Route reliability record
SEA→ONT
Seattle to Ontario
- Flights
- 2,550
- Period
- July 2025 to June 2026
- Distance
- 956 mi
- Carriers
- 4
This route lands a B, 5 points better than the national rate of 77.6%. About one flight in six arrive 15 minutes late or worse, and those flights are late by about 50 minutes on average. That is a workable record, not a spotless one. That rests on 2,550 flights, of which SkyWest Airlines flew 1,084 at 89.7% on time.
Set against the rest of the country, this route's record is 5 points better than the US average of 77.6%. That gap is worth roughly 53 fewer late arrivals for every thousand flights.
Key figures
On time
82.9%
Arrivals under 15 minutes late, of 2,518 completed flights
Avg delay
50 min
Across delayed arrivals only, not all flights
Cancelled
1.25%
National rate is 1.81%
Best carrier
SkyWest
89.7% on time across 1,084 flights
These figures cover 2,550 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 SEA to ONT
The choice of carrier is not cosmetic here. SkyWest Airlines arrives on time 89.7% of the time; Alaska Airlines manages 77.4%. Sample sizes sit next to every figure, because a two-point gap across 1,268 flights means less than a ten-point one.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| Delta Air Linesthin sample | 29 | 93.1% | 32 min | 0.00% | A+ |
| SkyWest Airlines | 1,084 | 89.7% | 52 min | 0.37% | A+ |
| Frontier Airlinesthin sample | 169 | 78.4% | 77 min | 1.18% | B |
| Alaska Airlines | 1,268 | 77.4% | 47 min | 2.05% | B |
The spread between SkyWest and Alaska is 12 points, wide enough that it should decide the booking. On a hundred trips that is 12 more arrivals inside the on-time window.
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 11 points, against 12 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am128 flights91.3%
- 8am–11am641 flights88.5%
- 11am–2pm497 flights80.7%
- 2pm–5pm550 flights80.1%
- 5pm–8pm68 flights83.8%
- 8pm–5am666 flights79.9%
Scale runs 70% to 95%. Rows with fewer than 30 flights show a dash.
Book the early departure. Flights leaving between 5am–8am arrive on time 91.3% of the time, against 79.9% for the overnight window. That is 11 points of difference for no extra money, and it comes from the same aircraft having had no earlier flight to fall behind on. 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 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.
December is the weak month at 72.2% on time, against 91.1% in September. Winter delays cluster: a single storm system can account for most of a month's shortfall, so an average December hides both very good weeks and genuinely unflyable days. July was the busiest month here, with 302 flights.
Across the week, Saturday is the strongest day at 86.8% on time and Friday the weakest at 79.2%. 8 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. Carrier accounts for 48% of the 22,000 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.
- 48%Carrier
- 31%Late-arriving aircraft
- 13%National Airspace System
- 7%Extreme weather
- 0%Security
The largest share of delay minutes, 48%, 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 10,000 of the 22,000 delay minutes reported here. The split is drawn from 21,659 reported delay minutes.
The practical consequence: the 5am–8am departure is the one that starts clean. Its aircraft has no earlier leg to be late from, which is exactly why that window shows 91.3% on time here.
Cancellations sit at 1.25% against a national 1.81%, which is ordinary. Diversions add another 0.00%.
Connections and stage length
What a delay here actually costs
Badly late arrivals are rare here: only 4.1% land an hour or more behind schedule, against 8.1% nationally. Connections built on this flight are about as safe as US domestic connections get, though a cancelled flight still ignores every layover you give it.
The 956 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 86.4% on time across 2,551 flights, 4 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 4 points of difference.
The sample behind these figures is 2,550 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. September runs 91.1% on time and December 72.2%, a swing of 19 points. If the dates move at all, move them out of December before you think about anything else. 2,518 completed flights over 12 months sit behind that.
Common questions
SEA to ONT questions
How often is SEA to ONT delayed?
About one flight in six arrive at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 17.1% of the 2,550 flights in this analysis, against 22.4% nationally. Delayed flights are late by about 50 minutes on average.
Which airline is most reliable on SEA to ONT?
SkyWest Airlines leads at 89.7% on time across 1,084 flights. Alaska Airlines is at the other end on 77.4%, a gap of 12 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 SEA to ONT?
The 5am–8am departure window is the strongest at 91.3% on time. The 8pm–5am window is the weakest at 79.9%, 11 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 December delays on SEA to ONT?
December is the weakest month at 72.2% on time, compared with 91.1% in September. 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
- ONT to SEA — the return legseparate record
- Seattle-Tacoma International (SEA) delay statistics
- Delta Air Lines on-time record93.1%
- SkyWest Airlines on-time record89.7%
- Frontier Airlines on-time record78.4%
- The best time of day to fly, according to 7.04 million flight records
- Worst months to fly in the US, ranked by the data
Direction matters. SEA to ONT and ONT to SEA 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.