Carrier route record
WNIAD→DEN
Southwest Airlines · Washington to Denver
- Flights
- 596
- Period
- September 2025 to August 2026
- Share of route
- 18%
- Carriers
- 2
Historical statistics for September 2025 to August 2026, not live status. For today’s flights, check with Southwest Airlines.
Southwest on IAD to DEN lands a B, about a point better than the national rate of 77.3%. About one flight in five arrives 15 minutes late or worse, and those flights are late by roughly 50 minutes on average. That rests on 596 flights, with a median arrival 1 minute early.
The US average over the same twelve months is 77.3%, 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
78.4%
Arrivals under 15 minutes late, of 589 completed flights
Avg delay
48 min
Across delayed arrivals only, not all flights
Cancelled
1.01%
National rate is 1.89%
Whole route
83.1%
Every carrier on IAD to DEN, 3,387 flights
These figures cover 596 flights between September 2025 and August 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.
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Carrier by carrier
Southwest against every carrier on IAD to DEN
Southwest flew 596 of the 3,387 flights on IAD to DEN, about 18% of it. Its 78.4% on time is 5 points worse than the route as a whole, which runs 83.1%. Ranked by on-time rate it is last of two carriers on the route, where the range runs from United at 84.1% to Southwest at 78.4%.
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 | 2,791 | 84.1% | 68 min | 0.50% | B+ |
| Southwest Airlines (this page) | 596 | 78.4% | 48 min | 1.01% | B |
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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 27 points, against half a point between the best and worst carrier.
- 5am–8am239 flights92.0%
- 8am–11am28 flights—
- 11am–2pm26 flights—
- 2pm–5pm91 flights67.8%
- 5pm–8pm181 flights65.0%
- 8pm–5am31 flights—
Scale runs 55% to 95%. Rows with fewer than 30 flights show a dash.
Book the early departure. Flights leaving between 5am–8am arrive on time 92.0% of the time, against 65.0% for the evening window. That is 27 points of difference for no extra money, and it comes from the same aircraft having had no earlier flight to fall behind on. The 5am–8am window is the busiest, with 239 flights scheduled in it.
Across the year
Southwest on IAD to DEN through the year
December is the weak month at 65.3% on time, against 87.0% in November. 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. September was the busiest month here, with 82 flights.
Across the week, Saturday is the strongest day at 83.3% on time and Thursday the weakest at 73.0%, a spread of 10 points.
Arrival by arrival
How late Southwest on IAD to DEN runs when it is late
Most of the lateness is short. 64 of the 127 late arrivals were under half an hour behind, and 27 reached an hour or more.
- 15+ min early92 flights15.6%
- Within 15 min370 flights62.8%
- 15–29 min late64 flights10.9%
- 30–59 min late36 flights6.1%
- 1–2 hours late17 flights2.9%
- 2+ hours late10 flights1.7%
Scale runs 0% to 100%.
Share of 589 completed flights by arrival against schedule.
A late pushback is often recovered here: of 130 departures that left 15 minutes late or more, 48 still arrived on time. In the other direction, 45 flights left on time yet arrived 15 minutes late or more, so that delay built up after pushback: in the air, in a holding pattern or on the ground at DEN.
The worst days
Worst days for Southwest on IAD to DEN
The latest arrival of the period was on 28 December 2025, 9 hours 24 minutes behind schedule; most of its delay minutes were filed as carrier-caused (crew, maintenance, fuelling, baggage). The five worst days are spread across December, April, March and September, with no single month standing out.
| Date | Arrived late by | Largest filed cause |
|---|---|---|
| 28 December 2025 | 9 hours 24 minutes | Carrier |
| 1 April 2026 | 5 hours 22 minutes | Carrier |
| 6 March 2026 | 3 hours 4 minutes | National Airspace System |
| 13 March 2026 | 2 hours 57 minutes | National Airspace System |
| 6 September 2025 | 2 hours 32 minutes | Extreme weather |
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 6,104 reported delay minutes.
- 34%Late-arriving aircraft
- 34%National Airspace System
- 29%Carrier
- 3%Extreme weather
- 0%Security
34% 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 2,057 minutes out of 6,104, against a national NAS share of 21%. The split is drawn from 6,104 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 92.0% on time here.
Cancellations sit at 1.01% against a national 1.89%, which is ordinary. Diversions add another 0.17%.
Connections and stage length
What a delay here costs
Badly late arrivals are rare here: only 4.6% land an hour or more behind schedule, against 8.3% nationally. Connections built on this flight are about as safe as US domestic connections get, though a cancelled flight still ignores every layover you give it.
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.
The return leg is a different operation and performs like one: it runs 67.7% on time across 595 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. 596 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. 596 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 78.4% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 589 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 DEN84.1%
- Southwest Airlines network-wide record
- Washington Dulles International (IAD) delay statistics
- Denver International (DEN) delay statistics
- Why the first flight of the morning is statistically safest
- The best time of day to fly, according to 7.05 million flight records
Source: US DOT Bureau of Transportation Statistics, on-time performance data, September 2025 to August 2026. Last updated ; recomputed monthly. How this data is collected.