Carrier route record

WNSEA→STL

Southwest Airlines · Seattle to St. Louis

Flights: 103
Period: July 2025 to June 2026
Share of route: 12%
Carriers: 2

Historical statistics for July 2025 to June 2026, not live status. For today’s flights, check with Southwest Airlines.

Southwest on SEA to STL draws a D+. About one flight in three arrives at least 15 minutes late, and the typical late arrival is about 40 minutes behind schedule. Build slack into anything you connect to. That rests on 103 flights, with a median arrival 8 minutes late.

The national on-time rate over the same period is 77.6%, so Southwest on SEA to STL runs 13 points behind the country as a whole. Across a thousand flights that is about 129 extra late arrivals.

On time

64.7%

Arrivals under 15 minutes late, of 102 completed flights

Avg delay

39 min

Across delayed arrivals only, not all flights

Cancelled

0.00%

National rate is 1.81%

Whole route

80.2%

Every carrier on SEA to STL, 883 flights

These figures cover 103 flights between July 2025 and June 2026. On-time means an arrival less than 15 minutes after the scheduled gate time, the same threshold the Department of Transportation uses in its own reports. Cancelled and diverted flights are excluded from the on-time rate and counted separately, which is why the cancellation column matters on its own.

Carrier by carrier

Southwest against every carrier on SEA to STL

Southwest flew 103 of the 883 flights on SEA to STL, about 12% of it. Its 64.7% on time is 16 points worse than the route as a whole, which runs 80.2%. Ranked by on-time rate it is last of two carriers on the route, where the range runs from Alaska at 82.2% to Southwest at 64.7%.

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.

On-time performance by carrier on SEA to STL
Airline	Flights	On time	Avg delay	Cancelled	Grade
Alaska Airlines	780	82.2%	43 min	0.26%	B+
Southwest Airlines (this page)thin sample	103	64.7%	39 min	0.00%	D+

Departure window

When Southwest runs best on this route

Departure time is the single lever a traveller controls, and it moves the odds more than most people expect. Each bar below is the on-time rate for flights scheduled to leave inside that window, across 103 flights.

There are not enough flights in each departure window to split Southwest's SEA to STL record by time of day without the percentages moving on noise. The headline rate of 64.7% is the reliable figure here.

Across the year

Southwest on SEA to STL through the year

Jul–Sep 202575 flights56.8%
Apr–Jun 202628 flights—

Scale runs 50% to 60%. Rows with fewer than 30 flights show a dash.

Monthly volumes are too low here to break the year into twelve reliable readings. What the twelve-month total says is 64.7% on time across 103 flights.

Split by quarter, Southwest on SEA to STL logged 75 flights in Jul–Sep 2025 and 28 flights in Apr–Jun 2026. Fewer than three of those quarters carried the 30 completed flights this site needs before printing a rate, so the 64.7% annual figure is the one to use.

Delay causes

Why Southwest runs late on this route

One category carries most of the weight here. Late-arriving aircraft accounts for 48% of the 1,408 delay minutes carriers reported, and the remaining four split what is left. The split is by minutes rather than by flights, so a few very long delays count for as much as many short ones.

Late-arriving aircraft
48%
Carrier
32%
National Airspace System
17%
Security
2%

48% 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 1,408 reported delay minutes here, that single category outweighs weather by 48 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 1,408 reported delay minutes.

Cancellations are rare at 0.00%, below the national 1.81% and low enough to earn the plus on the grade. Almost no flights are called off; the risk here is arriving late, not not arriving.

Connections and stage length

What a delay here costs

Badly late arrivals are rare here: only 4.9% 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 is a different operation and performs like one: it runs 54.4% on time across 103 flights, 10 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 10 points of difference.

One caution: this is a thin sample. 103 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.

What to do with this

Booking Southwest on SEA to STL

Put together: treat all of this as provisional. 103 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 64.7% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 102 completed flights over 12 months sit behind that.

Where to go next

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.

Key figures