Route reliability record
SEA→STL
Seattle to St. Louis
- Flights
- 883
- Period
- September 2025 to August 2026
- Distance
- 1,709 mi
- Carriers
- 2
This route earns a B+. More than four flights in five arrive inside the 15-minute window the DOT uses to define an on-time arrival, which puts it 7 points better than the national rate of 77.3%. When something does go wrong the average delay runs about 50 minutes. That rests on 883 SEA to STL flights, with a median arrival 10 minutes early.
Best time to fly SEA to STL: a 8am–11am departure, ideally on a Tuesday. Flights in that window arrived on time 90.6% of the time and Tuesday flights 92.6%, against 84.4% for the route overall.
Set against the rest of the country, this route's record is 7 points better than the US average of 77.3%. That gap is worth roughly 71 fewer late arrivals for every thousand flights.
Key figures
On time
84.4%
Arrivals under 15 minutes late, of 878 completed flights
Avg delay
50 min
Across delayed arrivals only, not all flights
Cancelled
0.34%
National rate is 1.89%
Best carrier
Alaska
85.8% on time across 780 flights
These figures cover 883 flights between September 2025 and August 2026. The median arrival is 10 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 SEA to STL
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 | 780 | 85.8% | 48 min | 0.26% | A+ |
| Southwest Airlinesthin sample | 103 | 73.5% | 55 min | 0.97% | C+ |
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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 half a point between the best and worst carrier.
- 5am–8am—
- 8am–11am407 flights90.6%
- 11am–2pm198 flights73.2%
- 2pm–5pm—
- 5pm–8pm278 flights83.4%
- 8pm–5am—
Scale runs 65% 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 73.2% on time against 90.6% for 8am–11am. 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 8am–11am window is the busiest, with 407 flights scheduled in it.
Explore
Filter SEA to STL 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 SEA to STL, the best departure window is 8am–11am at 90.6% on time and the worst is 11am–2pm at 73.2%, a wide gap of 17.4 points. All US flights leaving 8am–11am ran 83.9%.
- 5–8am—
- 8–11am90.6%
- 11am–2pm73.2%
- 2–5pm—
- 5–8pm83.4%
- 8pm–5am—
best all US flightsScale 60% to 95%. Cells under 30 flights show a dash.
Across the year
When delays peak on this route
This route's year swings by 27 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.
April is the best month on the calendar at 100.0% on time. Shoulder season thins the schedule and takes the summer weather with it, and the operation gets its margin back. July was the busiest month here, with 123 flights.
Across the week, Tuesday is the strongest day at 92.6% on time and Thursday the weakest at 76.3%, a spread of 16 points.
Delay causes
Why this route gets delayed
Carrier accounts for 42% of the 6,795 delay minutes carriers reported here, and the remaining four split what is left.
- 42%Carrier
- 35%Late-arriving aircraft
- 18%National Airspace System
- 4%Extreme weather
- 1%Security
35% of delay minutes trace back to a late-arriving aircraft, meaning the plane was already behind before it got here. Across 6,795 reported delay minutes here, that single category outweighs weather by 9 to one. The split is drawn from 6,795 reported delay minutes.
The largest share of delay minutes, 42%, falls under carrier control: crew scheduling, maintenance, fuelling, baggage. It accounts for 2,858 of the 6,795 delay minutes reported here.
Cancellations are rare at 0.34%, below the national 1.89% 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 actually costs
Badly late arrivals are rare here: only 2.7% 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.
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 is a different operation and performs like one: it runs 78.6% on time across 884 flights, 6 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 6 points of difference.
One caution: this is a thin sample. 883 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 STL after this flight
What to do with this
Booking this route
Put together: treat all of this as provisional. 883 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 84.4% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 878 completed flights over 12 months sit behind that.
Common questions
SEA to STL questions
How often is SEA to STL delayed?
About one flight in six arrives at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 15.6% of the 883 flights in this analysis, against 22.7% nationally. Delayed flights are late by about 50 minutes on average.
How reliable is SEA to STL compared with the rest of the country?
84.4% of flights arrive on time here against a national 77.3%, a difference of 7 points in its favour. That is worth a B+ on this site's grading scale.
What time of day has the fewest delays on SEA to STL?
The 8am–11am departure window is the strongest at 90.6% on time. The 11am–2pm window is the weakest at 73.2%, 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 September delays on SEA to STL?
September is the weakest month at 73.5% on time, compared with 100.0% in April. Across the year that is 27 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
- STL to SEA — the return legseparate record
- Seattle-Tacoma International (SEA) delay statistics
- St. Louis Lambert International (STL) delay statistics
- Alaska Airlines on SEA to STL85.8%
- Southwest Airlines on SEA to STL73.5%
- SEA to LAX, a related route82.0%
- DEN to STL, a related route77.7%
- SEA to PHX, a related route78.2%
- Alaska Airlines on-time record85.8%
- Southwest Airlines on-time record73.5%
- 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. SEA to STL and STL to SEA 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.