Airport month record

SEASEP

Seattle-Tacoma International in September 2025

Departures: 14,552
Month: September 2025
National, same month: 83.4%
Carriers: 10

Historical statistics for September 2025, not live status or a forecast for this year’s September. For today’s conditions at SEA, see live FAA status.

SEA in September lands a B+, level with the national rate of 83.4%. About one flight in six arrives 15 minutes late or worse, and those flights are late by about 50 minutes on average. That is a workable record, not a spotless one. That rests on 14,552 flights, of which United Airlines flew 831 at 88.5% on time.

United Airlines is the most reliable of the six carriers covered here at 88.5% on time across 831 flights.

On time

82.9%

Arrivals under 15 minutes late, of 14,477 completed departures

Avg delay

50 min

Across delayed arrivals only, not all flights

Cancelled

0.29%

Nationally in September: 0.51%

Whole year

79.4%

SEA across all twelve months

These figures cover 14,552 departures from Seattle-Tacoma International in September 2025. On time means an arrival less than 15 minutes after the scheduled gate time, the DOT's threshold, and the national comparison uses every US departure in the same month rather than the whole year.

Against the year

September next to SEA's other months

Against SEA's own year, September ran 4 points better than the twelve-month rate of 79.4%, which makes it number three of twelve months, best first. Cancellations were unusually low for SEA in September, at 0.29% against 0.99% across the year. Either side of it, August ran 73.6% on time and October 82.9%, so a trip moved by a few weeks would have landed in a worse stretch.

SEA month by month
Month	Flights	On time	Avg delay	Cancelled	Grade
July 2025	16,577	69.2%	57 min	1.38%	C
August 2025	15,970	73.6%	51 min	0.66%	C+
September 2025 (this page)	14,552	82.9%	50 min	0.29%	B+
October 2025	13,859	82.9%	58 min	1.16%	B
November 2025	12,616	81.9%	58 min	1.69%	B
December 2025	13,023	74.0%	59 min	1.02%	C
January 2026	11,927	81.6%	60 min	1.91%	B
February 2026	11,000	84.4%	56 min	0.78%	B+
March 2026	12,830	80.2%	64 min	2.23%	B
April 2026	12,864	85.6%	56 min	0.17%	A+
May 2026	14,564	82.2%	52 min	0.41%	B+
June 2026	15,402	78.1%	57 min	0.48%	B+

Departure window

The best time of day to leave SEA in September

Departure time matters more here than the choice of carrier. The gap between the best and worst window is 16 points, against 19 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.

5am–8am1,584 flights92.6%
8am–11am3,212 flights84.6%
11am–2pm2,778 flights78.7%
2pm–5pm2,319 flights81.1%
5pm–8pm2,183 flights76.6%
8pm–5am2,476 flights86.5%

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 92.6% of the time, against 76.6% for the evening window. That is 16 points of difference for no extra money, and it comes from the same aircraft having had no earlier flight to fall behind on. The 8am–11am window is the busiest, with 3,212 flights scheduled in it.

Across the week, Saturday is the strongest day at 87.6% on time and Thursday the weakest at 76.9%. 11 points is a smaller lever than departure hour, but it is free.

Carrier by carrier

Which airlines ran best at SEA in September

The choice of carrier is not cosmetic here. United Airlines arrives on time 88.5% of the time; Southwest Airlines manages 69.2%. Sample sizes sit next to every figure, because a two-point gap across 861 flights means less than a ten-point one.

