Route reliability record
ONT→SEA
Ontario to Seattle
- Flights
- 2,488
- Period
- September 2025 to August 2026
- Distance
- 956 mi
- Carriers
- 4
This route earns an A. More than four flights in five arrive inside the 15-minute window the DOT uses to define an on-time arrival, which puts it 9 points better than the national rate of 77.3%. When something does go wrong the average delay runs about an hour. That rests on 2,488 ONT to SEA flights, of which SkyWest Airlines flew 1,054 at 91.7% on time.
Best time to fly ONT to SEA: a 5am–8am departure, ideally on a Tuesday. Flights in that window arrived on time 93.8% of the time and Tuesday flights 88.7%, against 85.9% for the route overall.
Set against the rest of the country, this route's record is 9 points better than the US average of 77.3%. That gap is worth roughly 86 fewer late arrivals for every thousand flights.
Key figures
On time
85.9%
Arrivals under 15 minutes late, of 2,458 completed flights
Avg delay
67 min
Across delayed arrivals only, not all flights
Cancelled
1.13%
National rate is 1.89%
Best carrier
SkyWest
91.7% on time across 1,054 flights
These figures cover 2,488 flights between September 2025 and August 2026. The median arrival is 9 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.
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Carrier by carrier
Which airline is most reliable on ONT to SEA
The choice of carrier is not cosmetic here. SkyWest Airlines arrives on time 91.7% of the time; Alaska Airlines manages 82.2%. Sample sizes sit next to every figure, because a two-point gap across 1,257 flights means less than a ten-point one.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| SkyWest Airlines | 1,054 | 91.7% | 96 min | 0.28% | A+ |
| Delta Air Linesthin sample | 8 | 87.5% | 15 min | 0.00% | A+ |
| Alaska Airlines | 1,257 | 82.2% | 53 min | 1.75% | B |
| Frontier Airlinesthin sample | 169 | 77.1% | 80 min | 1.78% | B |
SkyWest leads Alaska by 10 points. Real, but small enough that a better departure time on the weaker carrier can outweigh it.
Flight by flight
Flights on ONT to SEA
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.
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Best ways to make this trip
The most reliable ways to fly Ontario to Seattle
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. 4 of the top 5 use a different airport at one end.
| Option | Airline | Success | On time | Cancelled | Best departure | Most punctual flight |
|---|---|---|---|---|---|---|
| 1. ONT → SEAOntario to Seattle | SkyWest Airlines | 91.4% | 91.7% | 0.28% | 2pm–5pm, Sat | OO 4031 17:50 |
| 2. LAX → SEALos Angeles to Seattle | United Airlines | 89.1% | 89.1% | 0.00% | 8am–11am, Sat | — |
| 3. SNA → SEASanta Ana to Seattle | Alaska Airlines | 86.9% | 87.9% | 1.13% | 8pm–5am, Wed | AS 1325 06:45 |
| 4. LAX → SEALos Angeles to Seattle | Delta Air Lines | 84.6% | 85.2% | 0.67% | 5am–8am, Tue | DL 2980 07:00 |
| 5. LAX → SEALos Angeles to Seattle | Alaska Airlines | 84.5% | 85.2% | 0.88% | 5am–8am, Mon | AS 1301 18:09 |
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 25 points, against 10 points between the best and worst carrier.
- 5am–8am715 flights93.8%
- 8am–11am125 flights86.4%
- 11am–2pm590 flights88.0%
- 2pm–5pm489 flights81.7%
- 5pm–8pm501 flights78.5%
- 8pm–5am68 flights69.1%
Scale runs 60% to 100%.
Book the early departure. Flights leaving between 5am–8am arrive on time 93.8% of the time, against 69.1% for the overnight window. That is 25 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 715 flights scheduled in it.
Explore
Filter ONT to SEA 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 SEA, the best departure window is 5am–8am at 93.8% on time and the worst is 8pm–5am at 69.1%, a wide gap of 24.7 points. All US flights leaving 5am–8am ran 89.3%.
- 5–8am93.8%
- 8–11am86.4%
- 11am–2pm88.0%
- 2–5pm81.7%
- 5–8pm78.5%
- 8pm–5am69.1%
best all US flightsScale 60% to 100%.
Across the year
When delays peak on this route
This route's year swings by 16 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.
August is the worst month at 79.3% on time, 16 points below February, the best at 95.6%. 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 256 flights.
Across the week, Tuesday is the strongest day at 88.7% on time and Monday the weakest at 82.8%, a spread of 6 points.
Delay causes
Why this route gets delayed
Carrier accounts for 46% of the 23,000 delay minutes carriers reported here, and the remaining four split what is left.
- 46%Carrier
- 37%Late-arriving aircraft
- 12%National Airspace System
- 5%Extreme weather
- 0%Security
37% of delay minutes trace back to a late-arriving aircraft, meaning the plane was already behind before it got here. Across 23,000 reported delay minutes here, that single category outweighs weather by 7 to one. The split is drawn from 23,115 reported delay minutes.
The largest share of delay minutes, 46%, falls under carrier control: crew scheduling, maintenance, fuelling, baggage. It accounts for 11,000 of the 23,000 delay minutes reported here.
Cancellations sit at 1.13% against a national 1.89%, which is ordinary. Diversions add another 0.08%.
Connections and stage length
What a delay here actually costs
Badly late arrivals are rare here: only 3.9% land an hour or more behind schedule, against 8.3% 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.
The return leg is a different operation and performs like one: it runs 82.6% on time across 2,487 flights, 3 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 3 points of difference.
The sample behind these figures is 2,488 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 SEA 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. February runs 95.6% on time and August 79.3%, a swing of 16 points. If the dates move at all, move them out of August before you think about anything else. 2,458 completed flights over 12 months sit behind that.
Common questions
ONT to SEA questions
How often is ONT to SEA delayed?
About one flight in seven arrives at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 14.1% of the 2,488 flights in this analysis, against 22.7% nationally. Delayed flights are late by about an hour on average.
Which airline is most reliable on ONT to SEA?
SkyWest Airlines leads at 91.7% on time across 1,054 flights. Alaska Airlines is at the other end on 82.2%, a gap of 10 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 SEA?
The 5am–8am departure window is the strongest at 93.8% on time. The 8pm–5am window is the weakest at 69.1%, 25 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 August delays on ONT to SEA?
August is the weakest month at 79.3% on time, compared with 95.6% in February. Across the year that is 16 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
- SEA to ONT — the return legseparate record
- Ontario International (ONT) delay statistics
- Seattle-Tacoma International (SEA) delay statistics
- SkyWest Airlines on ONT to SEA91.7%
- Alaska Airlines on ONT to SEA82.2%
- Frontier Airlines on ONT to SEA77.1%
- ONT to PHX, a related route82.4%
- LAX to SEA, a related route85.0%
- ONT to DEN, a related route81.8%
- SkyWest Airlines on-time record91.7%
- Delta Air Lines on-time record87.5%
- Alaska Airlines on-time record82.2%
- Why the first flight of the morning is statistically safest
- The best time of day to fly, according to 7.05 million flight records
Direction matters. ONT to SEA and SEA to ONT are scored separately because they are separate operations, with different departure banks and different aircraft. Compare the return leg.
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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.