Carrier route record

DLDEN→SEA

Delta Air Lines · Denver to Seattle

Flights: 868
Period: July 2025 to June 2026
Share of route: 14%
Carriers: 6

Historical statistics for July 2025 to June 2026, not live status. For today’s flights, check with Delta Air Lines.

Delta on DEN to SEA lands a B+, 2 points better than the national rate of 77.6%. About one flight in five arrives 15 minutes late or worse, and those flights are late by an hour and a half on average. That is a workable record, not a spotless one. That rests on 868 flights. The strongest carrier on the route with a full sample, United Airlines, flew 1,903 at 80.0% on time.

United Airlines is the most reliable of the four carriers covered here at 80.0% on time across 1,903 flights.

On time

79.2%

Arrivals under 15 minutes late, of 860 completed flights

Avg delay

89 min

Across delayed arrivals only, not all flights

Cancelled

0.92%

National rate is 1.81%

Whole route

73.9%

Every carrier on DEN to SEA, 6,309 flights

These figures cover 868 flights between July 2025 and June 2026. The median arrival is 7 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

Delta against every carrier on DEN to SEA

Delta flew 868 of the 6,309 flights on DEN to SEA, about 14% of it. Its 79.2% on time is 5 points better than the route as a whole, which runs 73.9%. Ranked by on-time rate it is number two of six carriers on the route, where the range runs from United at 80.0% to Southwest at 61.7%. One of those carriers flew fewer than 365 flights here, so treat their positions as rough.

The choice of carrier is not cosmetic here. United Airlines arrives on time 80.0% of the time; Southwest Airlines manages 61.7%. Sample sizes sit next to every figure, because a two-point gap across 1,341 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	1,903	80.0%	55 min	0.63%	B+
Delta Air Lines (this page)	868	79.2%	89 min	0.92%	B+
SkyWest Airlinesthin sample	220	75.8%	70 min	0.45%	B+
Frontier Airlines	419	74.9%	60 min	1.91%	C
Alaska Airlines	1,558	73.8%	57 min	0.96%	C+
Southwest Airlines	1,341	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.

Flight by flight

Delta flight numbers on DEN to SEA

DL 173106:0589.4% · 364 flights

Departure window

When Delta runs best on this route

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–8am364 flights89.4%
8am–11am—
11am–2pm267 flights80.5%
2pm–5pm—
5pm–8pm195 flights60.4%
8pm–5am42 flights69.0%

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

The 5pm–8pm window is where this operation falls apart: 60.4% on time, against 89.4% 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 5am–8am window is the busiest, with 364 flights scheduled in it.

Across the year

Delta on DEN to SEA through the year

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

December is the weak month at 58.6% on time, against 91.7% 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 93 flights.

Across the week, Wednesday is the strongest day at 83.5% on time and Thursday the weakest at 72.1%. 11 points is a smaller lever than departure hour, but it is free.

Delay causes

Why Delta runs late on this route

One category carries most of the weight here. Late-arriving aircraft accounts for 51% of the 16,000 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
51%
Carrier
39%
National Airspace System
8%
Extreme weather
2%

51% 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 16,000 reported delay minutes here, that single category outweighs weather by 24 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 15,887 reported delay minutes.

The largest share of delay minutes, 39%, 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 6,144 of the 16,000 delay minutes reported here.

Cancellations sit at 0.92% against a national 1.81%, which is ordinary. Diversions add another 0.00%.

Connections and stage length

What a delay here costs

7.2% 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 last an hour and a half, 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 73.3% on time across 869 flights, 6 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 6 points of difference.

One caution: this is a thin sample. 868 flights over twelve months is enough to clear this site's floor but not enough to pin the rate down tightly, and a single bad week moves it by more than a point. Treat the grade as a rough reading.

What to do with this

Booking Delta on DEN to SEA

Put together: treat all of this as provisional. 868 flights over twelve months is enough to clear this site's floor and not enough to be confident to better than a couple of points. The 79.2% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 860 completed flights over 12 months sit behind that.

Where to go next

Source: US DOT Bureau of Transportation Statistics, on-time performance data, July 2025 to June 2026. Last updated 15 September 2026; recomputed monthly. How this data is collected.

Key figures