Carrier route record
WNSEA→DEN
Southwest Airlines · Seattle to Denver
- Flights
- 1,345
- 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 SEA to DEN sits in the middle of the pack with a C+. About seven flights in ten arrive on time by the DOT's definition, 9 points worse than the national rate of 77.6%. The late ones land roughly 50 minutes behind schedule. That rests on 1,345 flights. The strongest carrier on the route with a full sample, United Airlines, flew 1,905 at 87.5% on time.
United Airlines is the most reliable of the four carriers covered here at 87.5% on time across 1,905 flights.
Key figures
On time
69.0%
Arrivals under 15 minutes late, of 1,325 completed flights
Avg delay
52 min
Across delayed arrivals only, not all flights
Cancelled
0.59%
National rate is 1.81%
Whole route
77.8%
Every carrier on SEA to DEN, 6,317 flights
These figures cover 1,345 flights between July 2025 and June 2026. On-time means an arrival less than 15 minutes after the scheduled gate time, the same threshold the Department of Transportation uses in its own reports. Cancelled and diverted flights are excluded from the on-time rate and counted separately, which is why the cancellation column matters on its own.
Carrier by carrier
Southwest against every carrier on SEA to DEN
Southwest flew 1,345 of the 6,317 flights on SEA to DEN, about 21% of it. Its 69.0% on time is 9 points worse than the route as a whole, which runs 77.8%. Ranked by on-time rate it is last of six carriers on the route, where the range runs from United at 87.5% to Southwest at 69.0%. 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 87.5% of the time; Delta Air Lines manages 73.3%. Sample sizes sit next to every figure, because a two-point gap across 869 flights means less than a ten-point one.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| United Airlines | 1,905 | 87.5% | 73 min | 0.84% | A+ |
| SkyWest Airlinesthin sample | 221 | 85.0% | 60 min | 0.45% | A+ |
| Alaska Airlines | 1,558 | 75.9% | 50 min | 0.83% | B+ |
| Frontier Airlines | 419 | 74.9% | 75 min | 1.91% | C |
| Delta Air Lines | 869 | 73.3% | 58 min | 0.92% | C+ |
| Southwest Airlines (this page) | 1,345 | 69.0% | 52 min | 0.59% | C+ |
The spread between United and Delta is 14 points, wide enough that it should decide the booking. On a hundred trips that is 14 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 64 points, against 14 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am382 flights95.5%
- 8am–11am314 flights82.1%
- 11am–2pm214 flights55.8%
- 2pm–5pm330 flights43.9%
- 5pm–8pm67 flights31.3%
- 8pm–5am38 flights44.7%
Scale runs 25% to 100%. Rows with fewer than 30 flights show a dash.
The 5pm–8pm window is where this operation falls apart: 31.3% on time, against 95.5% 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 382 flights scheduled in it.
Across the year
Southwest on SEA to DEN through the year
June is the worst month at 52.9% on time, 37 points below January, the best at 90.3%. 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 151 flights.
Across the week, Saturday is the strongest day at 78.5% on time and Sunday the weakest at 62.3%. 16 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 21,000 reported delay minutes.
- 38%Late-arriving aircraft
- 30%National Airspace System
- 28%Carrier
- 3%Extreme weather
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 21,000 reported delay minutes here, that single category outweighs weather by 11 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 21,311 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 95.5% on time here.
Cancellations are rare at 0.59%, 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
7.8% 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 50 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 61.7% on time across 1,341 flights, 7 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 7 points of difference.
The sample behind these figures is 1,345 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 SEA to DEN
Put together: the calendar is the biggest lever here, not the carrier or the hour. January runs 90.3% on time and June 52.9%, a swing of 37 points. If the dates move at all, move them out of June before you think about anything else. 1,325 completed flights over 12 months sit behind that.
Related
Where to go next
- SEA to DEN, every carrier
- Southwest on DEN to SEA, the return leg
- United Airlines on SEA to DEN87.5%
- SkyWest Airlines on SEA to DEN85.0%
- Alaska Airlines on SEA to DEN75.9%
- Frontier Airlines on SEA to DEN74.9%
- Delta Air Lines on SEA to DEN73.3%
- Southwest Airlines network-wide record
- Seattle-Tacoma International (SEA) delay statistics
- Denver International (DEN) delay statistics
- Why the first flight of the morning is statistically safest
- Same route, different airline: where the carrier you pick matters most
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.