Route reliability record
ONT→DEN
Ontario to Denver
- Flights
- 2,624
- Period
- September 2025 to August 2026
- Distance
- 819 mi
- Carriers
- 3
This route lands a B+, 5 points better than the national rate of 77.3%. Roughly one flight in five arrives 15 minutes late or worse, and those flights are late by a little over an hour on average. That rests on 2,624 ONT to DEN flights, of which United Airlines flew 797 at 87.1% on time.
Best time to fly ONT to DEN: a 5am–8am departure, ideally on a Sunday. Flights in that window arrived on time 92.8% of the time and Sunday flights 85.1%, against 81.8% for the route overall.
United Airlines is the most reliable of the three carriers covered here at 87.1% on time across 797 flights.
Key figures
On time
81.8%
Arrivals under 15 minutes late, of 2,606 completed flights
Avg delay
68 min
Across delayed arrivals only, not all flights
Cancelled
0.38%
National rate is 1.89%
Best carrier
United
87.1% on time across 797 flights
These figures cover 2,624 flights between September 2025 and August 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.
Fare search by Aviasales, a Travelpayouts partner. Booking through it may earn this site a commission at no cost to you; it changes no figure here.
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 87.1% of the time; Southwest Airlines manages 76.4%. Sample sizes sit next to every figure, because a two-point gap across 1,269 flights means less than a ten-point one.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| United Airlines | 797 | 87.1% | 83 min | 0.38% | A+ |
| Frontier Airlines | 558 | 86.5% | 99 min | 0.72% | A+ |
| Southwest Airlines | 1,269 | 76.4% | 55 min | 0.24% | B+ |
United leads Southwest by 11 points. Real, but small enough that a better departure time on the weaker carrier can outweigh it.
Flight by flight
Flights on ONT 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.
Timetable by Aviasales, a Travelpayouts partner. Booking through it may earn this site a commission at no cost to you; it changes no figure here.
Best ways to make this trip
The most reliable ways to fly Ontario to Denver
Every airline on this route and on the routes between nearby airports, ranked by how often a scheduled flight actually operated and arrived on time. 3 of the top 5 use a different airport at one end.
| Option | Airline | Success | On time | Cancelled | Best departure | Most punctual flight |
|---|---|---|---|---|---|---|
| 1. LAX → DENLos Angeles to Denver | United Airlines | 87.7% | 87.7% | 0.00% | 5am–8am, Sun | UA 1912 08:47 |
| 2. BUR → DENBurbank to Denver | United Airlines | 87.3% | 87.5% | 0.24% | 8am–11am, Sat | UA 1191 09:00 |
| 3. ONT → DENOntario to Denver | United Airlines | 86.8% | 87.1% | 0.38% | 5am–8am, Sun | UA 2446 05:00 |
| 4. SNA → DENSanta Ana to Denver | United Airlines | 86.7% | 86.9% | 0.26% | 8am–11am, Sat | UA 2656 18:26 |
| 5. ONT → DENOntario to Denver | Frontier Airlines | 85.9% | 86.5% | 0.72% | 5am–8am, Fri | F9 1500 06:00 |
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 32 points, against 11 points between the best and worst carrier.
- 5am–8am996 flights92.8%
- 8am–11am442 flights89.3%
- 11am–2pm583 flights73.1%
- 2pm–5pm434 flights60.8%
- 5pm–8pm91 flights75.6%
- 8pm–5am78 flights87.2%
Scale runs 50% to 100%.
Book the early departure. Flights leaving between 5am–8am arrive on time 92.8% of the time, against 60.8% 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 996 flights scheduled in it.
Explore
Filter ONT 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 ONT to DEN, the best departure window is 5am–8am at 92.8% on time and the worst is 2pm–5pm at 60.8%, a wide gap of 32.0 points. All US flights leaving 5am–8am ran 89.3%.
- 5–8am92.8%
- 8–11am89.3%
- 11am–2pm73.1%
- 2–5pm60.8%
- 5–8pm75.6%
- 8pm–5am87.2%
best all US flightsScale 50% to 95%.
Across the year
When delays peak on this route
This route's year swings by 15 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.
Delays peak in March, when the on-time rate falls to 73.0%. January is the strongest month at 88.0%. Moving a flexible trip across that boundary buys 15 points. June was the busiest month here, with 250 flights.
Across the week, Sunday is the strongest day at 85.1% on time and Wednesday the weakest at 78.7%, a spread of 6 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 32,000 reported delay minutes.
- 39%Late-arriving aircraft
- 34%National Airspace System
- 25%Carrier
- 1%Extreme weather
- 0%Security
39% of delay minutes trace back to a late-arriving aircraft, meaning the plane was already behind before it got here. Across 32,000 reported delay minutes here, that single category outweighs weather by 28 to one. The split is drawn from 32,219 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 11,000 minutes out of 32,000, against a national NAS share of 21%.
Cancellations are rare at 0.38%, 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
6.1% of arrivals land an hour or more late, close to the national 8.3%, so a 60 to 75 minute layover is reasonable here while anything under 45 minutes is a bet. Most delays on this route last a little over an hour, 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.
The return leg is a different operation and performs like one: it runs 74.8% on time across 2,623 flights, 7 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 7 points of difference.
The sample behind these figures is 2,624 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. January runs 88.0% on time and March 73.0%, a swing of 15 points. If the dates move at all, move them out of March before you think about anything else. 2,606 completed flights over 12 months sit behind that.
Common questions
ONT to DEN questions
How often is ONT to DEN delayed?
Roughly one flight in five arrives at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 18.2% of the 2,624 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 ONT to DEN?
United Airlines leads at 87.1% on time across 797 flights. Southwest Airlines is at the other end on 76.4%, 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 ONT to DEN?
The 5am–8am departure window is the strongest at 92.8% on time. The 2pm–5pm window is the weakest at 60.8%, 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 March delays on ONT to DEN?
March is the weakest month at 73.0% on time, compared with 88.0% in January. Across the year that is 15 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
- Ontario International (ONT) delay statistics
- Denver International (DEN) delay statistics
- United Airlines on ONT to DEN87.1%
- Frontier Airlines on ONT to DEN86.5%
- Southwest Airlines on ONT to DEN76.4%
- ONT to PHX, a related route82.4%
- PHX to DEN, a related route76.7%
- ONT to SEA, a related route85.9%
- United Airlines on-time record87.1%
- Frontier Airlines on-time record86.5%
- Southwest Airlines on-time record76.4%
- Same route, different airline: where the carrier you pick matters most
- Why the first flight of the morning is statistically safest
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.
Embed ONT to DEN’s on-time badge on your site
Paste this into a web page. The badge shows the current on-time rate, cancellation rate and grade, and updates every month. Preview it, or build a badge for another route or airport.
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.