 min | 0.88% | D+ |
The spread between United and JetBlue is 12 points, wide enough that it should decide the booking. On a hundred trips that is 12 more arrivals inside the on-time window.
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 52 points, against 12 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am363 flights88.6%
- 8am–11am2 flights—
- 11am–2pm147 flights66.4%
- 2pm–5pm170 flights49.4%
- 5pm–8pm228 flights36.6%
- 8pm–5am—
Scale runs 30% to 95%. Rows with fewer than 30 flights show a dash.
The 5pm–8pm window is where this operation falls apart: 36.6% on time, against 88.6% 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 DEN through the year
June is the worst month at 54.0% on time, 24 points below November, the best at 78.1%. Summer afternoon convection and peak-season schedules land on the same weeks every year, so this is a pattern you can book around rather than a run of bad luck. July was the busiest month here, with 88 flights.
Across the week, Tuesday is the strongest day at 67.9% on time and Friday the weakest at 61.0%. 7 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 51% of the 17,000 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.
- 51%Late-arriving aircraft
- 23%National Airspace System
- 21%Carrier
- 4%Extreme weather
51% 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 17,000 reported delay minutes here, that single category outweighs weather by 13 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 17,005 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 88.6% on time here.
Cancellations are rare at 0.88%, below the national 1.81% and low enough to earn the plus on the grade. Roughly one flight in 100 is called off; the risk here is arriving late, not not arriving.
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. 10.2% 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 59.7% on time across 910 flights, 5 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 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 BOS to DEN
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 64.8% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 898 completed flights over 12 months sit behind that.
Related
Where to go next
- BOS to DEN, every carrier
- Southwest on DEN to BOS, the return leg
- United Airlines on BOS to DEN83.0%
- Delta Air Lines on BOS to DEN81.4%
- JetBlue Airways on BOS to DEN70.9%
- Southwest Airlines network-wide record
- Boston Logan International (BOS) delay statistics
- Denver International (DEN) delay statistics
- Same route, different airline: where the carrier you pick matters most
- Why the first flight of the morning is statistically safest
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.