Route reliability record
DEN→SEA
Denver to Seattle
- Flights
- 6,328
- Period
- September 2025 to August 2026
- Distance
- 1,024 mi
- Carriers
- 6
This route sits in the middle of the pack with a C+. Close to three flights in four arrive on time by the DOT's definition, 3 points worse than the national rate of 77.3%. The late ones land about an hour behind schedule. That rests on 6,328 DEN to SEA flights, of which Frontier Airlines flew 439 at 82.7% on time.
Best time to fly DEN to SEA: a 5am–8am departure, ideally on a Saturday. Flights in that window arrived on time 89.9% of the time and Saturday flights 77.9%, against 74.5% for the route overall.
Frontier Airlines is the most reliable of the five carriers covered here at 82.7% on time across 439 flights.
Key figures
On time
74.5%
Arrivals under 15 minutes late, of 6,289 completed flights
Avg delay
58 min
Across delayed arrivals only, not all flights
Cancelled
0.60%
National rate is 1.89%
Best carrier
Frontier
82.7% on time across 439 flights
These figures cover 6,328 flights between September 2025 and August 2026. The median arrival is 2 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 DEN to SEA
The choice of carrier is not cosmetic here. Frontier Airlines arrives on time 82.7% of the time; Southwest Airlines manages 59.5%. 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 |
|---|---|---|---|---|---|
| Frontier Airlines | 439 | 82.7% | 54 min | 1.14% | B |
| United Airlines | 1,908 | 80.9% | 57 min | 0.47% | B+ |
| Delta Air Lines | 868 | 78.5% | 86 min | 0.92% | B+ |
| SkyWest Airlinesthin sample | 220 | 75.8% | 70 min | 0.45% | B+ |
| Alaska Airlines | 1,551 | 74.9% | 58 min | 0.71% | C+ |
| Southwest Airlines | 1,342 | 59.5% | 49 min | 0.30% | D+ |
The spread between Frontier and Southwest is 23 points, wide enough that it should decide the booking. On a hundred trips that is 23 more arrivals inside the on-time window.
Flight by flight
Flights on DEN to SEA
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 Denver to Seattle
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. DEN → SEADenver to Seattle | Frontier Airlines | 81.8% | 82.7% | 1.14% | 5am–8am, Thu | — |
| 2. DEN → SEADenver to Seattle | United Airlines | 80.5% | 80.9% | 0.47% | 5am–8am, Tue | UA 2271 07:45 |
| 3. DEN → SEADenver to Seattle | Delta Air Lines | 77.8% | 78.5% | 0.92% | 5am–8am, Sat | DL 1731 06:30 |
| 4. DEN → SEADenver to Seattle | Alaska Airlines | 74.4% | 74.9% | 0.71% | 5am–8am, Sat | AS 643 07:00 |
| 5. DEN → SEADenver to Seattle | Southwest Airlines | 59.3% | 59.5% | 0.30% | 5am–8am, Tue | — |
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 30 points, against 23 points between the best and worst carrier.
- 5am–8am1,073 flights89.9%
- 8am–11am1,024 flights81.1%
- 11am–2pm1,315 flights78.7%
- 2pm–5pm915 flights70.7%
- 5pm–8pm1,233 flights63.2%
- 8pm–5am768 flights59.6%
Scale runs 50% to 95%.
The 8pm–5am window is where this operation falls apart: 59.6% on time, against 89.9% 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 11am–2pm window is the busiest, with 1,315 flights scheduled in it.
Explore
Filter DEN to SEA 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 DEN to SEA, the best departure window is 5am–8am at 89.9% on time and the worst is 8pm–5am at 59.6%, a wide gap of 30.3 points. All US flights leaving 5am–8am ran 89.3%.
- 5–8am89.9%
- 8–11am81.1%
- 11am–2pm78.7%
- 2–5pm70.7%
- 5–8pm63.2%
- 8pm–5am59.6%
best all US flightsScale 50% to 95%.
Across the year
When delays peak on this route
This route's year swings by 25 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.
December is the weak month at 59.3% on time, against 84.0% in April. 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. July was the busiest month here, with 678 flights.
Across the week, Saturday is the strongest day at 77.9% on time and Thursday the weakest at 72.0%, a spread of 6 points.
Delay causes
Why this route gets delayed
Late-arriving aircraft accounts for 44% of the 93,000 delay minutes carriers reported here, and the remaining four split what is left.
- 44%Late-arriving aircraft
- 35%Carrier
- 19%National Airspace System
- 2%Extreme weather
- 0%Security
44% of delay minutes trace back to a late-arriving aircraft, meaning the plane was already behind before it got here. Across 93,000 reported delay minutes here, that single category outweighs weather by 19 to one. The split is drawn from 93,496 reported delay minutes.
The largest share of delay minutes, 35%, falls under carrier control: crew scheduling, maintenance, fuelling, baggage. It accounts for 33,000 of the 93,000 delay minutes reported here.
Cancellations are rare at 0.60%, 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
7.3% 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 78.3% on time across 6,333 flights, 4 points above 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 6,328 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 SEA 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 84.0% on time and December 59.3%, a swing of 25 points. If the dates move at all, move them out of December before you think about anything else. 6,289 completed flights over 12 months sit behind that.
Common questions
DEN to SEA questions
How often is DEN to SEA delayed?
About one flight in four arrives at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 25.5% of the 6,328 flights in this analysis, against 22.7% nationally. Delayed flights are late by about an hour on average.
Which airline is most reliable on DEN to SEA?
Frontier Airlines leads at 82.7% on time across 439 flights. Southwest Airlines is at the other end on 59.5%, a gap of 23 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 DEN to SEA?
The 5am–8am departure window is the strongest at 89.9% on time. The 8pm–5am window is the weakest at 59.6%, 30 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 December delays on DEN to SEA?
December is the weakest month at 59.3% on time, compared with 84.0% in April. Across the year that is 25 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
- SEA to DEN — the return legseparate record
- Denver International (DEN) delay statistics
- Seattle-Tacoma International (SEA) delay statistics
- Frontier Airlines on DEN to SEA82.7%
- United Airlines on DEN to SEA80.9%
- Delta Air Lines on DEN to SEA78.5%
- SkyWest Airlines on DEN to SEA75.8%
- Alaska Airlines on DEN to SEA74.9%
- Southwest Airlines on DEN to SEA59.5%
- DEN to PHX, a related route75.6%
- LAX to SEA, a related route85.0%
- DEN to SLC, a related route78.2%
- Frontier Airlines on-time record82.7%
- United Airlines on-time record80.9%
- Delta Air Lines on-time record78.5%
- Same route, different airline: where the carrier you pick matters most
- Why the first flight of the morning is statistically safest
Direction matters. DEN to SEA and SEA to DEN 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.