Route reliability record
STL→DEN
St. Louis to Denver
- Flights
- 3,655
- Period
- July 2025 to June 2026
- Distance
- 770 mi
- Carriers
- 4
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, 3 points worse than the national rate of 77.6%. The late ones land a little over an hour behind schedule. That rests on 3,655 flights, of which United Airlines flew 1,071 at 76.6% on time.
The US average over the same twelve months is 77.6%, so there is little to separate this route from the country as a whole. The variation that matters here is internal: which carrier, and what time of day.
Key figures
On time
74.9%
Arrivals under 15 minutes late, of 3,629 completed flights
Avg delay
70 min
Across delayed arrivals only, not all flights
Cancelled
0.41%
National rate is 1.81%
Best carrier
United
76.6% on time across 1,071 flights
These figures cover 3,655 flights between July 2025 and June 2026. Beyond the headline rate, 9.3% 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
Which airline is most reliable on STL to DEN
The two carriers here are closely matched: 3 points separate the best from the worst. At that spread, schedule and price are better tie-breakers than reliability. Sample sizes are shown so you can see which rows are worth weighing.
| 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 | 2,165 | 73.3% | 60 min | 0.23% | C+ |
No carrier stands out. United tops the table at 76.6%, 3 points clear of the bottom, which is inside the range this kind of sample moves year to year.
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 37 points, against 3 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am677 flights91.7%
- 8am–11am682 flights83.6%
- 11am–2pm562 flights80.5%
- 2pm–5pm757 flights64.9%
- 5pm–8pm589 flights54.4%
- 8pm–5am388 flights72.8%
Scale runs 45% to 95%. Rows with fewer than 30 flights show a dash.
The 5pm–8pm window is where this operation falls apart: 54.4% on time, against 91.7% 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 2pm–5pm window is the busiest, with 757 flights scheduled in it.
Across the year
When delays peak on this route
This route's year swings by 19 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.
July is the worst month at 65.5% on time, 19 points below February, the best at 84.8%. 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 358 flights.
Across the week, Tuesday is the strongest day at 79.8% on time and Monday the weakest at 70.7%. 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 64,000 reported delay minutes.
- 40%Late-arriving aircraft
- 33%National Airspace System
- 24%Carrier
- 3%Extreme weather
40% 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 64,000 reported delay minutes here, that single category outweighs weather by 12 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 63,999 reported delay minutes.
33% 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 21,000 minutes out of 64,000, against a national NAS share of 21%.
Cancellations are rare at 0.41%, 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 actually costs
For anyone connecting, the number to weigh is not the on-time rate but the tail behind it. 9.3% 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 78.1% on time across 3,662 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 3,655 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. February runs 84.8% on time and July 65.5%, a swing of 19 points. If the dates move at all, move them out of July before you think about anything else. 3,629 completed flights over 12 months sit behind that.
Common questions
STL to DEN questions
How often is STL to DEN 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 25.1% of the 3,655 flights in this analysis, against 22.4% nationally. Delayed flights are late by a little over an hour on average.
Which airline is most reliable on STL to DEN?
United Airlines leads at 76.6% on time across 1,071 flights. Southwest Airlines is at the other end on 73.3%, a gap of 3 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 STL to DEN?
The 5am–8am departure window is the strongest at 91.7% on time. The 5pm–8pm window is the weakest at 54.4%, 37 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 STL to DEN?
July is the weakest month at 65.5% on time, compared with 84.8% in February. Across the year that is 19 points between the best and worst months. Moving flexible dates out of that month is one of the few things that reliably helps.
Related
Where to go next
- DEN to STL — the return legseparate record
- St. Louis Lambert International (STL) delay statistics
- Denver International (DEN) delay statistics
- SkyWest Airlines on-time record87.1%
- United Airlines on-time record76.6%
- Frontier Airlines on-time record74.5%
- Why the first flight of the morning is statistically safest
- The best time of day to fly, according to 7.04 million flight records
Direction matters. STL to DEN and DEN to STL 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, July 2025 to June 2026. Updated monthly. How this data is collected.