Route reliability record

DEN→SEA

Denver to Seattle

Flights: 6,318
Period: June 2025 to May 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, 4 points worse than the national rate of 77.4%. The late ones land about an hour behind schedule. That rests on 6,318 flights, of which United Airlines flew 1,904 at 79.4% on time.

United Airlines is the most reliable of the five carriers covered here at 79.4% on time across 1,904 flights.

On time

73.0%

Arrivals under 15 minutes late, of 6,270 completed flights

Avg delay

60 min

Across delayed arrivals only, not all flights

Cancelled

0.73%

National rate is 1.80%

Best carrier

United

79.4% on time across 1,904 flights

These figures cover 6,318 flights between June 2025 and May 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.

Carrier by carrier

Which airline is most reliable on DEN to SEA

The choice of carrier is not cosmetic here. United Airlines arrives on time 79.4% of the time; Southwest Airlines manages 61.7%. Sample sizes sit next to every figure, because a two-point gap across 1,343 flights means less than a ten-point one.

On-time performance by carrier on DEN to SEA
Airline	Flights	On time	Avg delay	Cancelled	Grade
United Airlines	1,904	79.4%	56 min	0.63%	B+
Delta Air Lines	868	77.3%	98 min	0.92%	B+
SkyWest Airlinesthin sample	220	75.8%	70 min	0.45%	B+
Alaska Airlines	1,571	72.2%	58 min	0.89%	C+
Frontier Airlines	412	72.1%	63 min	1.70%	C
Southwest Airlines	1,343	61.7%	47 min	0.30%	D+

The spread between United and Southwest is 18 points, wide enough that it should decide the booking. On a hundred trips that is 18 more arrivals inside the on-time window.

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 18 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.

5am–8am994 flights89.2%
8am–11am932 flights78.6%
11am–2pm1,244 flights80.1%
2pm–5pm982 flights68.8%
5pm–8pm1,245 flights61.5%
8pm–5am921 flights59.9%

Scale runs 50% to 95%. Rows with fewer than 30 flights show a dash.

The 8pm–5am window is where this operation falls apart: 59.9% on time, against 89.2% 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.

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.

Darker means more delays. Figures are on-time percentages.April best, December worst

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.

Across the week, Saturday is the strongest day at 75.8% on time and Thursday the weakest at 69.2%. 7 points is a smaller lever than departure hour, but it is free.

Delay causes

Why this route gets delayed

One category carries most of the weight here. Late-arriving aircraft accounts for 46% of the 101,000 delay minutes carriers reported, and the remaining four split what is left. The split is by minutes rather than by flights, so a few very long delays count for as much as many short ones.

Late-arriving aircraft
46%
Carrier
34%
National Airspace System
17%
Extreme weather
2%
Security
0%

46% 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 101,000 reported delay minutes here, that single category outweighs weather by 22 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 largest share of delay minutes, 34%, falls under carrier control: crew scheduling, maintenance, fuelling, baggage. Unlike weather, that is a management outcome, and it is the category where the gap between two carriers on the same schedule is genuinely earned. It accounts for 34,000 of the 101,000 delay minutes reported here.

Cancellations are rare at 0.73%, below the national 1.80% 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

7.9% of arrivals land an hour or more late, close to the national 8.2%. 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 1,024 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. 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 77.1% on time across 6,325 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 6,318 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. 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.

Common questions

DEN to SEA questions

How often is DEN to SEA delayed?

About one flight in four arrive at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 27.0% of the 6,318 flights in this analysis, against 22.6% nationally. Delayed flights are late by about an hour on average.

Which airline is most reliable on DEN to SEA?

United Airlines leads at 79.4% on time across 1,904 flights. Southwest Airlines is at the other end on 61.7%, a gap of 18 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 89.2% on time. The 8pm–5am window is the weakest at 59.9%, 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 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.

Where to go next

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.

Source: US DOT Bureau of Transportation Statistics, on-time performance data, June 2025 to May 2026. Updated monthly. How this data is collected.

Key figures