Carrier route record
WNDEN→SEA
Southwest Airlines · Denver to Seattle
- Flights
- 1,341
- Period
- July 2025 to June 2026
- Share of route
- 21%
- Carriers
- 6
Historical statistics for July 2025 to June 2026, not live status. For today’s flights, check with Southwest Airlines.
Southwest on DEN to SEA draws a D+. Roughly one flight in three arrives at least 15 minutes late, and the typical late arrival is roughly 45 minutes behind schedule. Build slack into anything you connect to. That rests on 1,341 flights. The strongest carrier on the route with a full sample, United Airlines, flew 1,903 at 80.0% on time.
United Airlines is the most reliable of the four carriers covered here at 80.0% on time across 1,903 flights.
Key figures
On time
61.7%
Arrivals under 15 minutes late, of 1,336 completed flights
Avg delay
47 min
Across delayed arrivals only, not all flights
Cancelled
0.30%
National rate is 1.81%
Whole route
73.9%
Every carrier on DEN to SEA, 6,309 flights
These figures cover 1,341 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 DEN to SEA
Southwest flew 1,341 of the 6,309 flights on DEN to SEA, about 21% of it. Its 61.7% on time is 12 points worse than the route as a whole, which runs 73.9%. Ranked by on-time rate it is last of six carriers on the route, where the range runs from United at 80.0% to Southwest at 61.7%. One of those carriers flew fewer than 365 flights here, so treat their positions as rough.
The choice of carrier is not cosmetic here. United Airlines arrives on time 80.0% of the time; Alaska Airlines manages 73.8%. Sample sizes sit next to every figure, because a two-point gap across 1,558 flights means less than a ten-point one.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| United Airlines | 1,903 | 80.0% | 55 min | 0.63% | B+ |
| Delta Air Lines | 868 | 79.2% | 89 min | 0.92% | B+ |
| SkyWest Airlinesthin sample | 220 | 75.8% | 70 min | 0.45% | B+ |
| Frontier Airlines | 419 | 74.9% | 60 min | 1.91% | C |
| Alaska Airlines | 1,558 | 73.8% | 57 min | 0.96% | C+ |
| Southwest Airlines (this page) | 1,341 | 61.7% | 47 min | 0.30% | D+ |
United leads Alaska by 6 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 34 points, against 6 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am56 flights83.9%
- 8am–11am364 flights76.5%
- 11am–2pm126 flights59.5%
- 2pm–5pm210 flights57.1%
- 5pm–8pm232 flights50.0%
- 8pm–5am353 flights54.0%
Scale runs 40% to 90%. Rows with fewer than 30 flights show a dash.
The 5pm–8pm window is where this operation falls apart: 50.0% on time, against 83.9% 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 8am–11am window is the busiest, with 364 flights scheduled in it.
Across the year
Southwest on DEN to SEA through the year
December is the weak month at 37.9% on time, against 77.4% in January. Winter delays cluster: a single storm system can account for most of a month's shortfall, so an average December hides both very good weeks and genuinely unflyable days. July was the busiest month here, with 151 flights.
Across the week, Wednesday is the strongest day at 65.4% on time and Thursday the weakest at 57.0%. 8 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 47% of the 24,000 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.
- 47%Late-arriving aircraft
- 34%Carrier
- 18%National Airspace System
- 1%Extreme weather
- 0%Security
47% 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 47 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 23,868 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 83.9% on time here.
Cancellations are rare at 0.30%, 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
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 roughly 45 minutes, which a normal layover absorbs.
The 1,024 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 69.0% on time across 1,345 flights, 7 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 7 points of difference.
The sample behind these figures is 1,341 flights, enough that the headline rate is stable to within roughly a point. The hour-block and month splits divide that further, so read the thinner bars with more caution than the headline.
What to do with this
Booking Southwest on DEN to SEA
Put together: the calendar is the biggest lever here, not the carrier or the hour. January runs 77.4% on time and December 37.9%, a swing of 40 points. If the dates move at all, move them out of December before you think about anything else. 1,336 completed flights over 12 months sit behind that.
Related
Where to go next
- DEN to SEA, every carrier
- Southwest on SEA to DEN, the return leg
- United Airlines on DEN to SEA80.0%
- Delta Air Lines on DEN to SEA79.2%
- SkyWest Airlines on DEN to SEA75.8%
- Frontier Airlines on DEN to SEA74.9%
- Alaska Airlines on DEN to SEA73.8%
- Southwest Airlines network-wide record
- Denver International (DEN) delay statistics
- Seattle-Tacoma International (SEA) 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.