Carrier route record
WNIAD→DEN
Southwest Airlines · Washington to Denver
- Flights
- 762
- Period
- July 2025 to June 2026
- Share of route
- 21%
- Carriers
- 2
Historical statistics for July 2025 to June 2026, not live status. For today’s flights, check with Southwest Airlines.
Southwest on IAD to DEN lands a B, about a point worse than the national rate of 77.6%. About one flight in four arrives 15 minutes late or worse, and those flights are late by roughly 55 minutes on average. That is a workable record, not a spotless one. That rests on 762 flights, with a median arrival 1 minute early.
The US average over the same twelve months is 77.6%, so there is little to separate Southwest on IAD to DEN from the country as a whole. The variation that matters here is internal: which carrier, and what time of day.
Key figures
On time
76.2%
Arrivals under 15 minutes late, of 752 completed flights
Avg delay
53 min
Across delayed arrivals only, not all flights
Cancelled
1.18%
National rate is 1.81%
Whole route
81.7%
Every carrier on IAD to DEN, 3,560 flights
These figures cover 762 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 IAD to DEN
Southwest flew 762 of the 3,560 flights on IAD to DEN, about 21% of it. Its 76.2% on time is 6 points worse than the route as a whole, which runs 81.7%. Ranked by on-time rate it is last of two carriers on the route, where the range runs from United at 83.2% to Southwest at 76.2%.
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 |
|---|---|---|---|---|---|
| United Airlines | 2,798 | 83.2% | 71 min | 0.46% | B+ |
| Southwest Airlines (this page) | 762 | 76.2% | 53 min | 1.18% | B |
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 half a point between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am300 flights92.3%
- 8am–11am29 flights—
- 11am–2pm26 flights—
- 2pm–5pm135 flights67.9%
- 5pm–8pm222 flights60.7%
- 8pm–5am50 flights58.3%
Scale runs 50% to 100%. Rows with fewer than 30 flights show a dash.
The 8pm–5am window is where this operation falls apart: 58.3% on time, against 92.3% 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 300 flights scheduled in it.
Across the year
Southwest on IAD to DEN through the year
July is the worst month at 64.2% on time, 23 points below November, the best at 87.0%. 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 84 flights.
Across the week, Saturday is the strongest day at 81.8% on time and Thursday the weakest at 70.7%. 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 9,405 reported delay minutes.
- 36%National Airspace System
- 31%Carrier
- 30%Late-arriving aircraft
- 3%Extreme weather
- 1%Security
36% 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 3,351 minutes out of 9,405, against a national NAS share of 21%. The split is drawn from 9,405 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 92.3% on time here.
Cancellations sit at 1.18% against a national 1.81%, which is ordinary. Diversions add another 0.13%.
Connections and stage length
What a delay here costs
6.1% 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 55 minutes, which a normal layover absorbs.
The 1,452 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 65.4% on time across 761 flights, 11 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 11 points of difference.
One caution: this is a thin sample. 762 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 IAD to DEN
Put together: treat all of this as provisional. 762 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 76.2% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 752 completed flights over 12 months sit behind that.
Related
Where to go next
- IAD to DEN, every carrier
- Southwest on DEN to IAD, the return leg
- United Airlines on IAD to DEN83.2%
- Southwest Airlines network-wide record
- Washington Dulles International (IAD) delay statistics
- Denver International (DEN) delay statistics
- The best time of day to fly, according to 7.04 million flight records
- Worst months to fly in the US, ranked by the data
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.