Route reliability record
SEA→STL
Seattle to St. Louis
- Flights
- 883
- Period
- July 2025 to June 2026
- Distance
- 1,709 mi
- Carriers
- 2
This route lands a B+, 3 points better than the national rate of 77.6%. About one flight in five arrives 15 minutes late or worse, and those flights are late by roughly 40 minutes on average. That is a workable record, not a spotless one. That rests on 883 SEA to STL flights, with a median arrival 6 minutes early.
Best time to fly SEA to STL: a 8am–11am departure, ideally on a Wednesday. Flights in that window arrived on time 88.3% of the time and Wednesday flights 84.4%, against 80.2% 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
80.2%
Arrivals under 15 minutes late, of 878 completed flights
Avg delay
42 min
Across delayed arrivals only, not all flights
Cancelled
0.23%
National rate is 1.81%
Best carrier
Alaska
82.2% on time across 780 flights
These figures cover 883 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 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 | 82.2% | 43 min | 0.26% | B+ |
| Southwest Airlinesthin sample | 103 | 64.7% | 39 min | 0.00% | D+ |
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 24 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–8am—
- 8am–11am388 flights88.3%
- 11am–2pm217 flights64.5%
- 2pm–5pm—
- 5pm–8pm278 flights81.2%
- 8pm–5am—
Scale runs 55% 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 64.5% on time against 88.3% 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 388 flights scheduled in it.
Across the year
When delays peak on this route
This route's year swings by 37 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 63.1% on time, 37 points below April, the best at 100.0%. 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 123 flights.
Across the week, Wednesday is the strongest day at 84.4% on time and Thursday the weakest at 77.0%. 7 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 7,298 reported delay minutes.
- 41%Late-arriving aircraft
- 40%Carrier
- 19%National Airspace System
- 0%Security
- 0%Extreme weather
41% 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 7,298 reported delay minutes here, that single category outweighs weather by 41 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 7,298 reported delay minutes.
The largest share of delay minutes, 40%, 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 2,886 of the 7,298 delay minutes reported here.
Cancellations are rare at 0.23%, below the national 1.81% 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 3.0% 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 77.6% on time across 884 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. 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 80.2% 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 five arrives at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 19.8% of the 883 flights in this analysis, against 22.4% nationally. Delayed flights are late by roughly 40 minutes on average.
How reliable is SEA to STL compared with the rest of the country?
80.2% of flights arrive on time here against a national 77.6%, a difference of 3 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 88.3% on time. The 11am–2pm window is the weakest at 64.5%, 24 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 SEA to STL?
August is the weakest month at 63.1% on time, compared with 100.0% in April. Across the year that is 37 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 STL82.2%
- Southwest Airlines on SEA to STL64.7%
- SEA to LAX, a related route83.4%
- ATL to STL, a related route82.4%
- SEA to PHX, a related route79.2%
- Alaska Airlines on-time record82.2%
- Southwest Airlines on-time record64.7%
- 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. 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, July 2025 to June 2026. Last updated 15 September 2026; recomputed monthly. How this data is collected.