Carriers at SEA in September 2025
Airline	Flights	On time	Avg delay	Cancelled	Grade
Alaska Airlines	6,559	81.7%	42 min	0.41%	B+
Delta Air Lines	2,718	85.7%	66 min	0.07%	A+
SkyWest Airlines	2,658	87.3%	49 min	0.08%	A+
Southwest Airlines	861	69.2%	42 min	0.23%	C+
United Airlines	831	88.5%	62 min	0.00%	A+
American Airlines	547	80.9%	78 min	0.91%	B+
Hawaiian Airlinesthin sample	161	70.0%	69 min	0.62%	C+
Frontier Airlinesthin sample	115	75.2%	102 min	1.74%	B
JetBlue Airwaysthin sample	62	80.3%	47 min	1.61%	B
Envoy Airthin sample	40	87.2%	38 min	0.00%	A+

The spread between United and Southwest is 19 points, wide enough that it should decide the booking. On a hundred trips that is 19 more arrivals inside the on-time window.

Route by route

Busiest routes out of SEA in September

Routes from SEA in September 2025
Route	Flights	On time	Avg delay	Cancelled	Grade
SEA to ANC · Anchorage	624	88.4%	69 min	0.16%	A+
SEA to LAX · Los Angeles	605	88.7%	64 min	0.33%	A+
SEA to SFO · San Francisco	584	75.2%	48 min	0.68%	B+
SEA to DEN · Denver	574	75.9%	45 min	0.00%	B+
SEA to PHX · Phoenix	545	84.2%	53 min	0.73%	B+
SEA to ORD · Chicago	509	79.7%	45 min	0.00%	B+
SEA to GEG · Spokane	462	83.3%	47 min	0.22%	B+
SEA to LAS · Las Vegas	440	75.4%	56 min	0.00%	B+
SEA to PDX · Portland	433	84.1%	39 min	0.00%	B+
SEA to DFW · Dallas-Fort Worth	430	80.8%	63 min	1.40%	B
SEA to SLC · Salt Lake City	398	88.9%	45 min	0.25%	A+
SEA to SMF · Sacramento	398	84.1%	43 min	0.25%	B+
SEA to SAN · San Diego	387	81.7%	47 min	1.55%	B
SEA to SJC · San Jose	367	83.1%	46 min	0.00%	B+
SEA to BOI · Boisethin sample	334	86.2%	41 min	0.00%	A+
SEA to SNA · Santa Anathin sample	330	88.1%	40 min	0.61%	A+
SEA to ATL · Atlantathin sample	284	90.1%	117 min	0.00%	A+
SEA to JFK · New Yorkthin sample	270	76.4%	42 min	0.37%	B+
SEA to MSP · Minneapolisthin sample	264	85.9%	51 min	0.38%	A+
SEA to HNL · Honoluluthin sample	240	74.5%	61 min	0.42%	C+
SEA to FAI · Fairbanksthin sample	235	87.7%	48 min	0.00%	A+
SEA to BOS · Bostonthin sample	229	67.2%	60 min	0.00%	C+
SEA to EWR · Newarkthin sample	227	87.2%	73 min	0.00%	A+
SEA to ONT · Ontariothin sample	213	91.1%	47 min	0.00%	A+
SEA to EUG · Eugenethin sample	210	85.7%	45 min	0.00%	A+

Delay causes

What held SEA up in September

One category carries most of the weight here. Carrier accounts for 46% of the 124,000 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.

Carrier
46%
Late-arriving aircraft
27%
National Airspace System
24%
Extreme weather
3%
Security
0%

The largest share of delay minutes, 46%, 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 57,000 of the 124,000 delay minutes reported here. The split is drawn from 123,799 reported delay minutes.

Cancellations are rare at 0.29%, below the national 0.51% 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.

Badly late arrivals are rare here: only 3.6% land an hour or more behind schedule, against 5.4% 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.

What to do with this

Flying from SEA in September

For a September trip through SEA: this is one of the better months to be flying from here, 82.9% on time with 0.29% cancelled. That is a base rate for the month as a whole; a single storm can still spoil a particular day.

Seattle-Tacoma International (SEA), the whole year
SEA in July69.2%
SEA in August73.6%
SEA in October82.9%
SEA in November81.9%
SEA in December74.0%
SEA in January81.6%
SEA in February84.4%
SEA in March80.2%
SEA in April85.6%
SEA in May82.2%
SEA in June78.1%

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