Carrier route record
WNDEN→ONT
Southwest Airlines · Denver to Ontario
- Flights
- 1,267
- Period
- July 2025 to June 2026
- Share of route
- 47%
- Carriers
- 3
Historical statistics for July 2025 to June 2026, not live status. For today’s flights, check with Southwest Airlines.
Southwest on DEN to ONT sits in the middle of the pack with a C+. About seven flights in ten arrive on time by the DOT's definition, 10 points worse than the national rate of 77.6%. The late ones land roughly 50 minutes behind schedule. That rests on 1,267 flights. The strongest carrier on the route with a full sample, Frontier Airlines, flew 551 at 79.1% on time.
The national on-time rate over the same period is 77.6%, so Southwest on DEN to ONT runs 10 points behind the country as a whole. Across a thousand flights that is about 96 extra late arrivals.
Key figures
On time
68.0%
Arrivals under 15 minutes late, of 1,265 completed flights
Avg delay
49 min
Across delayed arrivals only, not all flights
Cancelled
0.16%
National rate is 1.81%
Whole route
73.4%
Every carrier on DEN to ONT, 2,671 flights
These figures cover 1,267 flights between July 2025 and June 2026. Beyond the headline rate, 8.5% 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
Southwest against every carrier on DEN to ONT
Southwest flew 1,267 of the 2,671 flights on DEN to ONT, about 47% of it. Its 68.0% on time is 5 points worse than the route as a whole, which runs 73.4%. Ranked by on-time rate it is last of three carriers on the route, where the range runs from Frontier at 79.1% to Southwest at 68.0%.
The two carriers here are closely matched: about a point 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 |
|---|---|---|---|---|---|
| Frontier Airlines | 551 | 79.1% | 62 min | 0.36% | B+ |
| United Airlines | 853 | 77.9% | 68 min | 0.00% | B+ |
| Southwest Airlines (this page) | 1,267 | 68.0% | 49 min | 0.16% | C+ |
No carrier stands out. Frontier tops the table at 79.1%, about a point clear of the bottom, which is inside the range this kind of sample moves year to year.
Departure window
When Southwest runs best on this route
Departure time matters more here than the choice of carrier. The gap between the best and worst window is 45 points, against about a point between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am38 flights78.9%
- 8am–11am334 flights90.1%
- 11am–2pm239 flights71.8%
- 2pm–5pm97 flights72.2%
- 5pm–8pm206 flights45.4%
- 8pm–5am353 flights55.2%
Scale runs 35% to 95%. Rows with fewer than 30 flights show a dash.
The 5pm–8pm window is where this operation falls apart: 45.4% on time, against 90.1% 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 353 flights scheduled in it.
Across the year
Southwest on DEN to ONT through the year
June is the worst month at 44.7% on time, 37 points below February, the best at 81.9%. 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 119 flights.
Across the week, Wednesday is the strongest day at 75.8% on time and Monday the weakest at 60.2%. 16 points is a smaller lever than departure hour, but it is free.
Delay causes
Why Southwest runs late on this route
One category carries most of the weight here. Late-arriving aircraft accounts for 60% of the 20,000 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.
- 60%Late-arriving aircraft
- 32%Carrier
- 8%National Airspace System
- 1%Extreme weather
- 0%Security
60% 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 20,000 reported delay minutes here, that single category outweighs weather by 60 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 19,681 reported delay minutes.
The practical consequence: the 8am–11am departure is the one that starts clean. Its aircraft has no earlier leg to be late from, which is exactly why that window shows 90.1% on time here.
Cancellations are rare at 0.16%, 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 costs
8.5% 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 last roughly 50 minutes, 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 75.9% on time across 1,267 flights, 8 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 8 points of difference.
The sample behind these figures is 1,267 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 Southwest on DEN to ONT
Put together: the calendar is the biggest lever here, not the carrier or the hour. February runs 81.9% on time and June 44.7%, a swing of 37 points. If the dates move at all, move them out of June before you think about anything else. 1,265 completed flights over 12 months sit behind that.
Related
Where to go next
- DEN to ONT, every carrier
- Southwest on ONT to DEN, the return leg
- Frontier Airlines on DEN to ONT79.1%
- United Airlines on DEN to ONT77.9%
- Southwest Airlines network-wide record
- Denver International (DEN) delay statistics
- Why the first flight of the morning is statistically safest
- The best time of day to fly, according to 7.04 million flight records
Source: US DOT Bureau of Transportation Statistics, on-time performance data, July 2025 to June 2026. Last updated 15 September 2026; recomputed monthly. How this data is collected.