Route reliability record
STL→SEA
St. Louis to Seattle
- Flights
- 884
- Period
- July 2025 to June 2026
- Distance
- 1,709 mi
- Carriers
- 2
This route lands a B+, level with the national rate of 77.6%. Roughly one flight in four arrives 15 minutes late or worse, and those flights are late by roughly 50 minutes on average. That is a workable record, not a spotless one. That rests on 884 STL to SEA flights, with a median arrival 3 minutes early.
Best time to fly STL to SEA: a 5am–8am departure, ideally on a Wednesday. Flights in that window arrived on time 90.3% of the time and Wednesday flights 85.5%, against 77.6% for the route overall.
The US average over the same twelve months is 77.6%, so there is little to separate this route from the country as a whole. The variation that matters here is internal: which carrier, and what time of day.
Key figures
On time
77.6%
Arrivals under 15 minutes late, of 874 completed flights
Avg delay
48 min
Across delayed arrivals only, not all flights
Cancelled
0.90%
National rate is 1.81%
Best carrier
Alaska
80.7% on time across 781 flights
These figures cover 884 flights between July 2025 and June 2026. The median arrival is 3 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 STL to SEA
Only one carrier clears this site's 365-flight floor here, so there is no comparison to draw. The table below is the full picture.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| Alaska Airlines | 781 | 80.7% | 49 min | 1.02% | B |
| Southwest Airlinesthin sample | 103 | 54.4% | 43 min | 0.00% | F+ |
Flight by flight
Flights on STL 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.
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 33 points, against half a point between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am322 flights90.3%
- 8am–11am9 flights—
- 11am–2pm51 flights56.9%
- 2pm–5pm346 flights78.5%
- 5pm–8pm135 flights57.8%
- 8pm–5am21 flights—
Scale runs 50% to 95%. Rows with fewer than 30 flights show a dash.
The weak window here is the middle of the day, not the evening: 11am–2pm runs 56.9% on time against 90.3% for 5am–8am. That shape usually means a departure bank rather than an accumulating delay, so the recovery later in the afternoon is real rather than noise. The 2pm–5pm window is the busiest, with 346 flights scheduled in it.
Across the year
When delays peak on this route
This route's year swings by 40 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 54.7% on time, against 94.8% in April. 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 123 flights.
Across the week, Wednesday is the strongest day at 85.5% on time and Saturday the weakest at 67.0%. 19 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 9,360 reported delay minutes.
- 36%Late-arriving aircraft
- 34%Carrier
- 26%National Airspace System
- 3%Extreme weather
- 1%Security
36% 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 9,360 reported delay minutes here, that single category outweighs weather by 12 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 9,360 reported delay minutes.
The largest share of delay minutes, 34%, 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 3,211 of the 9,360 delay minutes reported here.
Cancellations sit at 0.90% against a national 1.81%, which is ordinary. Diversions add another 0.23%.
Connections and stage length
What a delay here actually costs
Badly late arrivals are rare here: only 3.8% 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.
At 1,709 miles this is transcontinental flying, and long routes get a quiet structural advantage. Scheduled block time on a flight this length carries more padding in absolute minutes, so a twenty-minute late departure often still becomes an on-time arrival. Read the on-time rate here as slightly flattering next to a short-haul route with the same operational quality.
The return leg runs 80.2% on time across 883 flights, within 3 points of this direction. The two legs are scored separately because they usually differ; on this city pair they happen not to.
One caution: this is a thin sample. 884 flights over twelve months is enough to clear this site's floor but not enough to pin the rate down tightly, and a single bad week moves it by more than a point. Treat the grade as a rough reading.
Work out the risk of missing a connection in SEA after this flight
What to do with this
Booking this route
Put together: treat all of this as provisional. 884 flights over twelve months is enough to clear this site's floor and not enough to be confident to better than a couple of points. The 77.6% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 874 completed flights over 12 months sit behind that.
Common questions
STL to SEA questions
How often is STL to SEA delayed?
Roughly one flight in four arrives at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 22.4% of the 884 flights in this analysis, against 22.4% nationally. Delayed flights are late by roughly 50 minutes on average.
How reliable is STL to SEA compared with the rest of the country?
77.6% of flights arrive on time here against a national 77.6%, a difference of half a point in its favour. That is worth a B+ on this site's grading scale.
What time of day has the fewest delays on STL to SEA?
The 5am–8am departure window is the strongest at 90.3% on time. The 11am–2pm window is the weakest at 56.9%, 33 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 STL to SEA?
December is the weakest month at 54.7% on time, compared with 94.8% in April. Across the year that is 40 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 STL — the return legseparate record
- St. Louis Lambert International (STL) delay statistics
- Seattle-Tacoma International (SEA) delay statistics
- Alaska Airlines on STL to SEA80.7%
- Southwest Airlines on STL to SEA54.4%
- STL to ATL, a related route85.3%
- LAX to SEA, a related route85.4%
- STL to DEN, a related route74.9%
- Alaska Airlines on-time record80.7%
- Southwest Airlines on-time record54.4%
- Why the first flight of the morning is statistically safest
- The best time of day to fly, according to 7.04 million flight records
Direction matters. STL to SEA and SEA to STL 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, July 2025 to June 2026. Last updated 15 September 2026; recomputed monthly. How this data is collected.