Route reliability record
STL→DEN
St. Louis to Denver
- Flights
- 3,687
- Period
- September 2025 to August 2026
- Distance
- 770 mi
- Carriers
- 4
This route sits in the middle of the pack with a B+. Close to three flights in four arrive on time by the DOT's definition, 2 points worse than the national rate of 77.3%. The late ones land a little over an hour behind schedule. That rests on 3,687 STL to DEN flights, of which United Airlines flew 1,116 at 78.8% on time.
Best time to fly STL to DEN: a 5am–8am departure, ideally on a Tuesday. Flights in that window arrived on time 92.4% of the time and Tuesday flights 80.4%, against 75.0% for the route overall.
The US average over the same twelve months is 77.3%, 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
75.0%
Arrivals under 15 minutes late, of 3,662 completed flights
Avg delay
69 min
Across delayed arrivals only, not all flights
Cancelled
0.33%
National rate is 1.89%
Best carrier
United
78.8% on time across 1,116 flights
These figures cover 3,687 flights between September 2025 and August 2026. Beyond the headline rate, 9.4% of arrivals land a full hour or more behind schedule, against 8.3% nationally.
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Carrier by carrier
Which airline is most reliable on STL to DEN
The choice of carrier is not cosmetic here. United Airlines arrives on time 78.8% of the time; Southwest Airlines manages 72.6%. Sample sizes sit next to every figure, because a two-point gap across 2,190 flights means less than a ten-point one.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| SkyWest Airlinesthin sample | 89 | 90.9% | 113 min | 1.12% | A |
| United Airlines | 1,116 | 78.8% | 83 min | 0.63% | B+ |
| Frontier Airlinesthin sample | 292 | 74.5% | 90 min | 0.68% | C+ |
| Southwest Airlines | 2,190 | 72.6% | 60 min | 0.09% | C+ |
United leads Southwest by 6 points. Real, but small enough that a better departure time on the weaker carrier can outweigh it.
Flight by flight
Flights on STL to DEN
The flight numbers below flew this route at least 250 times in the period, busiest first. Each has its own record, with its schedule and delay causes.
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Best ways to make this trip
The most reliable ways to fly St. Louis to Denver
Every airline on this route, ranked by how often a scheduled flight actually operated and arrived on time, with its best departure time and most punctual flight number. No nearby airport pair beat these.
| Option | Airline | Success | On time | Cancelled | Best departure | Most punctual flight |
|---|---|---|---|---|---|---|
| 1. STL → DENSt. Louis to Denver | United Airlines | 78.3% | 78.8% | 0.63% | 5am–8am, Wed | — |
| 2. STL → DENSt. Louis to Denver | Southwest Airlines | 72.5% | 72.6% | 0.09% | 5am–8am, Tue | — |
Success is the share of scheduled flights that operated and arrived less than 15 minutes late, so a cancelled flight counts against it. “(route)” means the best time comes from every carrier on the route rather than this airline alone.
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 38 points, against 6 points between the best and worst carrier.
- 5am–8am624 flights92.4%
- 8am–11am690 flights83.8%
- 11am–2pm567 flights81.6%
- 2pm–5pm767 flights66.2%
- 5pm–8pm576 flights54.5%
- 8pm–5am463 flights70.7%
Scale runs 45% to 100%.
The 5pm–8pm window is where this operation falls apart: 54.5% on time, against 92.4% 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 2pm–5pm window is the busiest, with 767 flights scheduled in it.
Explore
Filter STL to DEN by airline, time, month and day
Pick a breakdown and a carrier. The dashed mark on each row is the same slice for every US flight.
On STL to DEN, the best departure window is 5am–8am at 92.4% on time and the worst is 5pm–8pm at 54.5%, a wide gap of 37.9 points. All US flights leaving 5am–8am ran 89.3%.
- 5–8am92.4%
- 8–11am83.8%
- 11am–2pm81.6%
- 2–5pm66.2%
- 5–8pm54.5%
- 8pm–5am70.7%
best all US flightsScale 45% to 95%.
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.
August is the worst month at 65.4% 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. July was the busiest month here, with 373 flights.
Across the week, Tuesday is the strongest day at 80.4% on time and Monday the weakest at 72.0%, a spread of 8 points.
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 63,000 reported delay minutes.
- 41%Late-arriving aircraft
- 33%National Airspace System
- 21%Carrier
- 5%Extreme weather
41% of delay minutes trace back to a late-arriving aircraft, meaning the plane was already behind before it got here. Across 63,000 reported delay minutes here, that single category outweighs weather by 8 to one. The split is drawn from 62,666 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 63,000, against a national NAS share of 21%.
Cancellations are rare at 0.33%, below the national 1.89% 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
9.4% of arrivals land a full hour or more behind schedule, against 8.3% nationally. Ninety minutes is the honest minimum for a connection 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.
The return leg runs 77.7% on time across 3,699 flights, within 3 points of this direction. The two legs are scored separately because they usually differ; on this city pair they happen not to.
The sample behind these figures is 3,687 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.
Work out the risk of missing a connection in DEN after this flight
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 August 65.4%, a swing of 19 points. If the dates move at all, move them out of August before you think about anything else. 3,662 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 arrives at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 25.0% of the 3,687 flights in this analysis, against 22.7% 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 78.8% on time across 1,116 flights. Southwest Airlines is at the other end on 72.6%, a gap of 6 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 92.4% on time. The 5pm–8pm window is the weakest at 54.5%, 38 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 August delays on STL to DEN?
August is the weakest month at 65.4% 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
- United Airlines on STL to DEN78.8%
- Frontier Airlines on STL to DEN74.5%
- Southwest Airlines on STL to DEN72.6%
- STL to ATL, a related route84.6%
- PHX to DEN, a related route76.7%
- STL to ORD, a related route72.5%
- SkyWest Airlines on-time record90.9%
- United Airlines on-time record78.8%
- Frontier Airlines on-time record74.5%
- Why the first flight of the morning is statistically safest
- How often a connection fails at the major US hubs
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.
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Source: US DOT Bureau of Transportation Statistics, on-time performance data, September 2025 to August 2026. Last updated ; recomputed monthly. How this data is collected.