Route reliability record
DEN→ONT
Denver to Ontario
- Flights
- 2,671
- Period
- July 2025 to June 2026
- Distance
- 819 mi
- Carriers
- 3
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, 4 points worse than the national rate of 77.6%. The late ones land about an hour behind schedule. That rests on 2,671 flights, of which Frontier Airlines flew 551 at 79.1% on time.
Frontier Airlines is the most reliable of the three carriers covered here at 79.1% on time across 551 flights.
Key figures
On time
73.4%
Arrivals under 15 minutes late, of 2,666 completed flights
Avg delay
56 min
Across delayed arrivals only, not all flights
Cancelled
0.15%
National rate is 1.81%
Best carrier
Frontier
79.1% on time across 551 flights
These figures cover 2,671 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
Which airline is most reliable on DEN to ONT
The choice of carrier is not cosmetic here. Frontier Airlines arrives on time 79.1% of the time; Southwest Airlines manages 68.0%. 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 |
|---|---|---|---|---|---|
| Frontier Airlines | 551 | 79.1% | 62 min | 0.36% | B+ |
| United Airlines | 853 | 77.9% | 68 min | 0.00% | B+ |
| Southwest Airlines | 1,267 | 68.0% | 49 min | 0.16% | C+ |
Frontier leads Southwest by 11 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 28 points, against 11 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am123 flights88.6%
- 8am–11am438 flights90.0%
- 11am–2pm621 flights81.5%
- 2pm–5pm219 flights69.3%
- 5pm–8pm604 flights64.0%
- 8pm–5am666 flights62.2%
Scale runs 55% to 95%. Rows with fewer than 30 flights show a dash.
The 8pm–5am window is where this operation falls apart: 62.2% on time, against 90.0% for the 8am–11am 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 8pm–5am window is the busiest, with 666 flights scheduled in it.
Across the year
When delays peak on this route
This route's year swings by 28 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.
June is the worst month at 55.8% on time, 28 points below November, the best at 83.6%. 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, Wednesday is the strongest day at 78.9% on time and Monday the weakest at 66.5%. 12 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 55% of the 40,000 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.
- 55%Late-arriving aircraft
- 34%Carrier
- 9%National Airspace System
- 1%Extreme weather
- 0%Security
55% 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 40,000 reported delay minutes here, that single category outweighs weather by 55 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 39,709 reported delay minutes.
The largest share of delay minutes, 34%, 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 14,000 of the 40,000 delay minutes reported here.
Cancellations are rare at 0.15%, 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
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 are the about 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 79.7% on time across 2,672 flights, 6 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 6 points of difference.
The sample behind these figures is 2,671 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. November runs 83.6% on time and June 55.8%, a swing of 28 points. If the dates move at all, move them out of June before you think about anything else. 2,666 completed flights over 12 months sit behind that.
Common questions
DEN to ONT questions
How often is DEN to ONT 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 26.6% of the 2,671 flights in this analysis, against 22.4% nationally. Delayed flights are late by about an hour on average.
Which airline is most reliable on DEN to ONT?
Frontier Airlines leads at 79.1% on time across 551 flights. Southwest Airlines is at the other end on 68.0%, a gap of 11 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 DEN to ONT?
The 8am–11am departure window is the strongest at 90.0% on time. The 8pm–5am window is the weakest at 62.2%, 28 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 June delays on DEN to ONT?
June is the weakest month at 55.8% on time, compared with 83.6% in November. Across the year that is 28 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
- ONT to DEN — the return legseparate record
- Denver International (DEN) delay statistics
- Frontier Airlines on-time record79.1%
- United Airlines on-time record77.9%
- Southwest Airlines on-time record68.0%
- 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. DEN to ONT and ONT to DEN 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.