Route reliability record
ONT→DEN
Ontario to Denver
- Flights
- 2,672
- Period
- July 2025 to June 2026
- Distance
- 819 mi
- Carriers
- 3
This route lands a B+, 2 points better than the national rate of 77.6%. About one flight in five arrive 15 minutes late or worse, and those flights are late by a little over an hour on average. That is a workable record, not a spotless one. That rests on 2,672 flights, of which United Airlines flew 853 at 85.1% on time.
United Airlines is the most reliable of the three carriers covered here at 85.1% on time across 853 flights.
Key figures
On time
79.7%
Arrivals under 15 minutes late, of 2,651 completed flights
Avg delay
70 min
Across delayed arrivals only, not all flights
Cancelled
0.45%
National rate is 1.81%
Best carrier
United
85.1% on time across 853 flights
These figures cover 2,672 flights between July 2025 and June 2026. The median arrival is 6 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
Which airline is most reliable on ONT to DEN
The choice of carrier is not cosmetic here. United Airlines arrives on time 85.1% of the time; Southwest Airlines manages 75.9%. Sample sizes sit next to every figure, because a two-point gap across 1,267 flights means less than a ten-point one.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| United Airlines | 853 | 85.1% | 85 min | 0.35% | A+ |
| Frontier Airlines | 552 | 80.1% | 98 min | 0.72% | B+ |
| Southwest Airlines | 1,267 | 75.9% | 53 min | 0.39% | B+ |
United leads Southwest by 9 points. Real, but small enough that a better departure time on the weaker carrier can outweigh it.
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 32 points, against 9 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am947 flights91.6%
- 8am–11am372 flights89.2%
- 11am–2pm579 flights74.2%
- 2pm–5pm437 flights59.8%
- 5pm–8pm203 flights65.2%
- 8pm–5am134 flights79.7%
Scale runs 50% to 95%. Rows with fewer than 30 flights show a dash.
Book the early departure. Flights leaving between 5am–8am arrive on time 91.6% of the time, against 59.8% for the afternoon window. That is 32 points of difference for no extra money, and it comes from the same aircraft having had no earlier flight to fall behind on. The 5am–8am window is the busiest, with 947 flights scheduled in it.
Across the year
When delays peak on this route
This route's year swings by 23 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 65.2% on time, 23 points below January, the best at 88.0%. 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 272 flights.
Across the week, Saturday is the strongest day at 83.2% on time and Monday the weakest at 77.7%. 6 points is a smaller lever than departure hour, but it is free.
Delay causes
Why this route gets delayed
One category carries most of the weight here. Late-arriving aircraft accounts for 43% of the 37,000 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.
- 43%Late-arriving aircraft
- 34%National Airspace System
- 22%Carrier
- 1%Extreme weather
- 0%Security
43% 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 37,000 reported delay minutes here, that single category outweighs weather by 30 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 37,392 reported delay minutes.
34% 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 13,000 minutes out of 37,000, against a national NAS share of 21%.
Cancellations are rare at 0.45%, 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
7.5% 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 are the a little over an hour kind, which a normal layover absorbs.
The 819 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 73.4% on time across 2,671 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.
The sample behind these figures is 2,672 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. January runs 88.0% on time and July 65.2%, a swing of 23 points. If the dates move at all, move them out of July before you think about anything else. 2,651 completed flights over 12 months sit behind that.
Common questions
ONT to DEN questions
How often is ONT to DEN delayed?
About one flight in five arrive at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 20.3% of the 2,672 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 ONT to DEN?
United Airlines leads at 85.1% on time across 853 flights. Southwest Airlines is at the other end on 75.9%, a gap of 9 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 ONT to DEN?
The 5am–8am departure window is the strongest at 91.6% on time. The 2pm–5pm window is the weakest at 59.8%, 32 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 ONT to DEN?
July is the weakest month at 65.2% on time, compared with 88.0% in January. Across the year that is 23 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 ONT — the return legseparate record
- Denver International (DEN) delay statistics
- United Airlines on-time record85.1%
- Frontier Airlines on-time record80.1%
- Southwest Airlines on-time record75.9%
- 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. ONT to DEN and DEN to ONT 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.