Route reliability record

ONT→SEA

Ontario to Seattle

Flights: 2,551
Period: July 2025 to June 2026
Distance: 956 mi
Carriers: 4

This route earns a 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 77.6%. When something does go wrong the average delay runs about an hour. That rests on 2,551 flights, of which SkyWest Airlines flew 1,085 at 92.1% on time.

Set against the rest of the country, this route's record is 9 points better than the US average of 77.6%. That gap is worth roughly 88 fewer late arrivals for every thousand flights.

On time

86.4%

Arrivals under 15 minutes late, of 2,516 completed flights

Avg delay

63 min

Across delayed arrivals only, not all flights

Cancelled

1.29%

National rate is 1.81%

Best carrier

SkyWest

92.1% on time across 1,085 flights

These figures cover 2,551 flights between July 2025 and June 2026. The median arrival is 8 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 ONT to SEA

The choice of carrier is not cosmetic here. SkyWest Airlines arrives on time 92.1% of the time; Alaska Airlines manages 81.7%. Sample sizes sit next to every figure, because a two-point gap across 1,268 flights means less than a ten-point one.

On-time performance by carrier on ONT to SEA
Airline	Flights	On time	Avg delay	Cancelled	Grade
Delta Air Linesthin sample	29	93.1%	43 min	0.00%	A+
SkyWest Airlines	1,085	92.1%	83 min	0.28%	A+
Frontier Airlinesthin sample	169	82.8%	96 min	0.00%	B+
Alaska Airlines	1,268	81.7%	52 min	2.37%	B

SkyWest leads Alaska by 10 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 17 points, against 10 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.

5am–8am715 flights93.9%
8am–11am181 flights87.3%
11am–2pm592 flights89.5%
2pm–5pm522 flights81.2%
5pm–8pm516 flights77.0%
8pm–5am25 flights—

Scale runs 70% to 100%. Rows with fewer than 30 flights show a dash.

Book the early departure. Flights leaving between 5am–8am arrive on time 93.9% of the time, against 77.0% for the evening window. That is 17 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.

Across the year

When delays peak on this route

This route's year swings by 14 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.

Darker means more delays. Figures are on-time percentages.February best, July worst

July is the worst month at 81.6% on time, 14 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 302 flights.

Across the week, Wednesday is the strongest day at 88.1% on time and Friday the weakest at 84.1%. 4 points is a smaller lever than departure hour, but it is free.

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 22,000 reported delay minutes.

Carrier
42%
Late-arriving aircraft
37%
National Airspace System
16%
Extreme weather
5%
Security
0%

37% 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 22,000 reported delay minutes here, that single category outweighs weather by 7 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 21,663 reported delay minutes.

The largest share of delay minutes, 42%, 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 9,012 of the 22,000 delay minutes reported here.

Cancellations sit at 1.29% against a national 1.81%, 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 3.6% 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 82.9% on time across 2,550 flights, 4 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 4 points of difference.

The sample behind these figures is 2,551 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. February runs 95.6% on time and July 81.6%, a swing of 14 points. If the dates move at all, move them out of July before you think about anything else. 2,516 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 arrive at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 13.6% of the 2,551 flights in this analysis, against 22.4% nationally. Delayed flights are late by about an hour on average.

Which airline is most reliable on ONT to SEA?

SkyWest Airlines leads at 92.1% on time across 1,085 flights. Alaska Airlines is at the other end on 81.7%, 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 93.9% on time. The 5pm–8pm window is the weakest at 77.0%, 17 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 July delays on ONT to SEA?

July is the weakest month at 81.6% on time, compared with 95.6% in February. Across the year that is 14 points between the best and worst months. Moving flexible dates out of that month is one of the few things that reliably helps.

Where to go next

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.

Source: US DOT Bureau of Transportation Statistics, on-time performance data, July 2025 to June 2026. Updated monthly. How this data is collected.

Key figures