Route reliability record
IAD→DEN
Washington to Denver
- Flights
- 3,560
- Period
- July 2025 to June 2026
- Distance
- 1,452 mi
- Carriers
- 2
This route lands a B+, 4 points better than the national rate of 77.6%. Roughly one flight in five arrive 15 minutes late or worse, and those flights are late by about an hour on average. That is a workable record, not a spotless one. That rests on 3,560 flights, of which United Airlines flew 2,798 at 83.2% on time.
Set against the rest of the country, this route's record is 4 points better than the US average of 77.6%. That gap is worth roughly 41 fewer late arrivals for every thousand flights.
Key figures
On time
81.7%
Arrivals under 15 minutes late, of 3,513 completed flights
Avg delay
66 min
Across delayed arrivals only, not all flights
Cancelled
0.62%
National rate is 1.81%
Best carrier
United
83.2% on time across 2,798 flights
These figures cover 3,560 flights between July 2025 and June 2026. The median arrival is 6 minutes early, which is the part an average hides: most flights here are fine, and the on-time rate is decided by a tail of badly late ones. On-time means under 15 minutes late, the DOT's own threshold.
Carrier by carrier
Which airline is most reliable on IAD to DEN
The choice of carrier is not cosmetic here. United Airlines arrives on time 83.2% of the time; Southwest Airlines manages 76.2%. Sample sizes sit next to every figure, because a two-point gap across 762 flights means less than a ten-point one.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| United Airlines | 2,798 | 83.2% | 71 min | 0.46% | B+ |
| Southwest Airlines | 762 | 76.2% | 53 min | 1.18% | B |
United leads Southwest by 7 points. Real, but small enough that a better departure time on the weaker carrier can outweigh it.
Departure window
The best time of day to leave
Departure time matters more here than the choice of carrier. The gap between the best and worst window is 25 points, against 7 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am411 flights92.1%
- 8am–11am759 flights88.5%
- 11am–2pm391 flights90.7%
- 2pm–5pm861 flights80.0%
- 5pm–8pm625 flights67.0%
- 8pm–5am513 flights77.3%
Scale runs 60% to 100%. Rows with fewer than 30 flights show a dash.
Book the early departure. Flights leaving between 5am–8am arrive on time 92.1% of the time, against 67.0% for the evening window. That is 25 points of difference for no extra money, and it comes from the same aircraft having had no earlier flight to fall behind on. The 2pm–5pm window is the busiest, with 861 flights scheduled in it.
Across the year
When delays peak on this route
This route's year swings by 23 points from best month to worst, which is wide. An annual average tells you very little about the week you are actually flying, so read the strip rather than the headline if your dates are fixed.
July is the worst month at 68.1% on time, 23 points below November, the best at 91.3%. 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 361 flights.
Across the week, Saturday is the strongest day at 84.0% on time and Thursday the weakest at 77.4%. 7 points is a smaller lever than departure hour, but it is free.
Delay causes
Why this route gets delayed
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 42,000 reported delay minutes.
- 36%Late-arriving aircraft
- 30%National Airspace System
- 26%Carrier
- 8%Extreme weather
- 0%Security
36% 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 42,000 reported delay minutes here, that single category outweighs weather by 5 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 42,398 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 92.1% on time here.
Cancellations are rare at 0.62%, 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 actually costs
6.2% 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 are the about an hour kind, 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 runs 80.3% on time across 3,558 flights, within about a point of this direction. The two legs are scored separately because they usually differ; on this city pair they happen not to.
The sample behind these figures is 3,560 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 this route
Put together: the calendar is the biggest lever here, not the carrier or the hour. November runs 91.3% on time and July 68.1%, a swing of 23 points. If the dates move at all, move them out of July before you think about anything else. 3,513 completed flights over 12 months sit behind that.
Common questions
IAD to DEN questions
How often is IAD to DEN delayed?
Roughly one flight in five arrive at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 18.3% of the 3,560 flights in this analysis, against 22.4% nationally. Delayed flights are late by about an hour on average.
Which airline is most reliable on IAD to DEN?
United Airlines leads at 83.2% on time across 2,798 flights. Southwest Airlines is at the other end on 76.2%, a gap of 7 points. Sample size matters as much as the percentage: a rate built from a few hundred flights moves around far more than one built from ten thousand.
What time of day has the fewest delays on IAD to DEN?
The 5am–8am departure window is the strongest at 92.1% on time. The 5pm–8pm window is the weakest at 67.0%, 25 points behind. Later departures inherit delays from earlier flights the same aircraft has already flown, which is why the curve slopes the way it does.
How bad are July delays on IAD to DEN?
July is the weakest month at 68.1% on time, compared with 91.3% in November. Across the year that is 23 points between the best and worst months. Moving flexible dates out of that month is one of the few things that reliably helps.
Related
Where to go next
- DEN to IAD — the return legseparate record
- Washington Dulles International (IAD) delay statistics
- Denver International (DEN) delay statistics
- United Airlines on-time record83.2%
- Southwest Airlines on-time record76.2%
- Why the first flight of the morning is statistically safest
- The best time of day to fly, according to 7.04 million flight records
Direction matters. IAD to DEN and DEN to IAD are scored separately because they are separate operations, with different departure banks and different aircraft. Compare the return leg.
Source: US DOT Bureau of Transportation Statistics, on-time performance data, July 2025 to June 2026. Updated monthly. How this data is collected.