Carrier route record

F9DEN→SEA

Frontier Airlines · Denver to Seattle

Flights: 419
Period: July 2025 to June 2026
Share of route: 7%
Carriers: 6

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

Frontier on DEN to SEA 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 77.6%. The late ones land about an hour behind schedule. That rests on 419 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

74.9%

Arrivals under 15 minutes late, of 411 completed flights

Avg delay

60 min

Across delayed arrivals only, not all flights

Cancelled

1.91%

National rate is 1.81%

Whole route

73.9%

Every carrier on DEN to SEA, 6,309 flights

These figures cover 419 flights between July 2025 and June 2026. Beyond the headline rate, 8.3% of arrivals land a full hour or more behind schedule. That is the number that wrecks a connection, and it is not visible in an on-time percentage alone.

Carrier by carrier

Frontier against every carrier on DEN to SEA

Frontier flew 419 of the 6,309 flights on DEN to SEA, about 7% of it. Its 74.9% on time is about level with the route as a whole at 73.9%. Ranked by on-time rate it is number four 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	868	79.2%	89 min	0.92%	B+
SkyWest Airlinesthin sample	220	75.8%	70 min	0.45%	B+
Frontier Airlines (this page)	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.

Departure window

When Frontier runs best on this route

Departure time matters more here than the choice of carrier. The gap between the best and worst window is 25 points, against 18 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.

5am–8am60 flights88.3%
8am–11am179 flights77.0%
11am–2pm46 flights86.0%
2pm–5pm21 flights—
5pm–8pm22 flights—
8pm–5am91 flights62.9%

Scale runs 55% to 95%. Rows with fewer than 30 flights show a dash.

The 8pm–5am window is where this operation falls apart: 62.9% on time, against 88.3% 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 8am–11am window is the busiest, with 179 flights scheduled in it.

Across the year

Frontier on DEN to SEA through the year

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

July is the worst month at 54.2% on time, 37 points below April, the best at 91.2%. 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. June was the busiest month here, with 57 flights.

Across the week, Wednesday is the strongest day at 79.5% on time and Friday the weakest at 68.1%. 11 points is a smaller lever than departure hour, but it is free.

Delay causes

Why Frontier runs late on this route

One category carries most of the weight here. Carrier accounts for 46% of the 6,197 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.

Carrier
46%
Late-arriving aircraft
38%
National Airspace System
16%
Extreme weather
0%

38% 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 6,197 reported delay minutes here, that single category outweighs weather by 38 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 6,197 reported delay minutes.

The largest share of delay minutes, 46%, 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 2,856 of the 6,197 delay minutes reported here.

Cancellations sit at 1.91% against a national 1.81%, which is ordinary. Diversions add another 0.00%.

Connections and stage length

What a delay here costs

8.3% 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 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. 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 runs 74.9% on time across 419 flights, within half a point of this direction. The two legs are scored separately because they usually differ; on this city pair they happen not to.

One caution: this is a thin sample. 419 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 Frontier on DEN to SEA

Put together: treat all of this as provisional. 419 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 74.9% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 411 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