Route reliability record
SEA→DEN
Seattle to Denver
- Flights
- 6,333
- Period
- September 2025 to August 2026
- Distance
- 1,024 mi
- Carriers
- 6
This route 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 about an hour on average. That rests on 6,333 SEA to DEN flights, of which United Airlines flew 1,910 at 88.2% on time.
Best time to fly SEA to DEN: a 5am–8am departure, ideally on a Saturday. Flights in that window arrived on time 94.0% of the time and Saturday flights 84.1%, against 78.3% for the route overall.
United Airlines is the most reliable of the five carriers covered here at 88.2% on time across 1,910 flights.
Key figures
On time
78.3%
Arrivals under 15 minutes late, of 6,257 completed flights
Avg delay
58 min
Across delayed arrivals only, not all flights
Cancelled
0.68%
National rate is 1.89%
Best carrier
United
88.2% on time across 1,910 flights
These figures cover 6,333 flights between September 2025 and August 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.
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Carrier by carrier
Which airline is most reliable on SEA to DEN
The choice of carrier is not cosmetic here. United Airlines arrives on time 88.2% of the time; Southwest Airlines manages 67.3%. Sample sizes sit next to every figure, because a two-point gap across 1,342 flights means less than a ten-point one.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| United Airlines | 1,910 | 88.2% | 74 min | 0.58% | A+ |
| SkyWest Airlinesthin sample | 221 | 85.0% | 60 min | 0.45% | A+ |
| Alaska Airlines | 1,552 | 78.2% | 48 min | 0.64% | B+ |
| Frontier Airlines | 439 | 76.0% | 74 min | 0.91% | B+ |
| Delta Air Lines | 869 | 72.9% | 57 min | 0.92% | C+ |
| Southwest Airlines | 1,342 | 67.3% | 55 min | 0.67% | C+ |
The spread between United and Southwest is 21 points, wide enough that it should decide the booking. On a hundred trips that is 21 more arrivals inside the on-time window.
Flight by flight
Flights on SEA to DEN
The flight numbers below flew this route at least 250 times in the period, busiest first. Each has its own record, with its schedule and delay causes.
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Best ways to make this trip
The most reliable ways to fly Seattle to Denver
Every airline on this route, ranked by how often a scheduled flight actually operated and arrived on time, with its best departure time and most punctual flight number. No nearby airport pair beat these.
| Option | Airline | Success | On time | Cancelled | Best departure | Most punctual flight |
|---|---|---|---|---|---|---|
| 1. SEA → DENSeattle to Denver | United Airlines | 87.7% | 88.2% | 0.58% | 5am–8am, Sat | UA 1505 08:40 |
| 2. SEA → DENSeattle to Denver | Alaska Airlines | 77.7% | 78.2% | 0.64% | 5am–8am, Sat | AS 682 07:05 |
| 3. SEA → DENSeattle to Denver | Frontier Airlines | 75.3% | 76.0% | 0.91% | 8pm–5am, Sun | F9 3406 12:35 |
| 4. SEA → DENSeattle to Denver | Delta Air Lines | 72.2% | 72.9% | 0.92% | 5am–8am, Wed | DL 2851 19:40 |
| 5. SEA → DENSeattle to Denver | Southwest Airlines | 66.8% | 67.3% | 0.67% | 5am–8am, Sat | — |
Success is the share of scheduled flights that operated and arrived less than 15 minutes late, so a cancelled flight counts against it. “(route)” means the best time comes from every carrier on the route rather than this airline alone.
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 29 points, against 21 points between the best and worst carrier.
- 5am–8am1,018 flights94.0%
- 8am–11am1,246 flights85.1%
- 11am–2pm1,447 flights75.7%
- 2pm–5pm1,323 flights65.2%
- 5pm–8pm796 flights74.3%
- 8pm–5am503 flights77.4%
Scale runs 55% to 100%.
Book the early departure. Flights leaving between 5am–8am arrive on time 94.0% of the time, against 65.2% for the afternoon window. That is 29 points of difference for no extra money, and it comes from the same aircraft having had no earlier flight to fall behind on. The 11am–2pm window is the busiest, with 1,447 flights scheduled in it.
Explore
Filter SEA to DEN by airline, time, month and day
Pick a breakdown and a carrier. The dashed mark on each row is the same slice for every US flight.
On SEA to DEN, the best departure window is 5am–8am at 94.0% on time and the worst is 2pm–5pm at 65.2%, a wide gap of 28.8 points. All US flights leaving 5am–8am ran 89.3%.
- 5–8am94.0%
- 8–11am85.1%
- 11am–2pm75.7%
- 2–5pm65.2%
- 5–8pm74.3%
- 8pm–5am77.4%
best all US flightsScale 55% to 100%.
Across the year
When delays peak on this route
This route's year swings by 24 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.
August is the worst month at 63.8% on time, 24 points below April, the best at 87.6%. 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 678 flights.
Across the week, Saturday is the strongest day at 84.1% on time and Sunday the weakest at 75.3%, a spread of 9 points.
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 79,000 reported delay minutes.
- 36%Late-arriving aircraft
- 30%Carrier
- 29%National Airspace System
- 4%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. Across 79,000 reported delay minutes here, that single category outweighs weather by 9 to one. The split is drawn from 79,078 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 94.0% on time here.
Cancellations are rare at 0.68%, below the national 1.89% 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
6.0% of arrivals land an hour or more late, close to the national 8.3%, so a 60 to 75 minute layover is reasonable here while anything under 45 minutes is a bet. Most delays on this route last about an hour, 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.
The return leg is a different operation and performs like one: it runs 74.5% on time across 6,328 flights, 4 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 4 points of difference.
The sample behind these figures is 6,333 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.
Work out the risk of missing a connection in DEN after this flight
What to do with this
Booking this route
Put together: the calendar is the biggest lever here, not the carrier or the hour. April runs 87.6% on time and August 63.8%, a swing of 24 points. If the dates move at all, move them out of August before you think about anything else. 6,257 completed flights over 12 months sit behind that.
Common questions
SEA to DEN questions
How often is SEA to DEN delayed?
About one flight in five arrives at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 21.7% of the 6,333 flights in this analysis, against 22.7% nationally. Delayed flights are late by about an hour on average.
Which airline is most reliable on SEA to DEN?
United Airlines leads at 88.2% on time across 1,910 flights. Southwest Airlines is at the other end on 67.3%, a gap of 21 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 SEA to DEN?
The 5am–8am departure window is the strongest at 94.0% on time. The 2pm–5pm window is the weakest at 65.2%, 29 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 August delays on SEA to DEN?
August is the weakest month at 63.8% on time, compared with 87.6% in April. Across the year that is 24 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 SEA — the return legseparate record
- Seattle-Tacoma International (SEA) delay statistics
- Denver International (DEN) delay statistics
- United Airlines on SEA to DEN88.2%
- SkyWest Airlines on SEA to DEN85.0%
- Alaska Airlines on SEA to DEN78.2%
- Frontier Airlines on SEA to DEN76.0%
- Delta Air Lines on SEA to DEN72.9%
- Southwest Airlines on SEA to DEN67.3%
- SEA to LAX, a related route82.0%
- PHX to DEN, a related route76.7%
- SEA to PHX, a related route78.2%
- United Airlines on-time record88.2%
- SkyWest Airlines on-time record85.0%
- Alaska Airlines on-time record78.2%
- Same route, different airline: where the carrier you pick matters most
- Why the first flight of the morning is statistically safest
Direction matters. SEA to DEN and DEN to SEA are scored separately because they are separate operations, with different departure banks and different aircraft. Compare the return leg.
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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.