Carrier route record

WNSTL→DEN

Southwest Airlines · St. Louis to Denver

Flights: 2,165
Period: July 2025 to June 2026
Share of route: 59%
Carriers: 4

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

Southwest on STL to DEN 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 77.6%. The late ones land about an hour behind schedule. That rests on 2,165 flights, with a median arrival 1 minute early.

The national on-time rate over the same period is 77.6%, so Southwest on STL to DEN runs 4 points behind the country as a whole. Across a thousand flights that is about 43 extra late arrivals.

On time

73.3%

Arrivals under 15 minutes late, of 2,156 completed flights

Avg delay

60 min

Across delayed arrivals only, not all flights

Cancelled

0.23%

National rate is 1.81%

Whole route

74.9%

Every carrier on STL to DEN, 3,655 flights

These figures cover 2,165 flights between July 2025 and June 2026. Beyond the headline rate, 9.1% 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

Southwest against every carrier on STL to DEN

Southwest flew 2,165 of the 3,655 flights on STL to DEN, most of the route. Its 73.3% on time is 2 points worse than the route as a whole, which runs 74.9%. Ranked by on-time rate it is last of four carriers on the route, where the range runs from SkyWest at 87.1% to Southwest at 73.3%. Two of those carriers flew fewer than 365 flights here, so treat their positions as rough.

Only one carrier clears this site's 365-flight floor here, so there is no comparison to draw. The table below is the full picture.

On-time performance by carrier on STL to DEN
Airline	Flights	On time	Avg delay	Cancelled	Grade
SkyWest Airlinesthin sample	142	87.1%	60 min	0.70%	A+
United Airlines	1,071	76.6%	91 min	0.56%	B+
Frontier Airlinesthin sample	277	74.5%	83 min	1.08%	C
Southwest Airlines (this page)	2,165	73.3%	60 min	0.23%	C+

Departure window

When Southwest runs best on this route

Departure time matters more here than the choice of carrier. The gap between the best and worst window is 47 points, against half a point between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.

5am–8am496 flights93.1%
8am–11am317 flights82.0%
11am–2pm346 flights80.9%
2pm–5pm386 flights64.0%
5pm–8pm388 flights46.5%
8pm–5am232 flights68.4%

Scale runs 40% to 100%. Rows with fewer than 30 flights show a dash.

The 5pm–8pm window is where this operation falls apart: 46.5% on time, against 93.1% 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 496 flights scheduled in it.

Across the year

Southwest on STL to DEN through the year

Darker means more delays. Figures are on-time percentages.January best, June worst

June is the worst month at 61.4% on time, 24 points below January, the best at 85.2%. 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. July was the busiest month here, with 211 flights.

Across the week, Tuesday is the strongest day at 79.8% on time and Monday the weakest at 69.1%. 11 points is a smaller lever than departure hour, but it is free.

Delay causes

Why Southwest runs late on this route

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 35,000 reported delay minutes.

Late-arriving aircraft
42%
National Airspace System
35%
Carrier
20%
Extreme weather
3%
Security
0%

42% 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 35,000 reported delay minutes here, that single category outweighs weather by 13 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 34,705 reported delay minutes.

35% 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 12,000 minutes out of 35,000, against a national NAS share of 21%.

Cancellations are rare at 0.23%, below the national 1.81% 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 costs

For anyone connecting, the number to weigh is not the on-time rate but the tail behind it. 9.1% of arrivals land a full hour or more behind schedule, against 8.1% nationally. A 45-minute layover does not survive that, and no amount of walking quickly changes it. Ninety minutes is the honest minimum here, and two hours if the onward flight is on a separate ticket.

The 770 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 76.4% on time across 2,172 flights, 3 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 3 points of difference.

The sample behind these figures is 2,165 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 Southwest on STL to DEN

Put together: the calendar is the biggest lever here, not the carrier or the hour. January runs 85.2% on time and June 61.4%, a swing of 24 points. If the dates move at all, move them out of June before you think about anything else. 2,156 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