Route reliability record

SEA→DEN

Seattle to Denver

Flights: 6,325
Period: June 2025 to May 2026
Distance: 1,024 mi
Carriers: 6

This route lands a B+, level with the national rate of 77.4%. Roughly one flight in four 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 6,325 flights, of which United Airlines flew 1,906 at 86.6% on time.

United Airlines is the most reliable of the five carriers covered here at 86.6% on time across 1,906 flights.

On time

77.1%

Arrivals under 15 minutes late, of 6,233 completed flights

Avg delay

60 min

Across delayed arrivals only, not all flights

Cancelled

0.82%

National rate is 1.80%

Best carrier

United

86.6% on time across 1,906 flights

These figures cover 6,325 flights between June 2025 and May 2026. The median arrival is 5 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 SEA to DEN

The choice of carrier is not cosmetic here. United Airlines arrives on time 86.6% of the time; Southwest Airlines manages 69.9%. Sample sizes sit next to every figure, because a two-point gap across 1,346 flights means less than a ten-point one.

On-time performance by carrier on SEA to DEN
Airline	Flights	On time	Avg delay	Cancelled	Grade
United Airlines	1,906	86.6%	78 min	0.84%	A+
SkyWest Airlinesthin sample	221	85.0%	60 min	0.45%	A+
Frontier Airlines	412	74.1%	75 min	1.70%	C
Alaska Airlines	1,571	74.0%	52 min	0.76%	C+
Delta Air Lines	869	72.5%	62 min	0.92%	C+
Southwest Airlines	1,346	69.9%	52 min	0.59%	C+

The spread between United and Southwest is 17 points, wide enough that it should decide the booking. On a hundred trips that is 17 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 26 points, against 17 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.

5am–8am1,160 flights90.4%
8am–11am1,275 flights82.8%
11am–2pm1,324 flights75.6%
2pm–5pm1,274 flights64.1%
5pm–8pm841 flights73.7%
8pm–5am451 flights73.5%

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 90.4% of the time, against 64.1% for the afternoon window. That is 26 points of difference for no extra money, and it comes from the same aircraft having had no earlier flight to fall behind on.

Across the year

When delays peak on this route

This route's year swings by 26 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, July worst

July is the worst month at 61.3% on time, 26 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.

Across the week, Saturday is the strongest day at 82.5% on time and Friday the weakest at 73.4%. 9 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 86,000 reported delay minutes.

National Airspace System
34%
Late-arriving aircraft
32%
Carrier
29%
Extreme weather
5%
Security
0%

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 29,000 minutes out of 86,000, against a national NAS share of 21%.

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 90.4% on time here.

Cancellations are rare at 0.82%, 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

7.1% 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 73.0% on time across 6,318 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 6,325 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 87.6% on time and July 61.3%, a swing of 26 points. If the dates move at all, move them out of July before you think about anything else.

Common questions

SEA to DEN questions

How often is SEA to DEN delayed?

Roughly 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 22.9% of the 6,325 flights in this analysis, against 22.6% nationally. Delayed flights are late by about an hour on average.

Which airline is most reliable on SEA to DEN?

United Airlines leads at 86.6% on time across 1,906 flights. Southwest Airlines is at the other end on 69.9%, a gap of 17 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 90.4% on time. The 2pm–5pm window is the weakest at 64.1%, 26 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 SEA to DEN?

July is the weakest month at 61.3% on time, compared with 87.6% in April. Across the year that is 26 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. 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.

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